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Title: A Popular History of Astronomy During the Nineteenth Century

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A POPULAR HISTORY OF
ASTRONOMY
DURING THE NINETEENTH
CENTURY

BY THE SAME AUTHOR

PROBLEMS IN
ASTROPHYSICS.
Demy 8vo., cloth. Containing over 100
Illustrations. Price 20s. net.

THE SYSTEM OF
THE STARS.
Second Edition. Thoroughly revised and largely
rewritten. Containing numerous and new
Illustrations. Demy 8vo., cloth. Price 20s. net.

MODERN
COSMOGONIES.
Crown 8vo., cloth. Price 3s. 6d. net.

A. AND C. BLACK, SOHO SQUARE, LONDON, W.

THE GREAT NEBULA IN ORION, 1883
See p. 408

A POPULAR

HISTORY OF ASTRONOMY
DURING

THE NINETEENTH CENTURY
BY

AGNES M. CLERKE
JUPITER 1879

SATURN 1885

LONDON
ADAM AND CHARLES BLACK
1908

First Edition, Post 8vo., published 1885

Second Edition, Post 8vo., published 1887

Third Edition, Demy 8vo., published 1893

Fourth Edition, Demy 8vo., published 1902

Fourth Edition, Post 8vo., reprinted February, 1908

PREFACE TO THE FOURTH EDITION

Since the third edition of the present work issued from the press, the
nineteenth century has run its course and finished its record. A new era has
dawned, not by chronological prescription alone, but to the vital sense of
humanity. Novel thoughts are rife; fresh impulses stir the nations; the
soughing of the wind of progress strikes every ear. "The old order
changeth" more and more swiftly as mental activity becomes intensified.
Already many of the scientific doctrines implicitly accepted fifteen years
ago begin to wear a superannuated aspect. Dalton's atoms are in process of
disintegration; Kirchhoff's theorem visibly needs to be modified; Clerk
Maxwell's medium no longer figures as an indispensable factotum;
"absolute zero" is known to be situated on an asymptote to the curve of
cold. Ideas, in short, have all at once become plastic, and none more
completely so than those relating to astronomy. The physics of the heavenly
bodies, indeed, finds its best opportunities in unlooked-for disclosures; for
it deals with transcendental conditions, and what is strange to terrestrial
experience may serve admirably to expound what is normal in the skies. In
celestial science especially, facts that appear subversive are often the most
illuminative, and the prospect of its advance widens and brightens with
each divagation enforced or permitted from the strait paths of rigid theory.

This readiness for innovation has undoubtedly its dangers and drawbacks.
To the historian, above all, it presents frequent occasions of embarrassment.
The writing of history is a strongly selective operation, the outcome being
valuable just in so far as the choice what to reject and what to include has
been judicious; and the task is no light one of discriminating between
barren speculations and ideas pregnant with coming truth. To the possession
of such prescience of the future as would be needed to do this effectually I

can lay no claim; but diligence and sobriety of thought are ordinarily within
reach, and these I shall have exercised to good purpose if I have succeeded
in rendering the fourth edition of A Popular History of Astronomy during
the Nineteenth Century not wholly unworthy of a place in the scientific
literature of the twentieth century.

My thanks are due to Sir David Gill for the use of his photograph of the
great comet of 1901, which I have added to my list of illustrations, and to
the Council of the Royal Astronomical Society for the loan of glass
positives needed for the reproduction of those included in the third edition.

London, July, 1902.

PREFACE TO THE FIRST EDITION

The progress of astronomy during the last hundred years has been rapid and
extraordinary. In its distinctive features, moreover, the nature of that
progress has been such as to lend itself with facility to untechnical
treatment. To this circumstance the present volume owes its origin. It
embodies an attempt to enable the ordinary reader to follow, with intelligent
interest, the course of modern astronomical inquiries, and to realize (so far
as it can at present be realized) the full effect of the comprehensive change
in the whole aspect, purposes, and methods of celestial science introduced
by the momentous discovery of spectrum analysis.

Since Professor Grant's invaluable work on the History of Physical
Astronomy was published, a third of a century has elapsed. During the
interval a so-called "new astronomy" has grown up by the side of the old.
One effect of its advent has been to render the science of the heavenly
bodies more popular, both in its needs and in its nature, than formerly. More
popular in its needs, since its progress now primarily depends upon the
interest in, and consequent efforts towards its advancement of the general
public; more popular in its nature, because the kind of knowledge it now
chiefly tends to accumulate is more easily intelligible—less remote from

ordinary experience—than that evolved by the aid of the calculus from
materials collected by the use of the transit-instrument and chronograph.

It has thus become practicable to describe in simple language the most
essential parts of recent astronomical discoveries, and, being practicable, it
could not be otherwise than desirable to do so. The service to astronomy
itself would be not inconsiderable of enlisting wider sympathies on its
behalf, while to help one single mind towards a fuller understanding of the
manifold works which have in all ages irresistibly spoken to man of the
glory of God might well be an object of no ignoble ambition.

The present volume does not profess to be a complete or exhaustive history
of astronomy during the period covered by it. Its design is to present a view
of the progress of celestial science, on its most characteristic side, since the
time of Herschel. Abstruse mathematical theories, unless in some of their
more striking results, are excluded from consideration. These, during the
eighteenth century, constituted the sum and substance of astronomy, and
their fundamental importance can never be diminished, and should never be
ignored. But as the outcome of the enormous development given to the
powers of the telescope in recent times, together with the swift advance of
physical science, and the inclusion, by means of the spectroscope, of the
heavenly bodies within the domain of its inquiries, much knowledge has
been acquired regarding the nature and condition of those bodies, forming,
it might be said, a science apart, and disembarrassed from immediate
dependence upon intricate, and, except to the initiated, unintelligible
formulæ. This kind of knowledge forms the main subject of the book now
offered to the public.

There are many reasons for preferring a history to a formal treatise on
astronomy. In a treatise, what we know is set forth. A history tells us, in
addition, how we came to know it. It thus places facts before us in the
natural order of their ascertainment, and narrates instead of enumerating.
The story to be told leaves the marvels of imagination far behind, and
requires no embellishment from literary art or high-flown phrases. Its best

ornament is unvarnished truthfulness, and this, at least, may confidently be
claimed to be bestowed upon it in the ensuing pages.

In them unity of treatment is sought to be combined with a due regard to
chronological sequence by grouping in separate chapters the various events
relating to the several departments of descriptive astronomy. The whole is
divided into two parts, the line between which is roughly drawn at the
middle of the present century. Herschel's inquiries into the construction of
the heavens strike the keynote of the first part; the discoveries of sun-spot
and magnetic periodicity and of spectrum analysis determine the character
of the second. Where the nature of the subject required it, however, this
arrangement has been disregarded. Clearness and consistency should
obviously take precedence of method. Thus, in treating of the telescopic
scrutiny of the various planets, the whole of the related facts have been
collected into an uninterrupted narrative. A division elsewhere natural and
helpful would here have been purely artificial, and therefore confusing.

The interests of students have been consulted by a full and authentic system
of references to the sources of information relied upon. Materials have been
derived, as a rule with very few exceptions, from the original authorities.
The system adopted has been to take as little as possible at second-hand.
Much pains have been taken to trace the origin of ideas, often obscurely
enunciated long before they came to resound through the scientific world,
and to give to each individual discoverer, strictly and impartially, his due.
Prominence has also been assigned to the biographical element, as
underlying and determining the whole course of human endeavour. The
advance of knowledge may be called a vital process. The lives of men are
absorbed into and assimilated by it. Inquiries into the kind and mode of the
surrender in each separate case must always possess a strong interest,
whether for study or for example.

The acknowledgments of the writer are due to Professor Edward S. Holden,
director of the Washburn Observatory, Wisconsin, and to Dr. Copeland,

chief astronomer of Lord Crawford's Observatory at Dunecht, for many
valuable communications.

London, September, 1885.

CONTENTS
INTRODUCTION

Three Kinds of Astronomy—Progress of the Science during the Eighteenth
Century—Popularity and Rapid Advance during the Nineteenth Century

PART I
PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE
NINETEENTH CENTURY

CHAPTER I
FOUNDATION OF SIDEREAL ASTRONOMY

State of Knowledge regarding the Stars in the Eighteenth Century—Career
of Sir William Herschel—Constitution of the Stellar System—Double
Stars—Herschel's Discovery of their Revolutions—His Method of
Star-gauging—Discoveries of Nebulæ—Theory of their Condensation
into Stars—Summary of Results

CHAPTER II

PROGRESS OF SIDEREAL ASTRONOMY

Exact Astronomy in Germany—Career of Bessel—His Fundamenta
Astronomiæ—Career of Fraunhofer—Parallaxes of Fixed Stars—
Translation of the Solar System—Astronomy of the Invisible—
Struve's Researches in Double Stars—Sir John Herschel's Exploration
of the Heavens—Fifty Years' Progress

CHAPTER III
PROGRESS OF KNOWLEDGE REGARDING THE SUN

Early Views as to the Nature of Sun-spots—Wilson's Observations and
Reasonings—Sir William Herschel's Theory of the Solar Constitution
—Sir John Herschel's Trade-Wind Hypothesis—Baily's Beads—Total
Solar Eclipse of 1842—Corona and Prominences—Eclipse of 1851

CHAPTER IV
PLANETARY DISCOVERIES

Bode's Law—Search for a Missing Planet—Its Discovery by Piazzi—
Further Discoveries of Minor Planets—Unexplained Disturbance of
Uranus—Discovery of Neptune—Its Satellite—An Eighth Saturnian
Moon—Saturn's Dusky Ring—The Uranian System

CHAPTER V
COMETS

Predicted Return of Halley's Comet—Career of Olbers—Acceleration of
Encke's Comet—Biela's Comet—Its Duplication—Faye's Comet—
Comet of 1811—Electrical Theory of Cometary Emanations—The

Earth in a Comet's Tail—Second Return of Halley's Comet—Great
Comet of 1843—Results to Knowledge

CHAPTER VI
INSTRUMENTAL ADVANCES

Two Principles of Telescopic Construction—Early Reflectors—Three
Varieties—Herschel's Specula—High Magnifying Powers—Invention
of the Achromatic Lens—Guinand's Optical Glass—The Great Rosse
Reflector—Its Disclosures—Mounting of Telescopes—Astronomical
Circles—Personal Equation

PART II
RECENT PROGRESS OF ASTRONOMY

CHAPTER I
FOUNDATION OF ASTRONOMICAL PHYSICS

Schwabe's Discovery of a Decennial Sun-spot Period—Coincidence with
Period of Magnetic Disturbance—Sun-spots and Weather—Spectrum
Analysis—Preliminary Inquiries—Fraunhofer Lines—Kirchhoff's
Principle—Anticipations—Elementary Principles of Spectrum
Analysis—Unity of Nature

CHAPTER II

SOLAR OBSERVATIONS AND THEORIES

Black Openings in Spots—Carrington's Observations—Rotation of the Sun
—Kirchhoff's Theory of the Solar Constitution—Faye's Views—Solar
Photography—Kew Observations—Spectroscopic Method—Cyclonic
Theory of Sun-spots—Volcanic Hypothesis—A Solar Outburst—Sun-
spot Periodicity—Planetary Influence—Structure of the Photosphere

CHAPTER III
RECENT SOLAR ECLIPSES

Expeditions to Spain—Great Indian Eclipse—New Method of Viewing
Prominences—Total Eclipse Visible in North America—Spectrum of
the Corona—Eclipse of 1870—Young's Reversing Layer—Eclipse of
1871—Corona of 1878—Varying Coronal Types—Egyptian Eclipse—
Daylight Coronal Photography—Observations at Caroline Island—
Photographs of Corona in 1886 and 1889—Eclipses of 1896, 1898,
1900, and 1901—Mechanical Theory of Corona—Electro-Magnetic
Theories—Nature of Corona

CHAPTER IV
SOLAR SPECTROSCOPY

Chemistry of Prominences—Study of their Forms—Two Classes—
Photographs and Spectrographs of Prominences—Their Distribution—
Structure of the Chromosphere—Spectroscopic Measurement of
Radial Movements—Spectroscopic Determination of Solar Rotation—
Velocities of Transport in the Sun—Lockyer's Theory of Dissociation
—Solar Constituents—Oxygen Absorption in Solar Spectrum

CHAPTER V

TEMPERATURE OF THE SUN

Thermal Power of the Sun—Radiation and Temperature—Estimates of
Solar Temperature—Rosetti's and Wilson's Results—Zöllner's Method
—Langley's Experiment at Pittsburg—The Sun's Atmosphere—
Langley's Bolometric Researches—Selective Absorption by our Air—
The Solar Constant

CHAPTER VI
THE SUN'S DISTANCE

Difficulty of the Problem—Oppositions of Mars—Transits of Venus—
Lunar Disturbance—Velocity of Light—Transit of 1874—
Inconclusive Result—Opposition of Mars in 1877—Measurements of
Minor Planets—Transit of 1882—Newcomb's Determination of the
Velocity of Light—Combined Result

CHAPTER VII
PLANETS AND SATELLITES

Schröter's Life and Work—Luminous Appearances during Transits of
Mercury—Mountains of Mercury—Intra-Mercurian Planets—
Schiaparelli's Results for the Rotation of Mercury and Venus—Illusory
Satellite—Mountains and Atmosphere of Venus—Ashen Light—
Solidity of the Earth—Variation of Latitude—Secular Changes of
Climate—Figure of the Globe—Study of the Moon's Surface—Lunar
Atmosphere—New Craters—Thermal Energy of Moonlight—Tidal
Friction

CHAPTER VIII

PLANETS AND SATELLITES—(continued)

Analogy between Mars and the Earth—Martian Snowcaps, Seas, and
Continents—Climate and Atmosphere—Schiaparelli's Canals—
Discovery of Two Martian Satellites—Photographic Detection of
Minor Planets—Orbit of Eros—Distribution of the Minor Planets—
Their Collective Mass and Estimated Diameters—Condition of Jupiter
—His Spectrum—Transits of his Satellites—Discovery of a Fifth
Satellite—The Great Red Spot—Constitution of Saturn's Rings—
Period of Rotation of the Planet—Variability of Japetus—Equatorial
Markings on Uranus—His Spectrum—Rotation of Neptune—Trans-
Neptunian Planets

CHAPTER IX
THEORIES OF PLANETARY EVOLUTION

Origin of the World according to Kant—Laplace's Nebular Hypothesis—
Maintenance of the Sun's Heat—Meteoric Hypothesis—Radiation as
an Effect of Contraction—Regenerative Theory—Faye's Scheme of
Planetary Development—Origin of the Moon—Effects of Tidal

CHAPTER X
RECENT COMETS

Donati's Comet—The Earth again Involved in a Comet's Tail—Comets of
the August and November Meteors—Star Showers—Comets and
Meteors—Biela's Comet and the Andromedes—Holmes's Comet—
Deflection of the Leonids—Orbits of Meteorites—Meteors with
Stationary Radiants—Spectroscopic Analysis of Cometary Light—
Comet of 1901—Coggia's Comet

CHAPTER XI
RECENT COMETS—(continued)

Forms of Comets' Tails—Electrical Repulsion—Brédikhine's Three Types
—Great Southern Comet—Supposed Previous Appearances—
Tebbutt's Comet and the Comet of 1807—Successful Photographs—
Schaeberle's Comet—Comet Wells—Sodium Blaze in Spectrum—
Great Comet of 1882—Transit across the Sun—Relation to Comets of
1843 and 1880—Cometary Systems—Spectral Changes in Comet of
1882—Brooks's Comet of 1889—Swift's Comet of 1892—Origin of
Comets

CHAPTER XII
STARS AND NEBULÆ

Stellar Chemistry—Four Orders of Stars—Their Relative Ages—Gaseous
Stars—Spectroscopic Star-Catalogues—Stellar Chemistry—Hydrogen
Spectrum in Stars—The Draper Catalogue—Velocities of Stars in Line
of Sight—Spectroscopic Binaries—Eclipses of Algol—Catalogues of
Variables—New Stars—Outbursts in Nebulæ—Nova Aurigæ—Nova
Persei—Gaseous Nebulæ—Variable Nebulæ—Movements of Nebulæ
—Stellar

Persei—Gaseous Nebulæ—Variable Nebulæ—Movements of Nebulæ—
Stellar and Nebular Photography—Nebulæ in the Pleiades—Photographic
Star-charting—Stellar Parallax—Double Stars—Stellar Photometry—Status
of Nebulæ—Photographs and Drawings of the Milky Way—Star Drift

CHAPTER XIII
METHODS OF RESEARCH

Development of Telescopic Power—Silvered Glass Reflectors—Giant
Refractors—Comparison with Reflectors—The Yerkes Telescope—
Atmospheric Disturbance—The Lick Observatory—Mechanical
Difficulties—The Equatoreal Coudé—The Photographic Camera—
Retrospect and Conclusion

APPENDIX
Chronology, 1774-1893—Chemical Elements in the Sun (Rowland, 1891)
—Epochs of Sun-spot Maximum and Minimum from 1610 to 1901—
Movements of Sun and Stars—List of Great Telescopes—List of
Observatories employed in the Construction of the Photographic Chart
and Catalogue of the Heavens

INDEX

LIST OF ILLUSTRATIONS

Photograph of the Great Nebula in Orion, 1883 Frontispiece

Photographs of Jupiter, 1879, and of Saturn, 1885 Vignette

Plate I. Photographs of the Solar Chromosphere and Prominences To
face p. 198

Plate II. Photograph of the Great Comet of May, 1901 (Taken at the
Royal Observatory, Cape of Good Hope)

Plate III. The Great Comet of September, (Photographed at the Cape of
Good Hope)

Plate IV. Photographs of Swift's Comet, 1892

Plate V. Photographic and Visual Spectrum of Nova Aurigæ

Plate VI. Photograph of the Milky Way in Sagittarius

HISTORY OF ASTRONOMY
DURING THE NINETEENTH CENTURY

INTRODUCTION

We can distinguish three kinds of astronomy, each with a different origin
and history, but all mutually dependent, and composing, in their
fundamental unity, one science. First in order of time came the art of
observing the returns, and measuring the places, of the heavenly bodies.
This was the sole astronomy of the Chinese and Chaldeans; but to it the
vigorous Greek mind added a highly complex geometrical plan of their
movements, for which Copernicus substituted a more harmonious system,
without as yet any idea of a compelling cause. The planets revolved in
circles because it was their nature to do so, just as laudanum sets to sleep
because it possesses a virtus dormitiva. This first and oldest branch is
known as "observational," or "practical astronomy." Its business is to note
facts as accurately as possible; and it is essentially unconcerned with
schemes for connecting those facts in a manner satisfactory to the reason.

The second kind of astronomy was founded by Newton. Its nature is best
indicated by the term "gravitational"; but it is also called "theoretical
astronomy."[1] It is based on the idea of cause; and the whole of its elaborate
structure is reared according to the dictates of a single law, simple in itself,
but the tangled web of whose consequences can be unravelled only by the
subtle agency of an elaborate calculus.

The third and last division of celestial science may properly be termed
"physical and descriptive astronomy." It seeks to know what the heavenly
bodies are in themselves, leaving the How? and

the Wherefore? of their movements to be otherwise answered. Now, such
inquiries became possible only through the invention of the telescope, so

that Galileo was, in point of fact, their originator. But Herschel first gave
them a prominence which the whole progress of science during the
nineteenth century served to confirm and render more exclusive.
Inquisitions begun with the telescope have been extended and made
effective in unhoped-for directions by the aid of the spectroscope and
photographic camera; and a large part of our attention in the present volume
will be occupied with the brilliant results thus achieved.

The unexpected development of this new physical-celestial science is the
leading fact in recent astronomical history. It was out of the regular course
of events. In the degree in which it has actually occurred it could certainly
not have been foreseen. It was a seizing of the prize by a competitor who
had hardly been thought qualified to enter the lists. Orthodox astronomers
of the old school looked with a certain contempt upon observers who spent
their nights in scrutinising the faces of the moon and planets rather than in
timing their transits, or devoted daylight energies, not to reductions and
computations, but to counting and measuring spots on the sun. They were
regarded as irregular practitioners, to be tolerated perhaps, but certainly not
encouraged.

The advance of astronomy in the eighteenth century ran in general an even
and logical course. The age succeeding Newton's had for its special task to
demonstrate the universal validity, and trace the complex results, of the law
of gravitation. The accomplishment of that task occupied just one hundred
years. It was virtually brought to a close when Laplace explained to the
French Academy, November 19, 1787, the cause of the moon's accelerated
motion. As a mere machine, the solar system, so far as it was then known,
was found to be complete and intelligible in all its parts; and in the
Mécanique Céleste its mechanical perfections were displayed under a form
of majestic unity which fitly commemorated the successive triumphs of
analytical genius over problems amongst the most arduous ever dealt with
by the mind of man.

Theory, however, demands a practical test. All its data are derived from
observation; and their insecurity becomes less tolerable as it advances
nearer to perfection. Observation, on the other hand, is the pitiless critic of
theory; it detects weak points, and provokes reforms which may be the
beginnings of discovery. Thus, theory and observation mutually act and
react, each alternately taking the lead in the endless race of improvement.

Now, while in France Lagrange and Laplace were bringing the gravitational
theory of the solar system to completion, work of a very different kind, yet
not less indispensable to the future welfare of astronomy, was being done in
England. The Royal Observatory at Greenwich is one of the few useful
institutions which date their origin from the reign of Charles II. The leading
position which it still occupies in the science of celestial observation was,
for near a century and a half after its foundation, an exclusive one.
Delambre remarked that, had all other materials of the kind been destroyed,
the Greenwich records alone would suffice for the restoration of astronomy.
The establishment was indeed absolutely without a rival.[2] Systematic
observations of sun, moon, stars, and planets were during the whole of the
eighteenth century made only at Greenwich. Here materials were
accumulated for the secure correction of theory, and here refinements were
introduced by which the exquisite accuracy of modern practice in
astronomy was eventually attained.

The chief promoter of these improvements was James Bradley. Few men
have possessed in an equal degree with him the power of seeing accurately,
and reasoning on what they see. He let nothing pass. The slightest
inconsistency between what appeared and what was to be expected roused
his keenest attention; and he never relaxed his mental grip of a subject until
it had yielded to his persistent inquisition. It was to these qualities that he
owed his discoveries of the aberration of light and the nutation of the earth's
axis. The first was announced in 1729. What is meant by it is that, owing to
the circumstance of light not being instantaneously transmitted, the
heavenly bodies appear shifted from their true places by an amount
depending upon the ratio which the velocity of light bears to the speed of

the earth in its orbit. Because light travels with enormous rapidity, the
shifting is very slight; and each star returns to its original position at the end
of a year.

Bradley's second great discovery was finally ascertained in 1748. Nutation
is a real "nodding" of the terrestrial axis produced by the dragging of the
moon at the terrestrial equatorial protuberance. From it results an apparent
displacement of the stars, each of them describing a little ellipse about its
true or "mean" position, in a period of nearly nineteen years.

Now, an acquaintance with the fact and the laws of each of these minute
irregularities is vital to the progress of observational astronomy; for without
it the places of the heavenly bodies could never be accurately known or
compared. So that Bradley, by their detection, at once raised the science to a
higher grade of precision. Nor was this the whole of his work. Appointed
Astronomer-Royal in 1742, he executed during the years 1750-62 a series
of observations which formed the real beginning of exact astronomy. Part of
their superiority must, indeed, be attributed to the co-operation of John
Bird, who provided Bradley in 1750 with a measuring instrument of till
then unequalled excellence. For not only was the art of observing in the
eighteenth century a peculiarly English art, but the means of observing were
furnished almost exclusively by British artists. John Dollond, the son of a
Spitalfields weaver, invented the achromatic lens in 1758, removing thereby
the chief obstacle to the development of the powers of refracting telescopes;
James Short, of Edinburgh, was without a rival in the construction of
reflectors; the sectors, quadrants, and circles of Graham, Bird, Ramsden,
and Cary were inimitable by Continental workmanship.

Thus practical and theoretical astronomy advanced on parallel lines in
England and France respectively, the improvement of their several tools—
the telescope and the quadrant on the one side, and the calculus on the other
—keeping pace. The whole future of the science seemed to be theirs. The
cessation of interest through a too speedy attainment of the perfection
towards which each spurred the other, appeared to be the only danger it held

in store for them. When all at once, a rival stood by their side—not, indeed,
menacing their progress, but threatening to absorb their popularity.

The rise of Herschel was the one conspicuous anomaly in the astronomical
history of the eighteenth century. It proved decisive of the course of events
in the nineteenth. It was unexplained by anything that had gone before; yet
all that came after hinged upon it. It gave a new direction to effort; it lent a
fresh impulse to thought. It opened a channel for the widespread public
interest which was gathering towards astronomical subjects to flow in.

Much of this interest was due to the occurrence of events calculated to
arrest the attention and excite the wonder of the uninitiated. The predicted
return of Halley's comet in 1759 verified, after an unprecedented fashion,
the computations of astronomers. It deprived such bodies for ever of their
portentous character; it ranked them as denizens of the solar system. Again,
the transits of Venus in 1761 and 1769 were the first occurrences of the kind
since the awakening of science to their consequence. Imposing
preparations, journeys to remote and hardly accessible regions, official
expeditions, international communications, all for the purpose of observing
them to the best advantage, brought their high significance vividly to the
public consciousness; a result aided by the facile pen of Lalande, in
rendering intelligible the means by which these elaborate arrangements
were to issue in an accurate knowledge of the sun's distance. Lastly,
Herschel's discovery of Uranus, March 13, 1781, had the surprising effect
of utter novelty. Since the human race had become acquainted with the
company of the planets, no addition had been made to their number. The
event thus broke with immemorial traditions, and seemed to show
astronomy as still young and full of unlooked-for possibilities.

Further popularity accrued to the science from the sequel of a career so
strikingly opened. Herschel's huge telescopes, his detection by their means
of two Saturnian and as many Uranian moons, his piercing scrutiny of the
sun, picturesque theory of its constitution, and sagacious indication of the
route pursued by it through space; his discovery of stellar revolving

systems, his bold soundings of the universe, his grandiose ideas, and the
elevated yet simple language in which they were conveyed—formed a
combination powerfully effective to those least susceptible of new
impressions. Nor was the evoked enthusiasm limited to the British Isles. In
Germany, Schröter followed—longo intervallo—in Herschel's track. Von
Zach set on foot from Gotha that general communication of ideas which
gives life to a forward movement. Bode wrote much and well for unlearned
readers. Lalande, by his popular lectures and treatises, helped to form an
audience which Laplace himself did not disdain to address in the Exposition
du Système du Monde.

This great accession of public interest gave the impulse to the
extraordinarily rapid progress of astronomy in the nineteenth century.
Official patronage combined with individual zeal sufficed for the elder
branches of the science. A few well-endowed institutions could accumulate
the materials needed by a few isolated thinkers for the construction of
theories of wonderful beauty and elaboration, yet precluded, by their
abstract nature, from winning general applause. But the new physical
astronomy depends for its prosperity upon the favour of the multitude
whom its striking results are well fitted to attract. It is, in a special manner,
the science of amateurs. It welcomes the most unpretending co-operation.
There is no one "with a true eye and a faithful hand" but can do good work
in watching the heavens. And not unfrequently, prizes of discovery which
the most perfect appliances failed to grasp, have fallen to the share of
ignorant or ill-provided assiduity.

Observers, accordingly, have multiplied; observatories have been founded
in all parts of the world; associations have been constituted for mutual help
and counsel. A formal astronomical congress met in 1789 at Gotha—then,
under Duke Ernest II. and Von Zach, the focus of German astronomy—and
instituted a combined search for the planet suspected to revolve
undiscovered between the orbits of Mars and Jupiter. The Astronomical
Society of London was established in 1820, and the similar German
institution in 1863. Both have been highly influential in promoting the

interests, local and general, of the science they are devoted to forward;
while functions corresponding to theirs have been discharged elsewhere by
older or less specially constituted bodies, and new ones of a more popular
character are springing up on all sides.

Modern facilities of communication have helped to impress more deeply
upon modern astronomy its associative character. The electric telegraph
gives a certain ubiquity which is invaluable to an observer of the skies.
With the help of a wire, a battery, and a code of signals, he sees whatever is
visible from any portion of our globe, depending, however, upon other eyes
than his own, and so entering as a unit into a widespread organisation of
intelligence. The press, again, has been a potent agent of co-operation. It
has mainly contributed to unite astronomers all over the world into a body
animated by the single aim of collecting "particulars" in their special branch
for what Bacon termed a History of Nature, eventually to be interpreted
according to the sagacious insight of some one among them gifted above
his fellows. The first really effective astronomical periodical was the
Monatliche Correspondenz, started by Von Zach in the year 1800. It was
followed in 1822 by the Astronomische Nachrichten, later by the Memoirs
and Monthly Notices of the Astronomical Society, and by the host of varied
publications which now, in every civilised country, communicate the
discoveries made in astronomy to divers classes of readers, and so
incalculably quicken the current of its onward flow.

Public favour brings in its train material resources. It is represented by
individual enterprise, and finds expression in an ample liberality. The first
regular observatory in the Southern Hemisphere was founded at Paramatta
by Sir Thomas Makdougall Brisbane in 1821. The Royal Observatory at the
Cape of Good Hope was completed in 1829. Similar establishments were
set to work by the East India Company at Madras, Bombay, and St. Helena,
during the first third of the nineteenth century. The organisation of
astronomy in the United States of America was due to a strong wave of
popular enthusiasm. In 1825 John Quincy Adams vainly urged upon
Congress the foundation of a National Observatory; but in 1843 the lectures

on celestial phenomena of Ormsby MacKnight Mitchel stirred an
impressionable audience to the pitch of providing him with the means of
erecting at Cincinnati the first astronomical establishment worthy the name
in that great country. On the 1st of January, 1882, no less than one hundred
and forty-four were active within its boundaries.

The apparition of the great comet of 1843 gave an additional fillip to the
movement. To the excitement caused by it the Harvard College Observatory
—called the "American Pulkowa"—directly owed its origin; and the
example was not ineffective elsewhere. The United States Naval
Observatory was built in 1844, Lieutenant Maury being its first Director.
Corporations, universities, municipalities, vied with each other in the
creation of such institutions; private subscriptions poured in; emissaries
were sent to Europe to purchase instruments and to procure instruction in
their use. In a few years the young Republic was, in point of astronomical
efficiency, at least on a level with countries where the science had been
fostered since the dawn of civilisation.

A vast widening of the scope of astronomy has accompanied, and in part
occasioned, the great extension of its area of cultivation which our age has
witnessed. In the last century its purview was a comparatively narrow one.
Problems lying beyond the range of the solar system were almost unheeded,
because they seemed inscrutable. Herschel first showed the sidereal
universe as accessible to investigation, and thereby offered to science new
worlds—majestic, manifold, "infinitely infinite" to our apprehension in
number, variety, and extent—for future conquest. Their gradual
appropriation has absorbed, and will long continue to absorb, the powers
which it has served to develop.

But this is not the only direction in which astronomy has enlarged, or rather
has levelled, its boundaries. The unification of the physical sciences is
perhaps the greatest intellectual feat of recent times. The process has
included astronomy; so that, like Bacon, she may now be said to have
"taken all knowledge" (of that kind) "for her province." In return, she

proffers potent aid for its increase. Every comet that approaches the sun is
the scene of experiments in the electrical illumination of rarefied matter,
performed on a huge scale for our benefit. The sun, stars, and nebulæ form
so many celestial laboratories, where the nature and mutual relations of the
chemical "elements" may be tried by more stringent tests than sublunary
conditions afford. The laws of terrestrial magnetism can be completely
investigated only with the aid of a concurrent study of the face of the sun.
The solar spectrum will perhaps one day, by its recurrent modifications, tell
us something of impending droughts, famines, and cyclones.

Astronomy generalises the results of the other sciences. She exhibits the
laws of Nature working over a wider area, and under more varied
conditions, than ordinary experience presents. Ordinary experience, on the
other hand, has become indispensable to her progress. She takes in at one
view the indefinitely great and the indefinitely little. The mutual revolutions
of the stellar multitude during tracts of time which seem to lengthen out to
eternity as the mind attempts to traverse them, she does not admit to be
beyond her ken; nor is she indifferent to the constitution of the minutest
atom of matter that thrills the ether into light. How she entered upon this
vastly expanded inheritance, and how, so far, she has dealt with it, is
attempted to be set forth in the ensuing chapters.

FOOTNOTES:
[1] The denomination "physical astronomy," first used by Kepler, and long
appropriated to this branch of the science, has of late been otherwise applied.

[2] Histoire de l'Astronomie au xviiie Siècle, p. 267.

PART I
PROGRESS OF ASTRONOMY DURING THE FIRST HALF OF THE
NINETEENTH CENTURY

CHAPTER I
FOUNDATION OF SIDEREAL ASTRONOMY

Until nearly a hundred years ago the stars were regarded by practical
astronomers mainly as a number of convenient fixed points by which the
motions of the various members of the solar system could be determined
and compared. Their recognised function, in fact, was that of milestones on
the great celestial highway traversed by the planets, as well as on the
byways of space occasionally pursued by comets. Not that curiosity as to
their nature, and even conjecture as to their origin, were at any period
absent. Both were from time to time powerfully stimulated by the
appearance of startling novelties in a region described by philosophers as
"incorruptible," or exempt from change. The catalogue of Hipparchus
probably, and certainly that of Tycho Brahe, some seventeen centuries later,
owed each its origin to the temporary blaze of a new star. The general
aspect of the skies was thus (however imperfectly) recorded from age to
age, and with improved appliances the enumeration was rendered more and
more accurate and complete; but the secrets of the stellar sphere remained
inviolate.

In a qualified though very real sense, Sir William Herschel may be called
the Founder of Sidereal Astronomy. Before his time some curious facts had
been noted, and some ingenious speculations hazarded, regarding the
condition of the stars, but not even the rudiments of systematic knowledge
had been acquired. The facts ascertained can be summed up in a very few
sentences.

Giordano Bruno was the first to set the suns of space in motion; but in
imagination only. His daring surmise was, however, confirmed in 1718,
when Halley announced[3] that Sirius, Aldebaran, Betelgeux, and Arcturus
had unmistakably shifted their quarters in the sky since Ptolemy assigned

their places in his catalogue. A similar conclusion was reached by J. Cassini
in 1738, from a comparison of his own observations with those made at
Cayenne by Richer in 1672; and Tobias Mayer drew up in 1756 a list
showing the direction and amount of about fifty-seven proper motions,[4]
founded on star-places determined by Olaus Römer fifty years previously.
Thus the stars were no longer regarded as "fixed," but the question
remained whether the movements perceived were real or only apparent; and
this it was not yet found possible to answer. Already, in the previous
century, the ingenious Robert Hooke had suggested an "alteration of the
very system of the sun,"[5] to account for certain suspected changes in
stellar positions; Bradley in 1748, and Lambert in 1761, pointed out that
such apparent displacements (by that time well ascertained) were in all
probability a combined effect of motions both of sun and stars; and Mayer
actually attempted the analysis, but without result.

On the 13th of August, 1596, David Fabricius, an unprofessional
astronomer in East Friesland, saw in the neck of the Whale a star of the
third magnitude, which by October had disappeared. It was, nevertheless,
visible in 1603, when Bayer marked it in his catalogue with the Greek letter
ο, and was watched, in 1638-39, through its phases of brightening and
apparent extinction by a Dutch professor named Holwarda.[6] From
Hevelius this first-known periodical star received the name of "Mira," or the
Wonderful, and Boulliaud in 1667 fixed the length of its cycle of change at
334 days. It was not a solitary instance. A star in the Swan was perceived by
Janson in 1600 to show fluctuations of light, and Montanari found in 1669
that Algol in Perseus shared the same peculiarity to a marked degree.
Altogether the class embraced in 1782 half-a-dozen members. When it is
added that a few star-couples had been noted in singularly, but it was
supposed accidentally, close juxtaposition, and that the failure of repeated
attempts to measure stellar parallaxes pointed to distances at least 400,000
times that of the earth from the sun,[7] the picture of sidereal science, when
the last quarter of the eighteenth century began, is practically complete. It
included three items of information: that the stars have motions, real or

apparent; that they are immeasurably remote; and that a few shine with a
periodically variable light. Nor were these scantily collected facts ordered
into any promise of further development. They lay at once isolated and
confused before the inquirer. They needed to be both multiplied and
marshalled, and it seemed as if centuries of patient toil must elapse before
any reliable conclusions could be derived from them. The sidereal world
was thus the recognised domain of far-reaching speculations, which
remained wholly uncramped by systematic research until Herschel entered
upon his career as an observer of the heavens.

The greatest of modern astronomers was born at Hanover, November 15,
1738. He was the fourth child of Isaac Herschel, a hautboy-player in the
band of the Hanoverian Guard, and was early trained to follow his father's
profession. On the termination, however, of the disastrous campaign of
1757, his parents removed him from the regiment, there is reason to believe,
in a somewhat unceremonious manner. Technically, indeed, he incurred the
penalties of desertion, remitted—according to the Duke of Sussex's
statement to Sir George Airy—by a formal pardon handed to him personally
by George III. on his presentation in 1782.[8] At the age of nineteen, then,
his military service having lasted four years, he came to England to seek his
fortune. Of the life of struggle and privation which ensued little is known
beyond the circumstances that in 1760 he was engaged in training the
regimental band of the Durham Militia, and that in 1765 he was appointed
organist at Halifax. In the following year he removed to Bath as oboist in
Linley's orchestra, and in October 1767 was promoted to the post of
organist in the Octagon Chapel. The tide of prosperity now began to flow
for him. The most brilliant and modish society in England was at that time
to be met at Bath, and the young Hanoverian quickly found himself a
favourite and the fashion in it. Engagements multiplied upon him. He
became director of the public concerts; he conducted oratorios, engaged
singers, organised rehearsals, composed anthems, chants, choral services,
besides undertaking private tuitions, at times amounting to thirty-five or

even thirty-eight lessons a week. He in fact personified the musical activity
of a place then eminently and energetically musical.

But these multifarious avocations did not take up the whole of his thoughts.
His education, notwithstanding the poverty of his family, had not been
neglected, and he had always greedily assimilated every kind of knowledge
that came in his way. Now that he was a busy and a prosperous man, it
might have been expected that he would run on in the deep professional
groove laid down for him. On the contrary, his passion for learning seemed
to increase with the diminution of the time available for its gratification. He
studied Italian, Greek, mathematics; Maclaurin's Fluxions served to
"unbend his mind"; Smith's Harmonics and Optics and Ferguson's
Astronomy were the nightly companions of his pillow. What he read
stimulated without satisfying his intellect. He desired not only to know, but
to discover. In 1772 he hired a small telescope, and through it caught a
preliminary glimpse of the rich and varied fields in which for so many years
he was to expatiate. Henceforward the purpose of his life was fixed: it was
to obtain "a knowledge of the construction of the heavens";[9] and this
sublime ambition he cherished to the end.

A more powerful instrument was the first desideratum; and here his
mechanical genius came to his aid. Having purchased the apparatus of a
Quaker optician, he set about the manufacture of specula with a zeal which
seemed to anticipate the wonders they were to disclose to him. It was not
until fifteen years later that his grinding and polishing machines were
invented, so the work had at that time to be entirely done by hand. During
this tedious and laborious process (which could not be interrupted without
injury, and lasted on one occasion sixteen hours), his strength was
supported by morsels of food put into his mouth by his sister,[10] and his
mind amused by her reading aloud to him the Arabian Nights, Don Quixote,
or other light works. At length, after repeated failures, he found himself
provided with a reflecting telescope—a 5-1/2-foot Gregorian—of his own
construction. A copy of his first observation with it, on the great Nebula in

Orion—an object of continual amazement and assiduous inquiry to him—is
preserved by the Royal Society. It bears the date March 4, 1774.[11]

In the following year he executed his first "review of the heavens,"
memorable chiefly as an evidence of the grand and novel conceptions
which already inspired him, and of the enthusiasm with which he delivered
himself up to their guidance. Overwhelmed with professional engagements,
he still contrived to snatch some moments for the stars; and between the
acts at the theatre was often seen running from the harpsichord to his
telescope, no doubt with that "uncommon precipitancy which accompanied
all his actions."[12] He now rapidly increased the power and perfection of
his telescopes. Mirrors of seven, ten, even twenty feet focal length, were
successively completed, and unprecedented magnifying powers employed.
His energy was unceasing, his perseverance indomitable. In the course of
twenty-one years no less than 430 parabolic specula left his hands. He had
entered upon his forty-second year when he sent his first paper to the
Philosophical Transactions; yet during the ensuing thirty-nine years his
contributions—many of them elaborate treatises—numbered sixty-nine,
forming a series of extraordinary importance to the history of astronomy. As
a mere explorer of the heavens his labours were prodigious. He discovered
2,500 nebulæ, 806 double stars, passed the whole firmament in review four
several times, counted the stars in 3,400 "gauge-fields," and executed a
photometric classification of the principal stars, founded on an elaborate
(and the first systematically conducted) investigation of their relative
brightness. He was as careful and patient as he was rapid; spared no time
and omitted no precaution to secure accuracy in his observations; yet in one
night he would examine, singly and attentively, up to 400 separate objects.

The discovery of Uranus was a mere incident of the scheme he had marked
out for himself—a fruit, gathered as it were by the way. It formed,
nevertheless, the turning-point in his career. From a star-gazing musician he
was at once transformed into an eminent astronomer; he was relieved from
the drudgery of a toilsome profession, and installed as Royal Astronomer,

with a modest salary of £200 a year; funds were provided for the
construction of the forty-foot reflector, from the great space-penetrating
power of which he expected unheard-of revelations; in fine, his future work
was not only rendered possible, but it was stamped as authoritative.[13] On
Whit-Sunday 1782, William and Caroline Herschel played and sang in
public for the last time in St. Margaret's Chapel, Bath; in August of the
same year the household was moved to Datchet, near Windsor, and on April
3, 1786, to Slough. Here happiness and honours crowded on the fortunate
discoverer. In 1788 he married Mary, only child of James Baldwin, a
merchant of the city of London, and widow of Mr. John Pitt—a lady whose
domestic virtues were enhanced by the possession of a large jointure. The
fruit of their union was one son, of whose work—the worthy sequel of his
father's—we shall have to speak further on. Herschel was created a Knight
of the Hanoverian Guelphic Order in 1816, and in 1821 he became the first
President of the Royal Astronomical Society, his son being its first Foreign
Secretary. But his health had now for some years been failing, and on
August 25, 1822, he died at Slough, in the eighty-fourth year of his age, and
was buried in Upton churchyard.

His epitaph claims for him the lofty praise of having "burst the barriers of
heaven." Let us see in what sense this is true.

The first to form any definite idea as to the constitution of the stellar system
was Thomas Wright, the son of a carpenter living at Byer's Green, near
Durham. With him originated what has been called the "Grindstone Theory"
of the universe, which regarded the Milky Way as the projection on the
sphere of a stratum or disc of stars (our sun occupying a position near the
centre), similar in magnitude and distribution to the lucid orbs of the
constellations.[14] He was followed by Kant,[15] who transcended the views
of his predecessor by assigning to nebulæ the position they long continued
to occupy, rather on imaginative than scientific grounds, of "island
universes," external to, and co-equal with, the Galaxy. Johann Heinrich
Lambert,[16] a tailor's apprentice from Mühlhausen, followed, but

independently. The conceptions of this remarkable man were grandiose, his
intuitions bold, his views on some points a singular anticipation of
subsequent discoveries. The sidereal world presented itself to him as a
hierarchy of systems, starting from the planetary scheme, rising to throngs
of suns within the circuit of the Milky Way—the "ecliptic of the stars," as
he phrased it—expanding to include groups of many Milky Ways; these
again combining to form the unit of a higher order of assemblage, and so
onwards and upwards until the mind reels and sinks before the immensity
of the contemplated creations.

"Thus everything revolves—the earth round the sun; the sun round the
centre of his system; this system round a centre common to it with other
systems; this group, this assemblage of systems, round a centre which is
common to it with other groups of the same kind; and where shall we have
done?"[17]

The stupendous problem thus speculatively attempted, Herschel undertook
to grapple with experimentally. The upshot of this memorable inquiry was
the inclusion, for the first time, within the sphere of human knowledge, of a
connected body of facts, and inferences from facts, regarding the sidereal
universe; in other words, the foundation of what may properly be called a
science of the stars.

Tobias Mayer had illustrated the perspective effects which must ensue in
the stellar sphere from a translation of the solar system, by comparing them
to the separating in front and closing up behind of trees in a forest to the eye
of an advancing spectator;[18] but the appearances which he thus correctly
described he was unable to detect. By a more searching analysis of a
smaller collection of proper motions, Herschel succeeded in rendering
apparent the very consequences foreseen by Mayer. He showed, for
example, that Arcturus and Vega did, in fact, appear to recede from, and
Sirius and Aldebaran to approach, each other by very minute amounts; and,
with a striking effort of divinatory genius, placed the "apex," or point of
direction of the sun's motion, close to the star λ in the constellation

Hercules,[19] within a few degrees of the spot indicated by later and
indefinitely more refined methods of research. He resumed the subject in
1805,[20] but though employing a more rigorous method, was scarcely so
happy in his result. In 1806,[21] he made a preliminary attempt to ascertain
the speed of the sun's journey, fixing it, by doubtless much too low an
estimate, at about three miles a second. Yet the validity of his general
conclusion as to the line of solar travel, though long doubted, has been
triumphantly confirmed. The question as to the "secular parallax" of the
fixed stars was in effect answered.

With their annual parallax, however, the case was very different. The search
for it had already led Bradley to the important discoveries of the aberration
of light and the nutation of the earth's axis; it was now about to lead
Herschel to a discovery of a different, but even more elevated character. Yet
in neither case was the object primarily sought attained.

From the very first promulgation of the Copernician theory the seeming
immobility of the stars had been urged as an argument against its truth; for
if the earth really travelled in a vast orbit round the sun, objects in
surrounding space should appear to change their positions, unless their
distances were on a scale which, to the narrow ideas of the universe then
prevailing, seemed altogether extravagant.[22] The existence of such
apparent or "parallactic" displacements was accordingly regarded as the
touchstone of the new views, and their detection became an object of
earnest desire to those interested in maintaining them. Copernicus himself
made the attempt; but with his "Triquetrum," a jointed wooden rule with the
divisions marked in ink, constructed by himself,[23] he was hardly able to
measure angles of ten minutes, far less fractions of a second. Galileo, a
more impassioned defender of the system, strained his ears, as it were, from
Arcetri, in his blind and sorrowful old age, for news of a discovery which
two more centuries had still to wait for. Hooke believed he had found a
parallax for the bright star in the Head of the Dragon; but was deceived.
Bradley convinced himself that such effects were too minute for his

instruments to measure. Herschel made a fresh attempt by a practically
untried method.

It is a matter of daily experience that two objects situated at different
distances seem to a beholder in motion to move relatively to each other.
This principle Galileo, in the third of his Dialogues on the Systems of the
World,[24] proposed to employ for the determination of stellar parallax; for
two stars, lying apparently close together, but in reality separated by a great
gulf of space, must shift their mutual positions when observed from
opposite points of the earth's orbit; or rather, the remoter forms a virtually
fixed point, to which the movements of the other can be conveniently
referred. By this means complications were abolished more numerous and
perplexing than Galileo himself was aware of, and the problem was reduced
to one of simple micrometrical measurement. The "double-star method"
was also suggested by James Gregory in 1675, and again by Wallis in 1693;
[25] Huygens first, and afterwards Dr. Long of Cambridge (about 1750),
made futile experiments with it; and it eventually led, in the hands of
Bessel, to the successful determination of the parallax of 61 Cygni.

Its advantages were not lost upon Herschel. His attempt to assign definite
distances to the nearest stars was no isolated effort, but part of the settled
plan upon which his observations were conducted. He proposed to sound
the heavens, and the first requisite was a knowledge of the length of his
sounding-line. Thus it came about that his special attention was early
directed to double stars.

"I resolved," he writes,[26] "to examine every star in the heavens with the
utmost attention and a very high power, that I might collect such materials
for this research as would enable me to fix my observations upon those that
would best answer my end. The subject has already proved so extensive,
and still promises so rich a harvest to those who are inclined to be diligent
in the pursuit, that I cannot help inviting every lover of astronomy to join
with me in observations that must inevitably lead to new discoveries."

The first result of these inquiries was a classed catalogue of 269 double
stars presented to the Royal Society in 1782, followed, after three years, by
an additional list of 434. In both these collections the distances separating
the individuals of each pair were carefully measured, and (with a few
exceptions) the angles made with the hour-circle by the lines joining their
centres (technically called "angles of position") were determined with the
aid of a "revolving-wire micrometer," specially devised for the purpose.
Moreover, an important novelty was introduced by the observation of the
various colours visible in the star-couples, the singular and vivid contrasts
of which were now for the first time described.

Double stars were at that time supposed to be a purely optical phenomenon.
Their components, it was thought, while in reality indefinitely remote from
each other, were brought into fortuitous contiguity by the chance of lying
nearly in the same line of sight from the earth. Yet Bradley had noticed a
change of 30°, between 1718 and 1759, in the position-angle of the two
stars forming Castor, and was thus within a hair's breadth of the discovery
of their physical connection.[27] While the Rev. John Michell, arguing by
the doctrine of probabilities, wrote as follows in 1767:—"It is highly
probable in particular, and next to a certainty in general, that such double
stars as appear to consist of two or more stars placed very near together, do
really consist of stars placed near together, and under the influence of some
general law."[28] And in 1784:[29] "It is not improbable that a few years may
inform us that some of the great number of double, triple stars, etc., which
have been observed by Mr. Herschel, are systems of bodies revolving about
each other."

This remarkable speculative anticipation had a practical counterpart in
Germany. Father Christian Mayer, a Jesuit astronomer at Mannheim, set
himself, in January 1776, to collect examples of stellar pairs, and shortly
after published the supposed discovery of "satellites" to many of the
principal stars.[30] But his observations were neither exact nor prolonged
enough to lead to useful results in such an inquiry. His disclosures were

derided; his planet-stars treated as results of hallucination. On n'a point cru
à des choses aussi extraordinaires, wrote Lalande[31] within one year of a
better-grounded announcement to the same effect.

Herschel at first shared the general opinion as to the merely optical
connection of double stars. Of this the purpose for which he made his
collection is in itself sufficient evidence, since what may be called the
differential method of parallaxes depends, as we have seen, for its efficacy
upon disparity of distance. It was "much too soon," he declared in 1782,[32]
"to form any theories of small stars revolving round large ones;" while in
the year following,[33] he remarked that the identical proper motions of the
two stars forming, to the naked eye, the single bright orb of Castor could
only be explained as both equally due to the "systematic parallax" caused
by the sun's movement in space. Plainly showing that the notion of a
physical tie, compelling the two bodies to travel together, had not as yet
entered into his speculations. But he was eminently open to conviction, and
had, moreover, by observations unparalleled in amount as well as in kind,
prepared ample materials for convincing himself and others. In 1802 he was
able to announce the fact of his discovery, and in the two ensuing years, to
lay in detail before the Royal Society proofs, gathered from the labours of a
quarter of a century, of orbital revolution in the case of as many as fifty
double stars, henceforth, he declared, to be held as real binary
combinations, "intimately held together by the bond of mutual attraction."
[34] The fortunate preservation in Dr. Maskelyne's note-book of a remark
made by Bradley about 1759, to the effect that the line joining the
components of Castor was an exact prolongation of that joining Castor with
Pollux, added eighteen years to the time during which the pair were under
scrutiny, and confirmed the evidence of change afforded by more recent
observations. Approximate periods were fixed for many of the revolving
suns—for Castor 342 years; for γ Leonis, 1200, δ Serpentis, 375, ε Bootis,
1681 years; ε Lyræ was noted as a "double-double-star," a change of
relative situation having been detected in each of the two pairs composing
the group; and the occultation was described of one star by another in the

course of their mutual revolutions, as exemplified in 1795 by the rapidly
circulating system of ζ Herculis.

Thus, by the sagacity and perseverance of a single observer, a firm basis
was at last provided upon which to raise the edifice of sidereal science. The
analogy long presumed to exist between the mighty star of our system and
the bright points of light spangling the firmament was shown to be no
fiction of the imagination, but a physical reality; the fundamental quality of
attractive power was proved to be common to matter so far as the telescope
was capable of exploring, and law, subordination, and regularity to give
testimony of supreme and intelligent design no less in those limitless
regions of space than in our narrow terrestrial home. The discovery was
emphatically (in Arago's phrase) "one with a future," since it introduced the
element of precise knowledge where more or less probable conjecture had
previously held almost undivided sway; and precise knowledge tends to
propagate itself and advance from point to point.

We have now to speak of Herschel's pioneering work in the skies. To
explore with line and plummet the shining zone of the Milky Way, to
delineate its form, measure its dimensions, and search out the intricacies of
its construction, was the primary task of his life, which he never lost sight
of, and to which all his other investigations were subordinate. He was
absolutely alone in this bold endeavour. Unaided, he had to devise methods,
accumulate materials, and sift out results. Yet it may safely be asserted that
all the knowledge we possess on this sublime subject was prepared, and the
greater part of it anticipated, by him.

The ingenious method of "star-gauging," and its issue in the delineation of
the sidereal system as an irregular stratum of evenly-scattered suns, is the
best-known part of his work. But it was, in truth, only a first rude
approximation, the principle of which maintained its credit in the literature
of astronomy a full half-century after its abandonment by its author. This
principle was the general equality of star distribution. If equal portions of
space really held equal numbers of stars, it is obvious that the number of

stars visible in any particular direction would be strictly proportional to the
range of the system in that direction, apparent accumulation being produced
by real extent. The process of "gauging the heavens," accordingly, consisted
in counting the stars in successive telescopic fields, and calculating thence
the depths of space necessary to contain them. The result of 3,400 such
operations was the plan of the Galaxy familiar to every reader of an
astronomical text-book. Widely-varying evidence was, as might have been
expected, derived from an examination of different portions of the sky.
Some fields of view were almost blank, while others (in or near the Milky
Way) blazed with the radiance of many hundred stars compressed into an
area about one-fourth that of the full-moon. In the most crowded parts
116,000 were stated to have been passed in review within a quarter of an
hour. Here the "length of his sounding-line" was estimated by Herschel at
about 497 times the distance of Sirius—in other words, the bounding orb, or
farthest sun of the system in that direction, so far as could be seen with the
20-foot reflector, was thus inconceivably remote. But since the distance of
Sirius, no less than of every other fixed star, was as yet an unknown
quantity, the dimensions inferred for the Galaxy were of course purely
relative; a knowledge of its form and structure might (admitting the truth of
the fundamental hypothesis) be obtained, but its real or absolute size
remained altogether undetermined.

Even as early as 1785, however, Herschel perceived traces of a tendency
which completely invalidated the supposition of any approach to an average
uniformity of distribution. This was the action of what he called a
"clustering power" in the Milky Way. "Many gathering clusters"[35] were
already discernible to him even while he endeavoured to obtain a "true
mean result" on the assumption that each star in space was separated from
its neighbours as widely as the sun from Sirius. "It appears," he wrote in
1789, "that the heavens consist of regions where suns are gathered into
separate systems"; and in certain assemblages he was able to trace "a course
or tide of stars setting towards a centre," denoting, not doubtfully, the
presence of attractive forces.[36] Thirteen years later, he described our sun

and his constellated companions as surrounded by "a magnificent collection
of innumerable stars, called the Milky Way, which must occasion a very
powerful balance of opposite attractions to hold the intermediate stars at
rest. For though our sun, and all the stars we see, may truly be said to be in
the plane of the Milky Way, yet I am now convinced, by a long inspection
and continued examination of it, that the Milky Way itself consists of stars
very differently scattered from those which are immediately about us."
"This immense aggregation," he added, "is by no means uniform. Its
component stars show evident signs of clustering together into many
separate allotments."[37]

The following sentences, written in 1811, contain a definite retractation of
the view frequently attributed to him:—

"I must freely confess," he says, "that by continuing my sweeps of the
heavens my opinion of the arrangement of the stars and their magnitudes,
and of some other particulars, has undergone a gradual change; and indeed,
when the novelty of the subject is considered, we cannot be surprised that
many things formerly taken for granted should on examination prove to be
different from what they were generally but incautiously supposed to be.
For instance, an equal scattering of the stars may be admitted in certain
calculations; but when we examine the Milky Way, or the closely
compressed clusters of stars of which my catalogues have recorded so many
instances, this supposed equality of scattering must be given up."[38]

Another assumption, the fallacy of which he had not the means of detecting
since become available, was retained by him to the end of his life. It was
that the brightness of a star afforded an approximate measure of its distance.
Upon this principle he founded in 1817 his method of "limiting apertures,"
[39] by which two stars, brought into view in two precisely similar
telescopes, were "equalised" by covering a certain portion of the object-
glass collecting the more brilliant rays. The distances of the orbs compared
were then taken to be in the ratio of the reduced to the original apertures of
the instruments with which they were examined. If indeed the absolute

lustre of each were the same, the result might be accepted with confidence;
but since we have no warrant for assuming a "standard star" to facilitate our
computations, but much reason to suppose an indefinite range, not only of
size but of intrinsic brilliancy, in the suns of our firmament, conclusions
drawn from such a comparison are entirely worthless.

In another branch of sidereal science besides that of stellar aggregation,
Herschel may justly be styled a pioneer. He was the first to bestow serious
study on the enigmatical objects known as "nebulæ." The history of the
acquaintance of our race with them is comparatively short. The only one
recognised before the invention of the telescope was that in the girdle of
Andromeda, certainly familiar in the middle of the tenth century to the
Persian astronomer Abdurrahman Al-Sûfi; and marked with dots on Spanish
and Dutch constellation-charts of the fourteenth and fifteenth centuries.[40]
Yet so little was it noticed that it might practically be said—as far as Europe
is concerned—to have been discovered in 1612 by Simon Marius (Mayer of
Genzenhausen), who aptly described its appearance as that of a "candle
shining through horn." The first mention of the great Orion nebula is by a
Swiss Jesuit named Cysatus, who succeeded Father Scheiner in the chair of
mathematics at Ingolstadt. He used it, apparently without any suspicion of
its novelty, as a term of comparison for the comet of December 1618.[41] A
novelty, nevertheless, to astronomers it still remained in 1656, when
Huygens discerned, "as it were, an hiatus in the sky, affording a glimpse of
a more luminous region beyond."[42] Halley in 1716 knew of six nebulæ,
which he believed to be composed of a "lucid medium" diffused through the
ether of space.[43] He appears, however, to have been unacquainted with
some previously noticed by Hevelius. Lacaille brought back with him from
the Cape a list of forty-two—the first-fruits of observation in Southern skies
—arranged in three numerically equal classes;[44] and Messier (nicknamed
by Louis XV. the "ferret of comets"), finding such objects a source of
extreme perplexity in the pursuit of his chosen game, attempted to eliminate
by methodising them, and drew up a catalogue comprising, in 1781, 103
entries.[45]

These preliminary attempts shrank into insignificance when Herschel began
to "sweep the heavens" with his giant telescopes. In 1786 he presented to
the Royal Society a descriptive catalogue of 1,000 nebulæ and clusters,
followed, three years later, by a second of as many more; to which he added
in 1802 a further gleaning of 500. On the subject of their nature his views
underwent a remarkable change. Finding that his potent instruments
resolved into stars many nebulous patches in which no signs of such a
structure had previously been discernible, he naturally concluded that
"resolvability" was merely a question of distance and telescopic power. He
was (as he said himself) led on by almost imperceptible degrees from
evident clusters, such as the Pleiades, to spots without a trace of stellar
formation, the gradations being so well connected as to leave no doubt that
all these phenomena were equally stellar. The singular variety of their
appearance was thus described by him:—

"I have seen," he says, "double and treble nebulæ variously arranged; large
ones with small, seeming attendants; narrow, but much extended lucid
nebulæ or bright dashes; some of the shape of a fan, resembling an electric
brush, issuing from a lucid point; others of the cometic shape, with a
seeming nucleus in the centre, or like cloudy stars surrounded with a
nebulous atmosphere; a different sort, again, contain a nebulosity of the
milky kind, like that wonderful, inexplicable phenomenon about θ Orionis;
while others shine with a fainter, mottled kind of light, which denotes their
being resolvable into stars."[46]

"These curious objects" he considered to be "no less than whole sidereal
systems,"[47] some of which might "well outvie our Milky Way in
grandeur." He admitted, however, a wide diversity in condition as well as
compass. The system to which our sun belongs he described as "a very
extensive branching congeries of many millions of stars, which probably
owes its origin to many remarkably large as well as pretty closely scattered
small stars, that may have drawn together the rest."[48] But the continued
action of this same "clustering power" would, he supposed, eventually lead

to the breaking-up of the original majestic Galaxy into two or three hundred
separate groups, already visibly gathering. Such minor nebulæ, due to the
"decay" of other "branching nebulæ" similar to our own, he recognised by
the score, lying, as it were, stratified in certain quarters of the sky. "One of
these nebulous beds," he informs us, "is so rich that in passing through a
section of it, in the time of only thirty-six minutes, I detected no less than
thirty-one nebulæ, all distinctly visible upon a fine blue sky." The stratum
of Coma Berenices he judged to be the nearest to our system of such layers;
nor did the marked aggregation of nebulæ towards both poles of the circle
of the Milky Way escape his notice.

By a continuation of the same process of reasoning, he was enabled (as he
thought) to trace the life-history of nebulæ from a primitive loose and
extended formation, through clusters of gradually increasing compression,
down to the kind named by him "Planetary" because of the defined and
uniform discs which they present. These he regarded as "very aged, and
drawing on towards a period of change or dissolution."[49]

"This method of viewing the heavens," he concluded, "seems to throw them
into a new kind of light. They now are seen to resemble a luxuriant garden
which contains the greatest variety of productions in different flourishing
beds; and one advantage we may at least reap from it is, that we can, as it
were, extend the range of our experience to an immense duration. For, to
continue the simile which I have borrowed from the vegetable kingdom, is
it not almost the same thing whether we live successively to witness the
germination, blooming, foliage, fecundity, fading, withering, and corruption
of a plant, or whether a vast number of specimens, selected from every
stage through which the plant passes in the course of its existence, be
brought at once to our view?"[50]

But already this supposed continuity was broken. After mature deliberation
on the phenomena presented by nebulous stars, Herschel was induced, in
1791, to modify essentially his original opinion.

"When I pursued these researches," he says, "I was in the situation of a
natural philosopher who follows the various species of animals and insects
from the height of their perfection down to the lowest ebb of life; when,
arriving at the vegetable kingdom, he can scarcely point out to us the
precise boundary where the animal ceases and the plant begins; and may
even go so far as to suspect them not to be essentially different. But,
recollecting himself, he compares, for instance, one of the human species to
a tree, and all doubt upon the subject vanishes before him. In the same
manner we pass through gentle steps from a coarse cluster of stars, such as
the Pleiades ... till we find ourselves brought to an object such as the nebula
in Orion, where we are still inclined to remain in the once adopted idea of
stars exceedingly remote and inconceivably crowded, as being the occasion
of that remarkable appearance. It seems, therefore, to require a more
dissimilar object to set us right again. A glance like that of the naturalist,
who casts his eye from the perfect animal to the perfect vegetable, is
wanting to remove the veil from the mind of the astronomer. The object I
have mentioned above is the phenomenon that was wanting for this
purpose. View, for instance, the 19th cluster of my 6th class, and afterwards
cast your eye on this cloudy star, and the result will be no less decisive than
that of the naturalist we have alluded to. Our judgment, I may venture to
say, will be, that the nebulosity about the star is not of a starry nature."[51]

The conviction thus arrived at of the existence in space of a widely diffused
"shining fluid" (a conviction long afterwards fully justified by the
spectroscope) led him into a field of endless speculation. What was its
nature? Should it "be compared to the coruscation of the electric fluid in the
aurora borealis? or to the more magnificent cone of the zodiacal light?"
Above all, what was its function in the cosmos? And on this point he
already gave a hint of the direction in which his mind was moving by the
remark that this self-luminous matter seemed "more fit to produce a star by
its condensation, than to depend on the star for its existence."[52]

This was not a novel idea. Tycho Brahe had tried to explain the blaze of the
star of 1572 as due to a sudden concentration of nebulous material in the
Milky Way, even pointing out the space left dark and void by the
withdrawal of the luminous stuff; and Kepler, theorising on a similar stellar
apparition in 1604, followed nearly in the same track. But under Herschel's
treatment the nebular origin of stars first acquired the consistency of a
formal theory. He meditated upon it long and earnestly, and in two elaborate
treatises, published respectively in 1811 and 1814, he at length set forth the
arguments in its favour. These rested entirely upon the "principle of
continuity." Between the successive classes of his assortment of developing
objects there was, as he said, "perhaps not so much difference as would be
in an annual description of the human figure, were it given from the birth of
a child till he comes to be a man in his prime."[53] From diffused nebulosity,
barely visible in the most powerful light-gathering instruments, but which
he estimated to cover nearly 152 square degrees of the heavens,[54] to
planetary nebulæ, supposed to be already centrally solid, instances were
alleged of every stage and phase of condensation. The validity of his
reasoning, however, was evidently impaired by his confessed inability to
distinguish between the dim rays of remote clusters and the milky light of
true gaseous nebulæ.

It may be said that such speculations are futile in themselves, and
necessarily barren of results. But they gratify an inherent tendency of the
human mind, and, if pursued in a becoming spirit, should be neither
reproved nor disdained. Herschel's theory still holds the field, the testimony
of recent discoveries with regard to it having proved strongly confirmatory
of its principle, although not of its details. Strangely enough, it seems to
have been propounded in complete independence of Laplace's nebular
hypothesis as to the origin of the solar system. Indeed, it dated, as we have
seen, in its first inception, from 1791, while the French geometrician's view
was not advanced until 1796.

We may now briefly sum up the chief results of Herschel's long years of
"watching the heavens." The apparent motions of the stars had been
disentangled; one portion being clearly shown to be due to a translation
towards a point in the constellation Hercules of the sun and his attendant
planets; while a large balance of displacement was left to be accounted for
by real movements, various in extent and direction, of the stars themselves.
By the action of a central force similar to, if not identical with, gravity, suns
of every degree of size and splendour, and sometimes brilliantly contrasted
in colour, were seen to be held together in systems, consisting of two, three,
four, even six members, whose revolutions exhibited a wide range of
variety both in period and in orbital form. A new department of physical
astronomy was thus created,[55] and rigid calculation for the first time made
possible within the astral region. The vast problem of the arrangement and
relations of the millions of stars forming the Milky Way was shown to be
capable of experimental treatment, and of at least partial solution,
notwithstanding the variety and complexity seen to prevail, to an extent
previously undreamt of, in the arrangement of that majestic system. The
existence of a luminous fluid, diffused through enormous tracts of space,
and intimately associated with stellar bodies, was virtually demonstrated,
and its place and use in creation attempted to be divined by a bold but
plausible conjecture. Change on a stupendous scale was inferred or
observed to be everywhere in progress. Periodical stars shone out and again
decayed; progressive ebbings or flowings of light were indicated as
probable in many stars under no formal suspicion of variability; forces were
everywhere perceived to be at work, by which the very structure of the
heavens themselves must be slowly but fundamentally modified. In all
directions groups were seen to be formed or forming; tides and streams of
suns to be setting towards powerful centres of attraction; new systems to be
in process of formation, while effete ones hastened to decay or regeneration
when the course appointed for them by Infinite Wisdom was run. And thus,
to quote the words of the observer who "had looked farther into space than
ever human being did before him,"[56] the state into which the incessant
action of the clustering power has brought the Milky Way at present, is a

kind of chronometer that may be used to measure the time of its past and
future existence; and although we do not know the rate of going of this
mysterious chronometer, it is nevertheless certain that, since the breaking-
up of the parts of the Milky Way affords a proof that it cannot last for ever,
it equally bears witness that its past duration cannot be admitted to be
infinite.[57]

FOOTNOTES:
[3] Phil. Trans., vol. xxx., p. 737.

[4] Out of eighty stars compared, fifty-seven were found to have changed their
places by more than 10″. Lesser discrepancies were at that time regarded as
falling within the limits of observational error. Tobiæ Mayeri Op. Inedita, t. i.,
pp. 80, 81, and Herschel in Phil. Trans., vol. lxxiii., pp. 275-278.

[5] Posthumous Works, p. 701.

[6] Arago in Annuaire du Bureau des Longitudes, 1842, p. 313.

[7] Bradley to Halley, Phil. Trans., vol. xxxv. (1728), p. 660. His observations
were directly applicable to only two stars, γ Draconis and η Ursæ Majoris, but
some lesser ones were included in the same result.

[8] Holden, Sir William Herschel, his Life and Works, p. 17.

[9] Phil. Trans., vol. ci., p. 269.

[10] Caroline Lucretia Herschel, born at Hanover, March 16, 1750, died in the
same place, January 9, 1848. She came to England in 1772, and was her
brother's devoted assistant, first in his musical undertakings, and afterwards,
down to the end of his life, in his astronomical labours.

[11] Holden, op. cit., p. 39.

[12] Memoir of Caroline Herschel, p. 37.

[13] See Holden's Sir William Herschel, p. 54.

[14] An Original Theory or New Hypothesis of the Universe, London, 1750. See
also De Morgan's summary of his views in Philosophical Magazine, April, 1848.

[15] Allgemeine Naturgeschichte und Theorie des Himmels, 1755.

[16] Cosmologische Briefe, Augsburg, 1761.

[17] The System of the World, p. 125, London, 1800 (a translation of
Cosmologische Briefe). Lambert regarded nebulæ as composed of stars crowded
together, but not as external universes. In the case of the Orion nebula, indeed,
he throws out such a conjecture, but afterwards suggests that it may form a
centre for that one of the subordinate systems composing the Milky Way to
which our sun belongs.

[18] Opera Inedita, t. i., p. 79.

[19] Phil. Trans., vol. lxxiii. (1783), p. 273. Pierre Prévost's similar
investigation, communicated to the Berlin Academy of Sciences four months
later, July 3, 1783, was inserted in the Memoirs of that body for 1781, and thus
seems to claim a priority not its due. Georg Simon Klügel at Halle gave about
the same time an analytical demonstration of Herschel's result. Wolf, Gesch. der
Astronomie, p. 733.

[20] Phil. Trans., vol. xcv., p. 233.

[21] Ibid., vol. xcvi., p. 205.

[22] "Ingens bolus devorandus est," Kepler admitted to Herwart in May, 1603.

[23] Described in "Præfatio Editoris" to De Revolutionibus, p. xix. (ed. 1854).

[24] Opere, t. i., p. 415.

[25] Phil. Trans., vol. xvii., p. 848.

[26] Ibid., vol. lxxii., p. 97.

[27] Doberck, Observatory, vol. ii., p. 110.

[28] Phil. Trans., vol. lvii., p. 249.

[29] Ibid., vol. lxxiv., p. 56.

[30] Beobachtungen von Fixsterntrabanten, 1778; and De Novis in Cœlo Sidereo
Phænomenis, 1779.

[31] Bibliographie, p. 569.

[32] Phil. Trans., vol. lxxii., p. 162.

[33] Ibid., vol. lxxiii., p. 272.

[34] Ibid., vol. xciii., p. 340.

[35] Phil. Trans., vol. lxxv., p. 255.

[36] Ibid., vol. lxxix., pp. 214, 222.

[37] Ibid., vol. xcii., pp. 479, 495.

[38] Phil. Trans., vol. ci., p. 269.

[39] Ibid., vol. cvii., p. 311.

[40] Bullialdus, De Nebulosâ Stellâ in Cingulo Andromedæ (1667); see also G.
P. Bond, Mém. Am. Ac., vol. iii., p. 75, Holden's Monograph on the Orion
Nebula, Washington Observations, vol. xxv., 1878 (pub. 1882), and Lady
Huggins's drawing, Atlas of Spectra, p. 119.

[41] Mathemata Astronomica, p. 75.

[42] Systema Saturnium, p. 9.

[43] Phil. Trans., vol. xxix., p. 390.

[44] Mém. Ac. des Sciences, 1755.

[45] Conn. des Temps, 1784 (pub. 1781), p. 227. A previous list of forty-five had
appeared in Mém. Ac. des Sciences, 1771.

[46] Phil. Trans., vol. lxxiv., p. 442.

[47] Ibid., vol. lxxix., p. 213.

[48] Ibid., vol. lxxv., p. 254.

[49] Ibid., vol. lxxix., p. 225.

[50] Phil. Trans., vol. lxxix., p. 226.

[51] Ibid., vol. lxxxi., p. 72.

[52] Ibid., p. 85.

[53] Phil. Trans., vol. ci., p. 271.

[54] Ibid., p. 277.

[55] J. Herschel, Phil. Trans., vol. cxvi., part iii., p. 1.

[56] His own words to the poet Campbell cited by Holden, Life and Works, p.
109.

[57] Phil. Trans., vol. civ., p. 283.

CHAPTER II
PROGRESS OF SIDEREAL ASTRONOMY

We have now to consider labours of a totally different character from those
of Sir William Herschel. Exploration and discovery do not constitute the
whole business of astronomy; the less adventurous, though not less arduous,
task of gaining a more and more complete mastery over the problems
immemorially presented to her, may, on the contrary, be said to form her
primary duty. A knowledge of the movements of the heavenly bodies has,
from the earliest times, been demanded by the urgent needs of mankind;
and science finds its advantage, as in many cases it has taken its origin, in
condescension to practical claims. Indeed, to bring such knowledge as near
as possible to absolute precision has been defined by no mean authority[58]
as the true end of astronomy.

Several causes concurred about the beginning of the last century to give a
fresh and powerful impulse to investigations having this end in view. The
rapid progress of theory almost compelled a corresponding advance in
observation; instrumental improvements rendered such an advance possible;
Herschel's discoveries quickened public interest in celestial inquiries; royal,
imperial, and grand-ducal patronage widened the scope of individual effort.
The heart of the new movement was in Germany. Hitherto the observatory
of Flamsteed and Bradley had been the acknowledged centre of practical
astronomy; Greenwich observations were the standard of reference all over
Europe; and the art of observing prospered in direct proportion to the
fidelity with which Greenwich methods were imitated. Dr. Maskelyne, who
held the post of Astronomer Royal during forty-six years (from 1765 to
1811), was no unworthy successor to the eminent men who had gone before
him. His foundation of the Nautical Almanac (in 1767) alone constitutes a
valid title to fame; he introduced at the Observatory the important

innovation of the systematic publication of results; and the careful and
prolonged series of observations executed by him formed the basis of the
improved theories, and corrected tables of the celestial movements, which
were rapidly being brought to completion abroad. His catalogue of thirty-
six "fundamental" stars was besides excellent in its way, and most
serviceable. Yet he was devoid of Bradley's instinct for divining the needs
of the future. He was fitted rather to continue a tradition than to found a
school. The old ways were dear to him; and, indefatigable as he was, a
definite purpose was wanting to compel him, by its exigencies, along the
path of progress. Thus, for almost fifty years after Bradley's death, the
acquisition of a small achromatic[59] was the only notable change made in
the instrumental equipment of the Observatory. The transit, the zenith
sector, and the mural quadrant, with which Bradley had done his
incomparable work, retained their places long after they had become
deteriorated by time and obsolete by the progress of invention; and it was
not until the very close of his career that Maskelyne, compelled by Pond's
detection of serious errors, ordered a Troughton's circle, which he did not
live to employ.

Meanwhile, the heavy national disasters with which Germany was
overwhelmed in the early part of the nineteenth century seemed to stimulate
rather than impede the intellectual revival already for some years in
progress there. Astronomy was amongst the first of the sciences to feel the
new impulse. By the efforts of Bode, Olbers, Schröter, and Von Zach, just
and elevated ideas on the subject were propagated, intelligence was
diffused, and a firm ground prepared for common action in mutual
sympathy and disinterested zeal. They received powerful aid through the
foundation, in 1804, by a young artillery officer named Von Reichenbach,
of an Optical and Mechanical Institute at Munich. Here the work of English
instrumental artists was for the first time rivalled, and that of English
opticians—when Fraunhofer entered the new establishment—far surpassed.
The development given to the refracting telescope by this extraordinary
man was indispensable to the progress of that fundamental part of

astronomy which consists in the exact determination of the places of the
heavenly bodies. Reflectors are brilliant engines of discovery, but they lend
themselves with difficulty to the prosaic work of measuring right ascensions
and polar distances. A signal improvement in the art of making and working
flint-glass thus most opportunely coincided with the rise of a German
school of scientific mechanicians, to furnish the instrumental means needed
for the reform which was at hand. Of the leader of that reform it is now time
to speak.

Friedrich Wilhelm Bessel was born at Minden, in Westphalia, July 22, 1784.
A certain taste for figures, coupled with a still stronger distaste for the Latin
accidence, directed his inclination and his father's choice towards a
mercantile career. In his fifteenth year, accordingly, he entered the house of
Kuhlenkamp and Sons, in Bremen, as an apprenticed clerk. He was now
thrown completely upon his own resources. From his father, a struggling
Government official, heavily weighted with a large family, he was well
aware that he had nothing to expect; his dormant faculties were roused by
the necessity for self-dependence, and he set himself to push manfully
forward along the path that lay before him. The post of supercargo on one
of the trading expeditions sent out from the Hanseatic towns to China and
the East Indies was the aim of his boyish ambition, for the attainment of
which he sought to qualify himself by the industrious acquisition of suitable
and useful knowledge. He learned English in two or three months; picked
up Spanish with the casual aid of a gunsmith's apprentice; studied the
geography of the distant lands which he hoped to visit; collected
information as to their climates, inhabitants, products, and the courses of
trade. He desired to add some acquaintance with the art (then much
neglected) of taking observations at sea; and thus, led on from navigation to
astronomy, and from astronomy to mathematics, he groped his way into a
new world.

It was characteristic of him that the practical problems of science should
have attracted him before his mind was as yet sufficiently matured to feel
the charm of its abstract beauties. His first attempt at observation was made

with a sextant, rudely constructed under his own directions, and a common
clock. Its object was the determination of the longitude of Bremen, and its
success, he tells us himself,[60] filled him with a rapture of delight, which,
by confirming his tastes, decided his destiny. He now eagerly studied
Bode's Jahrbuch and Von Zach's Monatliche Correspondenz, overcoming
each difficulty as it arose with the aid of Lalande's Traité d'Astronomie, and
supplying, with amazing rapidity, his early deficiency in mathematical
training. In two years he was able to attack a problem which would have
tasked the patience, if not the skill, of the most experienced astronomer.
Amongst the Earl of Egremont's papers Von Zach had discovered Harriot's
observations on Halley's comet at its appearance in 1607, and published
them as a supplement to Bode's Annual. With an elaborate care inspired by
his youthful ardour, though hardly merited by their loose nature, Bessel
deduced from them an orbit for that celebrated body, and presented the
work to Olbers, whose reputation in cometary researches gave a special
fitness to the proffered homage. The benevolent physician-astronomer of
Bremen welcomed with surprised delight such a performance emanating
from such a source. Fifteen years previously, the French Academy had
crowned a similar work; now its equal was produced by a youth of twenty,
busily engaged in commercial pursuits, self-taught, and obliged to snatch
from sleep the hours devoted to study. The paper was immediately sent to
Von Zach for publication, with a note from Olbers explaining the
circumstances of its author; and the name of Bessel became the common
property of learned Europe.

He had, however, as yet no intention of adopting astronomy as his
profession. For two years he continued to work in the counting-house by
day, and to pore over the Mécanique Céleste and the Differential Calculus
by night. But the post of assistant in Schröter's observatory at Lilienthal
having become vacant by the removal of Harding to Göttingen in 1805,
Olbers procured for him the offer of it. It was not without a struggle that he
resolved to exchange the desk for the telescope. His reputation with his
employers was of the highest; he had thoroughly mastered the details of the

business, which his keen practical intelligence followed with lively interest;
his years of apprenticeship were on the point of expiring, and an immediate,
and not unwelcome prospect of comparative affluence lay before him. The
love of science, however, prevailed; he chose poverty and the stars, and
went to Lilienthal with a salary of a hundred thalers yearly. Looking back
over his life's work, Olbers long afterwards declared that the greatest
service which he had rendered to astronomy was that of having discerned,
directed, and promoted the genius of Bessel.[61]

For four years he continued in Schröter's employment. At the end of that
time the Prussian Government chose him to superintend the erection of a
new observatory at Königsberg, which after many vexatious delays, caused
by the prostrate condition of the country, was finished towards the end of
1813. Königsberg was the first really efficient German observatory. It
became, moreover, a centre of improvement, not for Germany alone, but for
the whole astronomical world. During two-and-thirty years it was the scene
of Bessel's labours, and Bessel's labours had for their aim the
reconstruction, on an amended and uniform plan, of the entire science of
observation.

A knowledge of the places of the stars is the foundation of astronomy.[62]
Their configuration lends to the skies their distinctive features, and marks
out the shifting tracks of more mobile objects with relatively fixed, and
generally unvarying points of light. A more detailed and accurate
acquaintance with the stellar multitude, regarded from a purely
uranographical point of view, has accordingly formed at all times a primary
object of celestial science, and was, during the last century, cultivated with
a zeal and success by which all previous efforts were dwarfed into
insignificance. In Lalande's Histoire Céleste, published in 1801, the places
of no less than 47,390 stars were given, but in the rough, as it were, and
consequently needing laborious processes of calculation to render them
available for exact purposes. Piazzi set an example of improved methods of
observation, resulting in the publication, in 1803 and 1814, of two

catalogues of about 7,600 stars—the second being a revision and
enlargement of the first—which for their time were models of what such
works should be.[63] Stephen Groombridge at Blackheath was similarly and
most beneficially active. But something more was needed than the diligence
of individual observers. A systematic reform was called for; and it was this
which Bessel undertook and carried through.

Direct observation furnishes only what has been called the "raw material"
of the positions of the heavenly bodies.[64] A number of highly complex
corrections have to be applied before their mean can be disengaged from
their apparent places on the sphere. Of these, the most considerable and
familiar is atmospheric refraction, by which objects seem to stand higher in
the sky than they in reality do, the effect being evanescent at the zenith, and
attaining, by gradations varying with conditions of pressure and
temperature, a maximum at the horizon. Moreover, the points to which
measurements are referred are themselves in motion, either continually in
one direction, or periodically to and fro. The precession of the equinoxes is
slowly progressive, or rather retrogressive; the nutation of the pole
oscillatory in a period of about eighteen years. Added to which, the non-
instantaneous transmission of light, combined with the movement of the
earth in its orbit, causes a small annual displacement known as aberration.

Now it is easy to see that any uncertainty in the application of these
corrections saps the very foundations of exact astronomy. Extremely minute
quantities, it is true, are concerned; but the life and progress of modern
celestial science depends upon the sure recognition of extremely minute
quantities. In the early years of the nineteenth century, however, no uniform
system of "reduction" (so the complete correction of observational results is
termed) had been established. Much was left to the individual caprice of
observers, who selected for the several "elements" of reduction such values
as seemed best to themselves. Hence arose much hurtful confusion, tending
to hinder united action and mar the usefulness of laborious researches. For

this state of things, Bessel, by the exercise of consummate diligence,
sagacity, and patience, provided an entirely satisfactory remedy.

His first step was an elaborate investigation of the precious series of
observations made by Bradley at Greenwich from 1750 until his death in
1762. The catalogue of 3,222 stars which he extracted from them gave the
earliest example of the systematic reduction on a uniform plan of such a
body of work. It is difficult, without entering into details out of place in a
volume like the present, to convey an idea of the arduous nature of this task.
It involved the formation of a theory of the errors of each of Bradley's
instruments, and a difficult and delicate inquiry into the true value of each
correction to be applied, before the entries in the Greenwich journals could
be developed into a finished and authentic catalogue. Although completed
in 1813, it was not until five years later that the results appeared with the
proud, but not inappropriate title of Fundamenta Astronomiæ. The eminent
value of the work consisted in this, that by providing a mass of entirely
reliable information as to the state of the heavens at the epoch 1755, it
threw back the beginning of exact astronomy almost half a century. By
comparison with Piazzi's catalogues the amount of precession was more
accurately determined, the proper motions of a considerable number of stars
became known with certainty, and definite prediction—the certificate of
initiation into the secrets of Nature—at last became possible as regards the
places of the stars. Bessel's final improvements in the methods of reduction
were published in 1830 in his Tabulæ Regiomontanæ. They not only
constituted an advance in accuracy, but afforded a vast increase of facility
in application, and were at once and everywhere adopted. Thus astronomy
became a truly universal science; uncertainties and disparities were
banished, and observations made at all times and places rendered mutually
comparable.[65]

More, however, yet remained to be done. In order to verify with greater
strictness the results drawn from the Bradley and Piazzi catalogues, a third
term of comparison was wanted, and this Bessel undertook to supply. By a
course of 75,011 observations, executed during the years 1821-33, with the

utmost nicety of care, the number of accurately known stars was brought up
to above 50,000, and an ample store of trustworthy facts laid up for the use
of future astronomers. In this department Argelander, whom he attracted
from finance to astronomy, and trained in his own methods, was his
assistant and successor. The great "Bonn Durchmusterung,"[66] in which
324,198 stars visible in the northern hemisphere are enumerated, and the
corresponding "Atlas" published in 1857-63, constituting a picture of our
sidereal surroundings of heretofore unapproached completeness, may be
justly said to owe their origin to Bessel's initiative, and to form a sequel to
what he commenced.

But his activity was not solely occupied with the promotion of a
comprehensive reform in astronomy; it embraced special problems as well.
The long-baffled search for a parallax of the fixed stars was resumed with
fresh zeal as each mechanical or optical improvement held out fresh hopes
of a successful issue. Illusory results abounded. Piazza in 1805 perceived,
as he supposed, considerable annual displacements in Vega, Aldebaran,
Sirius, and Procyon; the truth being that his instruments were worn out with
constant use, and could no longer be depended upon.[67] His countryman,
Calandrelli, was similarly deluded. The celebrated controversy between the
Astronomer Royal and Dr. Brinkley, Director of the Dublin College
Observatory, turned on the same subject. Brinkley, who was in possession
of a first-rate meridian-circle, believed himself to have discovered relatively
large parallaxes for four of the brightest stars; Pond, relying on the
testimony of the Greenwich instruments, asserted their nullity. The dispute,
protracted for fourteen years, from 1810 until 1824, was brought to no
definite conclusion; but the strong presumption on the negative side was
abundantly justified in the event.

There was good reason for incredulity in the matter of parallaxes.
Announcements of their detection had become so frequent as to be
discredited before they were disproved; and Struve, who investigated the
subject at Dorpat in 1818-21, had clearly shown that the quantities

concerned were too small to come within the reliable measuring powers of
any instrument then in use. Already, however, the means were being
prepared of giving to those powers a large increase.

On the 21st July, 1801, two old houses in an alley of Munich tumbled
down, burying in their ruins the occupants, of whom one alone was
extricated alive, though seriously injured. This was an orphan lad of
fourteen named Joseph Fraunhofer. The Elector Maximilian Joseph was
witness of the scene, became interested in the survivor, and consoled his
misfortune with a present of eighteen ducats. Seldom was money better
bestowed. Part of it went to buy books and a glass-polishing machine, with
the help of which young Fraunhofer studied mathematics and optics, and
secretly exercised himself in the shaping and finishing of lenses; the
remainder purchased his release from the tyranny of one Weichselberger, a
looking-glass maker by trade, to whom he had been bound apprentice on
the death of his parents. A period of struggle and privation followed, during
which, however, he rapidly extended his acquirements; and was thus
eminently fitted for the task awaiting him, when, in 1806, he entered the
optical department of the establishment founded two years previously by
Von Reichenbach and Utzschneider. He now zealously devoted himself to
the improvement of the achromatic telescope; and, after a prolonged study
of the theory of lenses, and many toilsome experiments in the manufacture
of flint-glass, he succeeded in perfecting, December 12, 1817, an object-
glass of exquisite quality and finish, 9-1/2 inches in diameter, and of 14 feet
focal length.

This (as it was then considered) gigantic lens was secured by Struve for the
Russian Government, and the "great Dorpat refractor"—the first of the large
achromatics which have played such an important part in modern
astronomy—was, late in 1824, set up in the place which it still occupies. By
ingenious improvements in mounting and fitting, it was adapted to the
finest micrometrical work, and thus offered unprecedented facilities both
for the examination of double stars (in which Struve chiefly employed it),
and for such subtle measurements as might serve to reveal or disprove the

existence of a sensible stellar parallax. Fraunhofer, moreover, constructed
for the observatory at Königsberg the first really available heliometer. The
principle of this instrument (termed with more propriety a "divided object-
glass micrometer") is the separation, by a strictly measurable amount, of
two distinct images of the same object. If a double star, for instance, be
under examination, the two half-lenses into which the object-glass is
divided are shifted until the upper star (say) in one image is brought into
coincidence with the lower star in the other, when their distance apart
becomes known by the amount of motion employed.[68]

This virtually new engine of research was delivered and mounted in 1829,
three years after the termination of the life of its deviser. The Dorpat lens
had brought to Fraunhofer a title of nobility and the sole management of the
Munich Optical Institute (completely separated since 1814 from the
mechanical department). What he had achieved, however, was but a small
part of what he meant to achieve. He saw before him the possibility of
nearly quadrupling the light-gathering capacity of the great achromatic
acquired by Struve; he meditated improvements in reflectors as important
as those he had already effected in refractors; and was besides eagerly
occupied with investigations into the nature of light, the momentous
character of which we shall by-and-by have an opportunity of estimating.
But his health was impaired, it is said, from the weakening effects of his
early accident, combined with excessive and unwholesome toil, and, still
hoping for its restoration from a projected journey to Italy, he died of
consumption, June 7, 1826, aged thirty-nine years. His tomb in Munich
bears the concise eulogy, Approximavit sidera.

Bessel had no sooner made himself acquainted with the exquisite defining
powers of the Königsberg heliometer, than he resolved to employ them in
an attack upon the now secular problem of star-distances. But it was not
until 1837 that he found leisure to pursue the inquiry. In choosing his test-
star he adopted a new principle. It had hitherto been assumed that our
nearest neighbours in space must be found among the brightest ornaments
of our skies. The knowledge of stellar proper motions afforded by the

critical comparison of recent with earlier star-places, suggested a different
criterion of distance. It is impossible to escape from the conclusion that the
apparently swiftest-moving stars are, on the whole, also the nearest to us,
however numerous the individual exceptions to the rule. Now, as early as
1792,[69] Piazzi had noted as an indication of relative vicinity to the earth,
the unusually large proper motion (5·2′ annually) of a double star of the
fifth magnitude in the constellation of the Swan. Still more emphatically in
1812[70] Bessel drew the attention of astronomers to the fact, and 61 Cygni
became known as the "flying star." The seeming rate of its flight, indeed, is
of so leisurely a kind, that in a thousand years it will have shifted its place
by less than 3-1/2 lunar diameters, and that a quarter of a million would be
required to carry it round the entire circuit of the visible heavens.
Nevertheless, it has few rivals in rapidity of movement, the apparent
displacement of the vast majority of stars being, by comparison, almost
insensible.

This interesting, though inconspicuous object, then, was chosen by Bessel
to be put to the question with his heliometer, while Struve made a similar
and somewhat earlier trial with the bright gem of the Lyre, whose Arabic
title of the "Falling Eagle" survives as a time-worn remnant in "Vega." Both
astronomers agreed to use the "differential" method, for which their
instruments and the vicinity to their selected stars of minute, physically
detached companions offered special facilities. In the last month of 1838
Bessel made known the result of one year's observations, showing for 61
Cygni a parallax of about a third of a second (0·3136′).[71] He then had his
heliometer taken down and repaired, after which he resumed the inquiry,
and finally terminated a series of 402 measures in March 1840.[72] The
resulting parallax of 0·3483′ (corresponding to a distance about 600,000
times that of the earth from the sun), seemed to be ascertained beyond the
possibility of cavil, and is memorable as the first published instance of the
fathom-line, so industriously thrown into celestial space, having really and
indubitably touched bottom. It was confirmed in 1842-43 with curious

exactness by C. A. F. Peters at Pulkowa; but later researches showed that it
required increase to nearly half a second.[73]

Struve's measurements inspired less confidence. They extended over three
years (1835-38), but were comparatively few, and were frequently
interrupted. The parallax, accordingly, of about a quarter of a second
(0·2613′) which he derived from them for α Lyræ, and announced in 1840,
[74] has proved considerably too large.[75]

Meanwhile a result of the same kind, but of a more striking character than
either Bessel's or Struve's, had been obtained, one might almost say
casually, by a different method and in a distant region. Thomas Henderson,
originally an attorney's clerk in his native town of Dundee, had become
known for his astronomical attainments, and was appointed in 1831 to
direct the recently completed observatory at the Cape of Good Hope. He
began observing in April, 1832, and, the serious shortcomings of his
instrument notwithstanding, executed during the thirteen months of his
tenure of office a surprising amount of first-rate work. With a view to
correcting the declination of the lustrous double star α Centauri (which
ranks after Sirius and Canopus as the third brightest orb in the heavens), he
effected a number of successive determinations of its position, and on being
informed of its very considerable proper motion (3·6′ annually), he resolved
to examine the observations already made for possible traces of parallactic
displacement. This was done on his return to Scotland, where he filled the
office of Astronomer Royal from 1834 until his premature death in 1844.
The result justified his expectations. From the declination measurements
made at the Cape and duly reduced, a parallax of about one second of arc
clearly emerged (diminished by Gill's and Elkin's observations, 1882-1883,
to O·75′); but, by perhaps an excess of caution, was withheld from
publication until fuller certainty was afforded by the concurrent testimony
of Lieutenant Meadows's determinations of the same star's right ascension.
[76] When at last, January 9, 1839, Henderson communicated his discovery
to the Astronomical Society, he could no longer claim the priority which

was his due. Bessel had anticipated him with the parallax of 61 Cygni by
just two months.

Thus from three different quarters, three successful and almost
simultaneous assaults were delivered upon a long-beleaguered citadel of
celestial secrets. The same work has since been steadily pursued, with the
general result of showing that, as regards their overwhelming majority, the
stars are far too remote to show even the slightest trace of optical shifting
from the revolution of the earth in its orbit. In nearly a hundred cases,
however, small parallaxes have been determined, some certainly (that is,
within moderate limits of error), others more or less precariously. The list is
an instructive one, in its omissions no less than in its contents. It includes
stars of many degrees of brightness, from Sirius down to a nameless
telescopic star in the Great Bear;[77] yet the vicinity to the earth of this
minute object is so much greater than that of the brilliant Vega, that the
latter transported to its place would increase in lustre thirty-eight times.
Moreover, many of the brightest stars are found to have no sensible
parallax, while the majority of those ascertained to be nearest to the earth
are of fifth, sixth, even ninth magnitudes. The obvious conclusions follow
that the range of variety in the sidereal system is enormously greater than
had been supposed, and that estimates of distance based upon apparent
magnitude must be wholly futile. Thus, the splendid Canopus, Betelgeux,
and Rigel can be inferred, from their indefinite remoteness, to exceed our
sun thousands of times in size and lustre; while many inconspicuous
objects, which prove to be in our relative vicinity, must be notably his
inferiors. The limits of real stellar magnitude are then set very widely apart.
At the same time, the so-called "optical" and "geometrical" methods of
relatively estimating star-distances are both seen to have a foundation of
fact, although so disguised by complicated relations as to be of very
doubtful individual application. On the whole, the chances are in favour of
the superior vicinity of a bright star over a faint one; and, on the whole, the
stars in swiftest apparent motion are amongst those whose actual
remoteness is least. Indeed, there is no escape from either conclusion,

unless on the supposition of special arrangements in themselves highly
improbable, and, we may confidently say, non-existent.

The distances even of the few stars found to have measurable parallaxes are
on a scale entirely beyond the powers of the human mind to conceive. In the
attempt both to realize them distinctly, and to express them conveniently, a
new unit of length, itself of bewildering magnitude, has originated. This is
what we may call the light-journey of one year. The subtle vibrations of the
ether, propagated on all sides from the surface of luminous bodies, travel at
the rate of 186,300 miles a second, or (in round numbers) six billions of
miles a year. Four and a third such measures are needed to span the abyss
that separates us from the nearest fixed star. In other words, light takes four
years and four months to reach the earth from α Centauri; yet α Centauri
lies some ten billions of miles nearer to us (so far as is yet known) than any
other member of the sidereal system!

The determination of parallax leads, in the case of stars revolving in known
orbits, to the determination of mass; for the distance from the earth of the
two bodies forming a binary system being ascertained, the seconds of arc
apparently separating them from each other can be translated into millions
of miles; and we only need to add a knowledge of their period to enable us,
by an easy sum in proportion, to find their combined mass in terms of that
of the sun. Thus, since—according to Dr. Doberck's elements—the
components of α Centauri revolve round their common centre of gravity at
a mean distance nearly 25 times the radius of the earth's orbit, in a period of
88 years, the attractive force of the two together must be just twice the
solar. We may gather some idea of their relations by placing in imagination
a second luminary like our sun in circulation between the orbits of Neptune
and Uranus. But systems of still more majestic proportions are reduced by
extreme remoteness to apparent insignificance. A double star of the fourth
magnitude in Cassiopeia (Eta), to which a small parallax is ascribed on the
authority of O. Struve, appears to be above eight times as massive as the
central orb of our world; while a much less conspicuous pair—85 Pegasi—

exerts, if the available data can be depended upon, no less than thirteen
times the solar gravitating power.

Further, the actual rate of proper motions, so far as regards that part of them
which is projected upon the sphere, can be ascertained for stars at known
distance. The annual journey, for instance, of 61 Cygni across the line of
sight amounts to 1,000, and that of α Centauri to 446 millions of miles. A
small star, numbered 1,830 in Groombridge's Circumpolar Catalogue,
"devours the way" at the rate of at least 150 miles a second—a speed, in
Newcomb's opinion, beyond the gravitating power of the entire sidereal
system to control; and μ Cassiopeiæ possesses above two-thirds of that
surprising velocity; while for both objects, radial movements of just sixty
miles a second were disclosed by Professor Campbell's spectroscopic
measurements.

Herschel's conclusion as to the advance of the sun among the stars was not
admitted as valid by the most eminent of his successors. Bessel maintained
that there was absolutely no preponderating evidence in favour of its
supposed direction towards a point in the constellation Hercules.[78] Biot,
Burckhardt, even Herschel's own son, shared his incredulity. But the
appearance of Argelander's prize-essay in 1837[79] changed the aspect of the
question. Herschel's first memorable solution in 1783 was based upon the
motions of thirteen stars, imperfectly known; his second, in 1805, upon
those of no more than six. Argelander now obtained an entirely concordant
result from the large number of 390, determined with the scrupulous
accuracy characteristic of Bessel's work and his own. The reality of the fact
thus persistently disclosed could no longer be doubted; it was confirmed
five years later by the younger Struve, and still more strikingly in 1847[80]
by Galloway's investigations, founded exclusively on the apparent
displacements of southern stars. In 1859 and 1863, Sir George Airy and Mr.
Dunkin (1821-1898),[81] employing all the resources of modern science,
and commanding the wealth of material furnished by 1,167 proper motions
carefully determined by Mr. Main, reached conclusions closely similar to

that indicated nearly eighty years previously by the first great sidereal
astronomer; which Mr. Plummer's reinvestigation of the subject in 1883[82]
served but slightly to modify. Yet astronomers were not satisfied. Dr.
Auwers of Berlin completed in 1866 a splendid piece of work, for which he
received in 1888 the Gold Medal of the Royal Astronomical Society. It
consisted in reducing afresh, with the aid of the most refined modern data,
Bradley's original stars, and comparing their places thus obtained for the
year 1755 with those assigned to them from observations made at
Greenwich after the lapse of ninety years. In the interval, as was to be
anticipated, most of them were found to have travelled over some small
span of the heavens, and there resulted a stock of nearly three thousand
highly authentic proper motions. These ample materials were turned to
account by M. Ludwig Struve[83] for a discussion of the sun's motion, of
which the upshot was to shift its point of aim to the bordering region of the
constellations Hercules and Lyra. And the more easterly position of the
solar apex was fully confirmed by the experiments, with variously assorted
lists of stars, of Lewis Boss of Albany,[84] and Oscar Stumpe of Bonn.[85]
Fresh precautions of refinement were introduced into the treatment of the
subject by Ristenpart of Karlsruhe,[86] by Kapteyn of Groningen,[87] by
Newcomb[88] and Porter[89] in America, who ably availed themselves of the
copious materials accumulated before the close of the century. Their results,
although not more closely accordant than those of their predecessors,
combined to show that the journey of our system is directed towards a point
within a circle about ten degrees in radius, having the brilliant Vega for its
centre. To determine its rate was a still more arduous problem. It involved
the assumption, very much at discretion, of an average parallax for the stars
investigated; and Otto Struve's estimate of 154 million miles as the span
yearly traversed was hence wholly unreliable. Fortunately, however, as will
be seen further on, a method of determining the sun's velocity
independently of any knowledge of star-distances, has now become
available.

As might have been expected, speculation has not been idle regarding the
purpose and goal of the strange voyage of discovery through space upon
which our system is embarked; but altogether fruitlessly. The variety of the
conjectures hazarded in the matter is in itself a measure of their futility.
Long ago, before the construction of the heavens had as yet been made the
subject of methodical inquiry, Kant was disposed to regard Sirius as the
"central sun" of the Milky Way; while Lambert surmised that the vast Orion
nebula might serve as the regulating power of a subordinate group including
our sun. Herschel threw out the hint that the great cluster in Hercules might
prove to be the supreme seat of attractive force;[90] Argelander placed his
central body in the constellation Perseus;[91] Fomalhaut, the brilliant of the
Southern Fish, was set in the post of honour by Boguslawski of Breslau.
Mädler (who succeeded Struve at Dorpat in 1839) concluded from a more
formal inquiry that the ruling power in the sidereal system resided, not in
any single prepondering mass, but in the centre of gravity of the self-
controlled revolving multitude.[92] In the former case (as we know from the
example of the planetary scheme), the stellar motions would be most rapid
near the centre; in the latter, they would become accelerated with
remoteness from it.[93] Mädler showed that no part of the heavens could be
indicated as a region of exceptionally swift movements, such as would
result from the presence of a gigantic (though possibly obscure) ruling
body; but that a community of extremely sluggish movements undoubtedly
existed in and near the group of the Pleiades, where, accordingly, he placed
the centre of gravity of the Milky Way.[94] The bright star Alcyone thus
became the "central sun," but in a purely passive sense, its headship being
determined by its situation at the point of neutralisation of opposing
tendencies, and of consequent rest. By an avowedly conjectural method, the
solar period of revolution round this point was fixed at 18,200,000 years.

The scheme of sidereal government framed by the Dorpat astronomer was,
it may be observed, of the most approved constitutional type; deprivation,
rather than increase of influence accompanying the office of chief dignitary.
But while we are still ignorant, and shall perhaps ever remain so, of the

fundamental plan upon which the Galaxy is organised, recent investigations
tend more and more to exhibit it, not as monarchical (so to speak), but as
federative. The community of proper motions detected by Mädler in the
vicinity of the Pleiades may accordingly possess a significance altogether
different from what he imagined.

Bessel's so-called "foundation of an Astronomy of the Invisible" now
claims attention.[95] His prediction regarding the planet Neptune does not
belong to the present division of our subject; a strictly analogous discovery
in the sidereal system was, however, also very clearly foreshadowed by
him. His earliest suspicions of non-uniformity in the proper motion of
Sirius dated from 1834; they extended to Procyon in 1840; and after a series
of refined measurements with the new Repsold circle, he announced in
1844 his conclusion that these irregularities were due to the presence of
obscure bodies round which the two bright Dog-stars revolved as they
pursued their way across the sphere.[96] He even assigned to each an
approximate period of half a century. "I adhere to the conviction," he wrote
later to Humboldt, "that Procyon and Sirius form real binary systems,
consisting of a visible and an invisible star. There is no reason to suppose
luminosity an essential quality of cosmical bodies. The visibility of
countless stars is no argument against the invisibility of countless others."
[97]

An inference so contradictory to received ideas obtained little credit, until
Peters found, in 1851,[98] that the apparent anomalies in the movements of
Sirius could be completely explained by an orbital revolution in a period of
fifty years. Bessel's prevision was destined to be still more triumphantly
vindicated. On the 31st of January, 1862, while in the act of trying a new
18-inch refractor, Mr. Alvan G. Clark (one of the celebrated firm of
American opticians) actually discovered the hypothetical Sirian companion
in the precise position required by theory. It has now been watched through
nearly an entire revolution (period 49·4 years), and proves to be very
slightly luminous in proportion to its mass. Its attractive power, in fact, is

nearly half that of its primary, while it emits only 1/10000th of its light.
Sirius itself, on the other hand, possesses a far higher radiative intensity
than our sun. It gravitates—admitting Sir David Gill's parallax of 0·38′ to
be exact—like two suns, but shines like twenty. Possibly it is much
distended by heat, and undoubtedly its atmosphere intercepts a very much
smaller proportion of its light than in stars of the solar class. As regards
Procyon, visual verification was awaited until November 13, 1896, when
Professor Schaeberle, with the great Lick refractor, detected the long-sought
object in the guise of a thirteenth-magnitude star. Dr. See's calculations[99]
showed it to possess one-fifth the mass of its primary, or rather more than
half that of our sun.[100] Yet it gives barely 1/20000th of the sun's light, so
that it is still nearer to total obscurity than the dusky satellite of Sirius. The
period of forty years assigned to the system by Auwers in 1862[101] appears
to be singularly exact.

But Bessel was not destined to witness the recognition of "the invisible" as
a legitimate and profitable field for astronomical research. He died March
17, 1846, just six months before the discovery of Neptune, of an obscure
disease, eventually found to be occasioned by an extensive fungus-growth
in the stomach. The place which he left vacant was not one easy to fill. His
life's work might be truly described as "epoch-making." Rarely indeed shall
we find one who reconciled with the same success the claims of theoretical
and practical astronomy, or surveyed the science which he had made his
own with a glance equally comprehensive, practical, and profound.

The career of Friedrich Georg Wilhelm Struve illustrates the maxim that
science differentiates as it develops. He was, while much besides, a
specialist in double stars. His earliest recorded use of the telescope was to
verify Herschel's conclusion as to the revolving movement of Castor, and he
never varied from the predilection which this first observation at once
indicated and determined. He was born at Altona, of a respectable yeoman
family, April 15, 1793, and in 1811 took a degree in philology at the new
Russian University of Dorpat. He then turned to science, was appointed in

1813 to a professorship of astronomy and mathematics, and began regular
work in the Dorpat Observatory just erected by Parrot for Alexander I. It
was not, however, until 1819 that the acquisition of a 5-foot refractor by
Troughton enabled him to take the position-angles of double stars with
regularity and tolerable precision. The resulting catalogue of 795 stellar
systems gave the signal for a general resumption of the Herschelian labours
in this branch. His success, so far, and the extraordinary facilities for
observation afforded by the Fraunhofer achromatic encouraged him to
undertake, February 11, 1825, a review of the entire heavens down to 15°
south of the celestial equator, which occupied more than two years, and
yielded, from an examination of above 120,000 stars, a harvest of about
2,200 previously unnoticed composite objects. The ensuing ten years were
devoted to delicate and patient measurements, the results of which were
embodied in Mensuræ Micrometricæ, published at St. Petersburg in 1837.
This monumental work gives the places, angles of position, distances,
colours, and relative brightness of 3,112 double and multiple stars, all
determined with the utmost skill and care. The record is one which gains in
value with the process of time, and will for ages serve as a standard of
reference by which to detect change or confirm discovery.

It appears from Struve's researches that about one in forty of all stars down
to the ninth magnitude is composite, but that the proportion is doubled in
the brighter orders.[102] This he attributed to the difficulty of detecting the
faint companions of very remote orbs. It was also noticed, both by him and
Bessel, that double stars are in general remarkable for large proper motions.
Struve's catalogue included no star of which the components were more
than 32′ apart, because beyond that distance the chances of merely optical
juxtaposition become considerable; but the immense preponderance of
extremely close over (as it were) loosely yoked bodies is such as to
demonstrate their physical connection, even if no other proof were
forthcoming. Many stars previously believed to be single divided under the
scrutiny of the Dorpat refractor; while in some cases, one member of a
supposed binary system revealed itself as double, thus placing the surprised

observer in the unexpected presence of a triple group of suns. Five instances
were noted of two pairs lying so close together as to induce a conviction of
their mutual dependence;[103] besides which, 124 examples occurred of
triple, quadruple, and multiple combinations, the reality of which was open
to no reasonable doubt.[104]

It was first pointed out by Bessel that the fact of stars exhibiting a common
proper motion might serve as an unfailing test of their real association into
systems. This was, accordingly, one of the chief criteria employed by Struve
to distinguish true binaries from merely optical couples. On this ground
alone, 61 Cygni was admitted to be a genuine double star; and it was shown
that, although its components appeared to follow almost strictly rectilinear
paths, yet the probability of their forming a connected pair is actually
greater than that of the sun rising to-morrow morning.[105] Moreover, this
tie of an identical movement was discovered to unite bodies[106] far beyond
the range of distance ordinarily separating the members of binary systems,
and to prevail so extensively as to lead to the conclusion that single do not
outnumber conjoined stars more than twice or thrice.[107]

In 1835 Struve was summoned by the Emperor Nicholas to superintend the
erection of a new observatory at Pulkowa, near St. Petersburg, destined for
the special cultivation of sidereal astronomy. Boundless resources were
placed at his disposal, and the institution created by him was acknowledged
to surpass all others of its kind in splendour, efficiency, and completeness.
Its chief instrumental glory was a refractor of fifteen inches aperture by
Merz and Mahler (Fraunhofer's successors), which left the famous Dorpat
telescope far behind, and remained long without a rival. On the completion
of this model establishment, August 19, 1839, Struve was installed as its
director, and continued to fulfil the important duties of the post with his
accustomed vigour until 1858, when illness compelled his virtual
resignation in favour of his son Otto Struve, born at Dorpat in 1819. He
died November 23, 1864.

An inquiry into the laws of stellar distribution, undertaken during the early
years of his residence at Pulkowa, led Struve to confirm in the main the
inferences arrived at by Herschel as to the construction of the heavens.
According to his view, the appearance known as the Milky Way is produced
by a collection of irregularly condensed star-clusters, within which the sun
is somewhat eccentrically placed. The nebulous ring which thus integrates
the light of countless worlds was supposed by him to be made up of stars
scattered over a bent or "broken plane," or to lie in two planes slightly
inclined to each other, our system occupying a position near their
intersection.[108] He further attempted to show that the limits of this vast
assemblage must remain for ever shrouded from human discernment, owing
to the gradual extinction of light in its passage through space,[109] and
sought to confer upon this celebrated hypothesis a definiteness and certainty
far beyond the aspirations of its earlier advocates, Chéseaux and Olbers; but
arbitrary assumptions vitiated his reasonings on this, as well as on some
other points.[110]

In his special line as a celestial explorer of the most comprehensive type,
Sir William Herschel had but one legitimate successor, and that successor
was his son. John Frederick William Herschel was born at Slough, March
17, 1792, graduated with the highest honours from St. John's College,
Cambridge, in 1813, and entered upon legal studies with a view to being
called to the Bar. But his share in an early compact with Peacock and
Babbage, "to do their best to leave the world wiser than they found it," was
not thus to be fulfilled. The acquaintance of Dr. Wollaston decided his
scientific vocation. Already, in 1816, we find him reviewing some of his
father's double stars; and he completed in 1820 the 18-inch speculum which
was to be the chief instrument of his investigations. Soon afterwards, he
undertook, in conjunction with Mr. (later Sir James) South, a series of
observations, issuing in the presentation to the Royal Society of a paper[111]
containing micrometrical measurements of 380 binary stars, by which the
elder Herschel's inferences of orbital motion were, in many cases, strikingly
confirmed. A star in the Northern Crown, for instance (η Coronæ), had

completed more than one entire circuit since its first discovery; another, τ
Ophiuchi, had closed up into apparent singleness; while the motion of a
third, ξ Ursæ Majoris, in an obviously eccentric orbit, was so rapid as to
admit of being traced and measured from month to month.

It was from the first confidently believed that the force retaining double
stars in curvilinear paths was identical with that governing the planetary
revolutions. But that identity was not ascertained until Savary of Paris
showed, in 1827,[112] that the movements of the above-named binary in the
Great Bear could be represented with all attainable accuracy by an ellipse
calculated on orthodox gravitational principles with a period of 58-1/4
years. Encke followed at Berlin with a still more elegant method; and Sir
John Herschel, pointing out the uselessness of analytical refinements where
the data were necessarily so imperfect, described in 1832 a graphical
process by which "the aid of the eye and hand" was brought in "to guide the
judgment in a case where judgment only, and not calculation, could be of
any avail."[113] Improved methods of the same kind were published by Dr.
See in 1893,[114] and by Mr. Burnham in 1894;[115] and our acquaintance
with stellar orbits is steadily gaining precision, certainty, and extent.

In 1825 Herschel undertook, and executed with great assiduity during the
ensuing eight years, a general survey of the northern heavens, directed
chiefly towards the verification of his father's nebular discoveries. The
outcome was a catalogue of 2,306 nebulæ and clusters, of which 525 were
observed for the first time, besides 3,347 double stars discovered almost
incidentally.[116] "Strongly invited," as he tells us himself, "by the peculiar
interest of the subject, and the wonderful nature of the objects which
presented themselves," he resolved to attempt the completion of the survey
in the southern hemisphere. With this noble object in view, he embarked his
family and instruments on board the Mount Stewart Elphinstone, and, after
a prosperous voyage, landed at Cape Town on the 16th of January, 1834.
Choosing as the scene of his observations a rural spot under the shelter of
Table Mountain, he began regular "sweeping" on the 5th of March. The site

of his great reflector is now marked with an obelisk, and the name of
Feldhausen has become memorable in the history of science; for the four
years' work done there may truly be said to open the chapter of our
knowledge as regards the southern skies.

The full results of Herschel's journey to the Cape were not made public
until 1847, when a splendid volume[117] embodying them was brought out
at the expense of the Duke of Northumberland. They form a sequel to his
father's labours such as the investigations of one man have rarely received
from those of another. What the elder observer did for the northern heavens,
the younger did for the southern, and with generally concordant results.
Reviving the paternal method of "star-gauging," he showed, from a count of
2,299 fields, that the Milky Way surrounds the solar system as a complete
annulus of minute stars; not, however, quite symmetrically, since the sun
was thought to lie somewhat nearer to those portions visible in the southern
hemisphere, which display a brighter lustre and a more complicated
structure than the northern branches. The singular cosmical agglomerations
known as the "Magellanic Clouds" were now, for the first time, submitted to
a detailed, though admittedly incomplete, examination, the almost
inconceivable richness and variety of their contents being such that a
lifetime might with great profit be devoted to their study. In the Greater
Nubecula, within a compass of forty-two square degrees, Herschel reckoned
278 distinct nebulæ and clusters, besides fifty or sixty outliers, and a large
number of stars intermixed with diffused nebulosity—in all, 919 catalogued
objects, and, for the Lesser Cloud, 244. Yet this was only the most
conspicuous part of what his twenty-foot revealed. Such an extraordinary
concentration of bodies so various led him to the inevitable conclusion that
"the Nubeculæ are to be regarded as systems sui generis, and which have no
analogues in our hemisphere."[118] He noted also the blankness of
surrounding space, especially in the case of Nubecula Minor, "the access to
which on all sides," he remarked, "is through a desert;" as if the cosmical
material in the neighbourhood had been swept up and garnered in these
mighty groups.[119]

Of southern double stars, he discovered and gave careful measurements of
2,102, and described 1,708 nebulæ, of which at least 300 were new. The list
was illustrated with a number of drawings, some of them extremely
beautiful and elaborate.

Sir John Herschel's views as to the nature of nebulæ were considerably
modified by Lord Rosse's success in "resolving" with his great reflectors a
crowd of these objects into stars. His former somewhat hesitating belief in
the existence of phosphorescent matter, "disseminated through extensive
regions of space in the manner of a cloud or fog,"[120] was changed into a
conviction that no valid distinction could be established between the
faintest wisp of cosmical vapour just discernible in a powerful telescope,
and the most brilliant and obvious cluster. He admitted, however, an
immense range of possible variety in the size and mode of aggregation of
the stellar constituents of various nebulæ. Some might appear nebulous
from the closeness of their parts; some from their smallness. Others, he
suggested, might be formed of "discrete luminous bodies floating in a non-
luminous medium;"[121] while the annular kind probably consisted of
"hollow shells of stars."[122] That a physical, and not merely an optical,
connection unites nebulæ with the embroidery (so to speak) of small stars
with which they are in many instances profusely decorated, was evident to
him, as it must be to all who look as closely and see as clearly as he did. His
description of No. 2,093 in his northern catalogue as "a network or tracery
of nebula following the lines of a similar network of stars,"[123] would alone
suffice to dispel the idea of accidental scattering; and many other examples
of a like import might be quoted. The remarkably frequent occurrence of
one or more minute stars in the close vicinity of "planetary" nebulæ led him
to infer their dependent condition; and he advised the maintenance of a
strict watch for evidences of circulatory movements, not only over these
supposed stellar satellites, but also over the numerous "double nebulæ," in
which, as he pointed out, "all the varieties of double stars as to distance,
position, and relative brightness, have their counterparts." He, moreover,
investigated the subject of nebular distribution by the simple and effectual

method of graphic delineation or "charting," and succeeded in showing that
while a much greater uniformity of scattering prevails in the southern than
in the northern heavens, a condensation is nevertheless perceptible about
the constellations Pisces and Cetus, roughly corresponding to the "nebular
region" in Virgo by its vicinity (within 20° or 30°) to the opposite pole of
the Milky Way. He concluded "that the nebulous system is distinct from the
sidereal, though involving, and perhaps to a certain extent intermixed with,
the latter."[124]

Towards the close of his residence at Feldhausen, Herschel was fortunate
enough to witness one of those singular changes in the aspect of the
firmament which occasionally challenge the attention even of the incurious,
and excite the deepest wonder of the philosophical observer. Immersed
apparently in the Argo nebula is a star denominated η Carinæ. When Halley
visited St. Helena in 1677, it seemed of the fourth magnitude; but Lacaille
in the middle of the following century, and others after him, classed it as of
the second. In 1827 the traveller Burchell, being then at St. Paul, near Rio
Janeiro, remarked that it had unexpectedly assumed the first rank—a
circumstance the more surprising to him because he had frequently, when in
Africa during the years 1811 to 1815, noted it as of only fourth magnitude.
This observation, however, did not become generally known until later.
Herschel, on his arrival at Feldhausen, registered the star as a bright second,
and had no suspicion of its unusual character until December 16, 1837,
when he suddenly perceived its light to be almost tripled. It then far
outshone Rigel in Orion, and on the 2nd of January following it very nearly
matched α Centauri. From that date it declined; but a second and even
brighter maximum occurred in April, 1843, when Maclear, then director of
the Cape Observatory, saw it blaze out with a splendour approaching that of
Sirius. Its waxings and wanings were marked by curious "trepidations" of
brightness extremely perplexing to theory. In 1863 it had sunk below the
fifth magnitude, and in 1869 was barely visible to the naked eye; yet it was
not until eighteen years later that it touched a minimum of 7·6 magnitude.
Soon afterwards a recovery of brightness set in, but was not carried very

far; and the star now shines steadily as of the seventh magnitude, its reddish
light contrasting effectively with the silvery rays of the surrounding nebula.
An attempt to include its fluctuations within a cycle of seventy years[125]
has signally failed; the extent and character of the vicissitudes to which it is
subject stamping it rather as a species of connecting link between periodic
and temporary stars.[126]

Among the numerous topics which engaged Herschel's attention at the Cape
was that of relative stellar brightness. Having contrived an "astrometer" in
which an "artificial star," formed by the total reflection of moonlight from
the base of a prism, served as a standard of comparison, he was able to
estimate the lustre of the natural stars examined by the distances at which
the artificial object appeared equal respectively to each. He thus constructed
a table of 191 of the principal stars,[127] both in the northern and southern
hemispheres, setting forth the numerical values of their apparent brightness
relatively to that of α Centauri, which he selected as a unit of measurement.
Further, the light of the full moon being found by him to exceed that of his
standard star 27,408 times, and Dr. Wollaston having shown that the light of
the full moon is to that of the sun as 1:801,072[128] (Zöllner made the ratio
1:618,000), it became possible to compare stellar with solar radiance.
Hence was derived, in the case of the few stars at ascertained distances, a
knowledge of real lustre. Alpha Centauri, for example, emits less than
twice, Capella one hundred times as much light as our sun; while Arcturus,
at its enormous distance, must display the splendour of 1,300 such
luminaries.

Herschel returned to England in the spring of 1838, bringing with him a
wealth of observation and discovery such as had perhaps never before been
amassed in so short a time. Deserved honours awaited him. He was created
a baronet on the occasion of the Queen's coronation (he had been knighted
in 1831); universities and learned societies vied with each other in
showering distinctions upon him; and the success of an enterprise in which
scientific zeal was tinctured with an attractive flavour of adventurous

romance, was justly regarded as a matter of national pride. His career as an
observing astronomer was now virtually closed, and he devoted his leisure
to the collection and arrangement of the abundant trophies of his father's
and his own activity. The resulting great catalogue of 5,079 nebulæ
(including all then certainly known), published in the Philosophical
Transactions for 1864, is, and will probably long remain, the fundamental
source of information on the subject;[129] but he unfortunately did not live
to finish the companion work on double stars, for which he had
accumulated a vast store of materials.[130] He died at Collingwood in Kent,
May 11, 1871, in the eightieth year of his age, and was buried in
Westminster Abbey, close beside the grave of Sir Isaac Newton.

The consideration of Sir John Herschel's Cape observations brings us to the
close of the period we are just now engaged in studying. They were given to
the world, as already stated, three years before the middle of the century,
and accurately represent the condition of sidereal science at that date.
Looking back over the fifty years traversed, we can see at a glance how
great was the stride made in the interval. Not alone was acquaintance with
individual members of the cosmos vastly extended, but their mutual
relations, the laws governing their movements, their distances from the
earth, masses, and intrinsic lustre, had begun to be successfully
investigated. Begun to be; for only regarding a scarcely perceptible minority
had even approximate conclusions been arrived at. Nevertheless the whole
progress of the future lay in that beginning; it was the thin end of the wedge
of exact knowledge. The principle of measurement had been substituted for
that of probability; a basis had been found large and strong enough to
enable calculation to ascend from it to the sidereal heavens; and refinements
had been introduced, fruitful in performance, but still more in promise.
Thus, rather the kind than the amount of information collected was
significant for the time to come—rather the methods employed than the
results actually secured rendered the first half of the nineteenth century of
epochal importance in the history of our knowledge of the stars.

FOOTNOTES:
[58] Bessel, Populäre Vorlesungen, pp. 6, 408.

[59] Fitted to the old transit instrument, July 11, 1772.

[60] Briefwechsel mit Olbers, p. xvi.
[61] R. Wolf, Gesch. der Astron., p. 518.

[62] Bessel, Pop. Vorl., p. 22.

[63] A new reduction of the observations upon which they were founded was
undertaken in 1896 by Herman S. Davis, of the U.S. Coast Survey.

[64] Bessel, Pop. Vorl., p. 440.

[65] Durège, Bessel's Leben und Wirken, p. 28.

[66] Bonner Beobachtungen, Bd. iii.-v., 1859-62.

[67] Bessel, Pop. Vorl., p. 238.

[68] The heads of the screws applied to move the halves of the object-glass in
the Königsberg heliometer are of so considerable a size that a thousandth part of
a revolution, equivalent to 1/20 of a second of arc, can be measured with the
utmost accuracy. Main, R. A. S. Mem., vol. xii., p. 53.
[69] Specola Astronomica di Palermo, lib. vi., p. 10, note.

[70] Monatliche Correspondenz, vol. xxvi., p. 162.

[71] Astronomische Nachrichten, Nos. 365-366. It should be explained that what
is called the "annual parallax" of a star is only half its apparent displacement. In
other words, it is the angle subtended at the distance of that particular star by the
radius of the earth's orbit.

[72] Astr. Nach., Nos. 401-402.

[73] Sir R. Ball's measurements at Dunsink gave to 61 Cygni a parallax of 0·47′;
Professor Pritchard obtained, by photographic determinations, one of 0·43′.

[74] Additamentum in Mensuras Micrometricas, p. 28.

[75] Elkin's corrected result (in 1897) for the parallax of Vega is 0·082′.

[76] Mem. Roy. Astr. Soc., vol. xi., p. 61.

[77] That numbered 21,185 in Lalande's Hist. Cél., found by Argelander to have
a proper motion of 4·734′, and by Winnecke a parallax of O·511′. Month. Not.,

vol. xviii., p. 289.

[78] Fund. Astr., p. 309.

[79] Mém. Prés. à l'Ac. de St. Pétersb., t. iii.

[80] Phil. Trans., vol. cxxxvii., p. 79.

[81] Mem. Roy. Astr. Soc., vols. xxviii. and xxxii.

[82] Ibid., vol. xlvii., p. 327.

[83] Mémoires de St. Pétersbourg, t. xxxv., No. 3, 1887; revised in Astr. Nach.,
Nos. 3,729-30, 1901.

[84] Astronomical Journal, Nos. 213, 501.

[85] Astr. Nach., Nos. 2,999, 3,000.

[86] Veröffentlichungen der Grossh. Sternwarte zu Karlsruhe, Bd. iv., 1892.

[87] Proceedings Amsterdam Acad. of Sciences, Jan. 27, 1900.

[88] Astr. Jour., No. 457.

[89] Ibid., Nos. 276, 497.

[90] Phil. Trans., vol. xcvi., p. 230.

[91] Mém. Prés. à l'Ac. de St. Pétersbourg, t. iii., p. 603 (read Feb. 5, 1837).

[92] Die Centralsonne, Astr. Nach., Nos. 566-567, 1846.

[93] Sir J. Herschel, note to Treatise on Astronomy, and Phil. Trans., vol. cxxiii.,
part ii., p. 502.

[94] The position is (as Sir J. Herschel pointed out, Outlines of Astronomy, p.
631, 10th ed.) placed beyond the range of reasonable probability by its
remoteness (fully 26°) from the galactic plane.

[95] Mädler in Westermann's Jahrbuch, 1867, p. 615.
[96] Letter from Bessel to Sir J. Herschel, Month. Not., vol. vi., p. 139.

[97] Wolf, Gesch. d. Astr., p. 743, note.

[98] Astr. Nach., Nos. 745-748.

[99] Astr. Jour., No. 440.

[100] Adopting Elkin's revised parallax for Procyon of 0·325′.

[101] Astr. Nach., Nos. 1371-1373.

[102] Ueber die Doppelsterne, Bericht, 1827, p. 22.

[103] Ueber die Doppelsterne, Bericht, 1827, p. 25.

[104] Mensuræ Micr., p. xcix.

[105] Stellarum Fixarum imprimis Duplicium et Multiplicum Positiones Mediæ,
pp. cxc., cciii.

[106] For instance, the southern stars, 36A Ophiuchi (itself double) and 30
Scorpii, which are 12′ 10″ apart. Ibid., p. cciii.

[107] Stellarum Fixarum, etc., p. ccliii.

[108] Études d'Astronomie Stellaire, 1847, p. 82.

[109] Ibid., p. 86.

[110] See Encke's criticism in Astr. Nach., No. 622.

[111] Phil. Trans., vol. cxiv., part iii., 1824.

[112] Conn. d. Temps, 1830.
[113] R. A. S. Mem., vol. v., p. 178, 1833.

[114] Astr. and Astrophysics, vol. xii., p. 581.

[115] Popular Astr., vol. i., p. 243.

[116] Phil. Trans., vol. cxxiii., and Results, etc., Introd.
[117] Results of Astronomical Observations made during the years 1834-8 at the
Cape of Good Hope.

[118] Results, etc., p. 147.
[119] See Proctor's Universe of Stars, p. 92.

[120] A Treatise on Astronomy, 1833, p. 406.

[121] Results, etc., p. 139.

[122] Ibid., pp. 24, 142.

[123] Phil. Trans., vol. cxxiii., p. 503.

[124] Results, etc., p. 136.

[125] Loomis, Month. Not., vol. xxix., p. 298.

[126] See the Author's System of the Stars, pp. 116-120.

[127] Outlines of Astr., App. I.

[128] Phil. Trans., vol. cxix., p. 27.

[129] Dr. Dreyer's New General Catalogue, published in 1888 as vol. xlix. of the
Royal Astronomical Society's Memoirs, is an enlargement of Herschel's work. It
includes 7,840 entries, and was supplemented, in 1895, by an "Index Catalogue"
of 1,529 nebulæ discovered 1888 to 1894. Mem. R. A. S., vol. li.

[130] A list of 10,320 composite stars was drawn out by him in order of right
ascension, and has been published in vol. xl. of Mem. R. A. S.; but the data
requisite for their formation into a catalogue were not forthcoming. See Main's
and Pritchard's Preface to above, and Dunkin's Obituary Notices, p. 73.

CHAPTER III
PROGRESS OF KNOWLEDGE REGARDING THE SUN

The discovery of sun-spots in 1610 by Fabricius and Galileo first opened a
way for inquiry into the solar constitution; but it was long before that way
was followed with system or profit. The seeming irregularity of the
phenomena discouraged continuous attention; casual observations were
made the basis of arbitrary conjectures, and real knowledge received little
or no increase. In 1620 we find Jean Tarde, Canon of Sarlat, arguing that
because the sun is "the eye of the world," and the eye of the world cannot
suffer from ophthalmia, therefore the appearances in question must be due,
not to actual specks or stains on the bright solar disc, but to the transits of a
number of small planets across it! To this new group of heavenly bodies he
gave the name of "Borbonia Sidera," and they were claimed in 1633 for the
House of Hapsburg, under the title of "Austriaca Sidera" by Father
Malapertius, a Belgian Jesuit.[131] A similar view was temporarily
maintained against Galileo by the justly celebrated Father Scheiner of
Ingolstadt, and later by William Gascoigne, the inventor of the micrometer;
but most of those who were capable of thinking at all on such subjects (and
they were but few) adhered either to the cloud theory or to the slag theory
of sun-spots. The first was championed by Galileo, the second by Simon
Marius, "astronomer and physician" to the brother Margraves of
Brandenburg. The latter opinion received a further notable development
from the fact that in 1618, a year remarkable for the appearance of three
bright comets, the sun was almost free from spots; whence it was inferred
that the cindery refuse from the great solar conflagration, which usually
appeared as dark blotches on its surface, was occasionally thrown off in the
form of comets, leaving the sun, like a snuffed taper, to blaze with renewed
brilliancy.[132]

In the following century, Derham gathered from observations carried on
during the years 1703-11, "That the spots on the sun are caused by the
eruption of some new volcano therein, which at first pouring out a
prodigious quantity of smoke and other opacous matter, causeth the spots;
and as that fuliginous matter decayeth and spendeth itself, and the volcano
at last becomes more torrid and flaming, so the spots decay, and grow to
umbræ, and at last to faculæ."[133]

The view, confidently upheld by Lalande,[134] that spots were rocky
elevations uncovered by the casual ebbing of a luminous ocean, the
surrounding penumbræ representing shoals or sandbanks, had even less to
recommend it than Derham's volcanic theory. Both were, however,
significant of a growing tendency to bring solar phenomena within the
compass of terrestrial analogies.

For 164 years, then, after Galileo first levelled his telescope at the setting
sun, next to nothing was learned as to its nature; and the facts immediately
ascertained, of its rotation on an axis nearly erect to the plane of the
ecliptic, in a period of between twenty-five and twenty-six days, and of the
virtual limitation of the spots to a so-called "royal" zone extending some
thirty degrees north and south of the solar equator, gained little either in
precision or development from five generations of astronomers.

But in November, 1769, a spot of extraordinary size engaged the attention
of Alexander Wilson, professor of astronomy in the University of Glasgow.
He watched it day by day, and to good purpose. As the great globe slowly
revolved, carrying the spot towards its western edge, he was struck with the
gradual contraction and final disappearance of the penumbra on the side
next the centre of the disc; and when on the 6th of December the same spot
re-emerged on the eastern limb, he perceived, as he had anticipated, that the
shady zone was now deficient on the opposite side, and resumed its original
completeness as it returned to a central position. In other spots subsequently
examined by him, similar perspective effects were visible, and he proved in
1774,[135] by strict geometrical reasoning, that they could only arise in vast

photospheric excavations. It was not, indeed, the first time that such a view
had been suggested. Father Scheiner's later observations plainly
foreshadowed it;[136] a conjecture to the same effect was emitted by
Leonard Rost of Nuremburg early in the eighteenth century;[137] both by
Lahire in 1703 and by J. Cassini in 1719 spots had been seen as notches on
the solar limb; while in 1770 Pastor Schülen of Essingen, from the careful
study of phenomena similar to those noted by Wilson, concluded their
depressed nature.[138] Modern observations, nevertheless, prove those
phenomena to be by no means universally present.

Wilson's general theory of the sun was avowedly tentative. It took the
modest form of an interrogatory. "Is it not reasonable to think," he asks,
"that the great and stupendous body of the sun is made up of two kinds of
matter, very different in their qualities; that by far the greater part is solid
and dark, and that this immense and dark globe is encompassed with a thin
covering of that resplendent substance from which the sun would seem to
derive the whole of his vivifying heat and energy?"[139] He further suggests
that the excavations or spots may be occasioned "by the working of some
sort of elastic vapour which is generated within the dark globe," and that the
luminous matter, being in some degree fluid, and being acted upon by
gravity, tends to flow down and cover the nucleus. From these hints,
supplemented by his own diligent observations and sagacious reasonings,
Herschel elaborated a scheme of solar constitution which held its ground
until the physics of the sun were revolutionised by the spectroscope.

A cool, dark, solid globe, its surface diversified with mountains and valleys,
clothed in luxuriant vegetation, and "richly stored with inhabitants,"
protected by a heavy cloud-canopy from the intolerable glare of the upper
luminous region, where the dazzling coruscations of a solar aurora some
thousands of miles in depth evolved the stores of light and heat which
vivify our world—such was the central luminary which Herschel
constructed with his wonted ingenuity, and described with his wonted
eloquence.

"This way of considering the sun and its atmosphere," he says,[140]
"removes the great dissimilarity we have hitherto been used to find between
its condition and that of the rest of the great bodies of the solar system. The
sun, viewed in this light, appears to be nothing else than a very eminent,
large, and lucid planet, evidently the first, or, in strictness of speaking, the
only primary one of our system; all others being truly secondary to it. Its
similarity to the other globes of the solar system with regard to its solidity,
its atmosphere, and its diversified surface, the rotation upon its axis, and the
fall of heavy bodies, leads us on to suppose that it is most probably also
inhabited, like the rest of the planets, by beings whose organs are adapted to
the peculiar circumstances of that vast globe."

We smile at conclusions which our present knowledge condemns as
extravagant and impossible, but such incidental flights of fancy in no way
derogate from the high value of Herschel's contributions to solar science.
The cloud-like character which he attributed to the radiant shell of the sun
(first named by Schröter the "photosphere") is borne out by all recent
investigations; he observed its mottled or corrugated aspect, resembling, as
he described it, the roughness on the rind of an orange; showed that
"faculæ" are elevations or heaped-up ridges of the disturbed photospheric
matter; and threw out the idea that spots may ensue from an excess of the
ordinary luminous emissions. A certain "empyreal" gas was, he supposed
(very much as Wilson had done), generated in the body of the sun, and
rising everywhere by reason of its lightness, made for itself, when in
moderate quantities, small openings or "pores,"[141] abundantly visible as
dark points on the solar disc. But should an uncommon quantity be formed,
"it will," he maintained, "burst through the planetary[142] regions of clouds,
and thus will produce great openings; then, spreading itself above them, it
will occasion large shallows (penumbræ), and mixing afterwards gradually
with other superior gases, it will promote the increase, and assist in the
maintenance, of the general luminous phenomena."[143]

This partial anticipation of the modern view that the solar radiations are
maintained by some process of circulation within the solar mass, was
reached by Herschel through prolonged study of the phenomena in
question. The novel and important idea contained in it, however, it was at
that time premature to attempt to develop. But though many of the subtler
suggestions of Herschel's genius passed unnoticed by his contemporaries,
the main result of his solar researches was an unmistakable one. It was
nothing less than the definitive introduction into astronomy of the
paradoxical conception of the central fire and hearth of our system as a
cold, dark, terrestrial mass, wrapt in a mantle of innocuous radiance—an
earth, so to speak, within—a sun without.

Let us pause for a moment to consider the value of this remarkable
innovation. It certainly was not a step in the direction of truth. On the
contrary, the crude notions of Anaxagoras and Xeno approached more
nearly to what we now know of the sun, than the complicated structure
devised for the happiness of a nobler race of beings than our own by the
benevolence of eighteenth-century astronomers. And yet it undoubtedly
constituted a very important advance in science. It was the first earnest
attempt to bring solar phenomena within the compass of a rational system;
to put together into a consistent whole the facts ascertained; to fabricate, in
short, a solar machine that would in some fashion work. It is true that the
materials were inadequate and the design faulty. The resulting construction
has not proved strong enough to stand the wear and tear of time and
discovery, but has had to be taken to pieces and remodelled on a totally
different plan. But the work was not therefore done in vain. None of
Bacon's aphorisms show a clearer insight into the relations between the
human mind and the external world than that which declares "Truth to
emerge sooner from error than from confusion."[144] A definite theory (even
if a false one) gives holding-ground to thought. Facts acquire a meaning
with reference to it. It affords a motive for accumulating them and a means
of co-ordinating them; it provides a framework for their arrangement, and a
receptacle for their preservation, until they become too strong and

numerous to be any longer included within arbitrary limits, and shatter the
vessel originally framed to contain them.

Such was the purpose subserved by Herschel's theory of the sun. It helped
to clarify ideas on the subject. The turbid sense of groping and viewless
ignorance gave place to the lucidity of a possible scheme. The persuasion of
knowledge is a keen incentive to its increase. Few men care to investigate
what they are obliged to admit themselves entirely ignorant of; but once
started on the road of knowledge, real or supposed, they are eager to pursue
it. By the promulgation of a confident and consistent view regarding the
nature of the sun, accordingly, research was encouraged, because it was
rendered hopeful, and inquirers were shown a path leading indefinitely
onwards where an impassable thicket had before seemed to bar the way.

We have called the "terrestrial" theory of the sun's nature an innovation, and
so, as far as its general acceptance is concerned, it may justly be termed;
but, like all successful innovations, it was a long time brewing. It is
extremely curious to find that Herschel had a predecessor in its advocacy
who never looked through a telescope (nor, indeed, imagined the possibility
of such an instrument), who knew nothing of sun-spots, was still (mistaken
assertions to the contrary notwithstanding) in the bondage of the geocentric
system, and regarded nature from the lofty standpoint of an idealist
philosophy. This was the learned and enlightened Cardinal Cusa, a
fisherman's son from the banks of the Moselle, whose distinguished career
in the Church and in literature extended over a considerable part of the
fifteenth century (1401-64). In his singular treatise De Doctâ Ignorantiâ,
one of the most notable literary monuments of the early Renaissance, the
following passage occurs:—"To a spectator on the surface of the sun, the
splendour which appears to us would be invisible, since it contains, as it
were, an earth for its central mass, with a circumferential envelope of light
and heat, and between the two an atmosphere of water and clouds and
translucent air." The luminary of Herschel's fancy could scarcely be more
clearly portrayed; some added words, however, betray the origin of the
Cardinal's idea. "The earth also," he says, "would appear as a shining star to

any one outside the fiery element." It was, in fact, an extension to the sun of
the ancient elemental doctrine; but an extension remarkable at that period,
as premonitory of the tendency, so powerfully developed by subsequent
discoveries, to assimilate the orbs of heaven to the model of our
insignificant planet, and to extend the brotherhood of our system and our
species to the farthest limit of the visible or imaginable universe.

In later times we find Flamsteed communicating to Newton, March 7, 1681,
his opinion "that the substance of the sun is terrestrial matter, his light but
the liquid menstruum encompassing him."[145] Bode in 1776 arrived
independently at the conclusion that "the sun is neither burning nor
glowing, but in its essence a dark planetary body, composed like our earth
of land and water, varied by mountains and valleys, and enveloped in a
vaporous atmosphere";[146] and the learned in general applauded and
acquiesced. The view, however, was in 1787 still so far from popular, that
the holding of it was alleged as a proof of insanity in Dr. Elliot when
accused of a murderous assault on Miss Boydell. His friend Dr. Simmons
stated on his behalf that he had received from him in the preceding January
a letter giving evidence of a deranged mind, wherein he asserted "that the
sun is not a body of fire, as hath been hitherto supposed, but that its light
proceeds from a dense and universal aurora, which may afford ample light
to the inhabitants of the surface beneath, and yet be at such a distance aloft
as not to annoy them. No objection, he saith, ariseth to that great luminary's
being inhabited; vegetation may obtain there as well as with us. There may
be water and dry land, hills and dales, rain and fair weather; and as the
light, so the season must be eternal, consequently it may easily be
conceived to be by far the most blissful habitation of the whole system!"
The Recorder, nevertheless, objected that if an extravagant hypothesis were
to be adduced as proof of insanity, the same might hold good with regard to
some other speculators, and desired Dr. Simmons to tell the court what he
thought of the theories of Burnet and Buffon.[147]

Eight years later, this same "extravagant hypothesis," backed by the
powerful recommendation of Sir William Herschel, obtained admittance to
the venerable halls of science, there to abide undisturbed for nearly seven
decades. Individual objectors, it is true, made themselves heard, but their
arguments had little effect on the general body of opinion. Ruder blows
were required to shatter an hypothesis flattering to human pride of invention
in its completeness, in the plausible detail of observations by which it
seemed to be supported, and in its condescension to the natural pleasure in
discovering resemblance under all but total dissimilarity.

Sir John Herschel included among the results of his multifarious labours at
the Cape of Good Hope a careful study of the sun-spots conspicuously
visible towards the end of the year 1836 and in the early part of 1837. They
were remarkable, he tells us, for their forms and arrangement, as well as for
their number and size; one group, measured on the 29th of March in the
latter year, covering (apart from what may be called its outlying
dependencies) the vast area of five square minutes or 3,780 million square
miles.[148] We have at present to consider, however, not so much these
observations in themselves, as the chain of theoretical suggestions by which
they were connected. The distribution of spots, it was pointed out, on two
zones parallel to the equator, showed plainly their intimate connection with
the solar rotation, and indicated as their cause fluid circulations analogous
to those producing the terrestrial trade and anti-trade winds.

"The spots, in this view of the subject," he went on to say,[149] "would come
to be assimilated to those regions on the earth's surface where, for the
moment, hurricanes and tornadoes prevail; the upper stratum being
temporarily carried downwards, displacing by its impetus the two strata of
luminous matter beneath, the upper of course to a greater extent than the
lower, and thus wholly or partially denuding the opaque surface of the sun
below. Such processes cannot be unaccompanied by vorticose motions,
which, left to themselves, die away by degrees and dissipate, with the
peculiarity that their lower portions come to rest more speedily than their

upper, by reason of the greater resistance below, as well as the remoteness
from the point of action, which lies in a higher region, so that their centres
(as seen in our waterspouts, which are nothing but small tornadoes) appear
to retreat upwards. Now this agrees perfectly with what is observed during
the obliteration of the solar spots, which appear as if filled in by the
collapse of their sides, the penumbra closing in upon the spot and
disappearing after it."

But when it comes to be asked whether a cause can be found by which a
diversity of solar temperature might be produced corresponding with that
which sets the currents of the terrestrial atmosphere in motion, we are
forced to reply that we know of no such cause. For Sir John Herschel's
hypothesis of an increased retention of heat at the sun's equator, due to the
slightly spheroidal or bulging form of its outer atmospheric envelope,
assuredly gives no sufficient account of such circulatory movements as he
supposed to exist. Nevertheless, the view that the sun's rotation is intimately
connected with the formation of spots is so obviously correct, that we can
only wonder it was not thought of sooner, while we are even now unable to
explain with any certainty how it is so connected.

Mere scrutiny of the solar surface, however, is not the only means of solar
observation. We have a satellite, and that satellite from time to time acts
most opportunely as a screen, cutting off a part or the whole of those
dazzling rays in which the master-orb of our system veils himself from
over-curious regards. The importance of eclipses to the study of the solar
surroundings is of comparatively recent recognition; nevertheless, much of
what we know concerning them has been snatched, as it were, by surprise
under favour of the moon. In former times, the sole astronomical use of
such incidents was the correction of the received theories of the solar and
lunar movements; the precise time of their occurrence was the main fact to
be noted, and subsidiary phenomena received but casual attention. Now,
their significance as a geometrical test of tabular accuracy is altogether
overshadowed by the interest attaching to the physical observations for
which they afford propitious occasions. This change may be said to date, in

its pronounced form, from the great eclipse of 1842. Although a necessary
consequence of the general direction taken by scientific progress, it remains
associated in a special manner with the name of Francis Baily.

The "philosopher of Newbury" was by profession a London stockbroker,
and a highly successful one. Nevertheless, his services to science were
numerous and invaluable, though not of the brilliant kind which attract
popular notice. Born at Newbury in Berkshire, April 28, 1774, and placed in
the City at the age of fourteen, he derived from the acquaintance of Dr.
Priestley a love of science which never afterwards left him. It was, however,
no passion such as flames up in the brain of the destined discoverer, but a
regulated inclination, kept well within the bounds of an actively pursued
commercial career. After travelling for a year or two in what were then the
wilds of North America, he went on the Stock Exchange in 1799, and
earned during twenty-four years of assiduous application to affairs a high
reputation for integrity and ability, to which corresponded an ample fortune.
In the meantime the Astronomical Society (largely through his co-
operation) had been founded; he had for three years acted as its secretary,
and he now felt entitled to devote himself exclusively to a subject which
had long occupied his leisure hours. He accordingly in 1825 retired from
business, purchased a house in Tavistock Place, and fitted up there a small
observatory. He was, however, by preference a computator rather than an
observer. What Sir John Herschel calls the "archæology of practical
astronomy" found in him an especially zealous student. He re-edited the
star-catalogues of Ptolemy, Ulugh Beigh, Tycho Brahe, Hevelius, Halley,
Flamsteed, Lacaille, and Mayer; calculated the eclipse of Thales and the
eclipse of Agathocles, and vindicated the memory of the first Astronomer
Royal. But he was no less active in meeting present needs than in revising
past performances. The subject of the reduction of observations, then, as we
have already explained,[150] in a state of deplorable confusion, attracted his
most earnest attention, and he was close on the track of Bessel when made
acquainted with the method of simplification devised at Königsberg.
Anticipated as an inventor, he could still be of eminent use as a promoter of

these valuable improvements; and, carrying them out on a large scale in the
star-catalogue of the Astronomical Society (published in 1827), "he put" (in
the words of Herschel) "the astronomical world in possession of a power
which may be said, without exaggeration, to have changed the face of
sidereal astronomy."[151]

His reputation was still further enhanced by his renewal, with vastly
improved apparatus, of the method, first used by Henry Cavendish in 1797-
98, for determining the density of the earth. From a series of no less than
2,153 delicate and difficult experiments, conducted at Tavistock Place
during the years 1838-42, he concluded our planet to weigh 5·66 as much
as a globe of water of the same bulk; and this result slightly corrected is still
accepted as a very close approximation of the truth.

What we have thus glanced at is but a fragment of the truly surprising mass
of work accomplished by Baily in the course of a variously occupied life. A
rare combination of qualities fitted him for his task. Unvarying health,
undisturbed equanimity, methodical habits, the power of directed and
sustained thought, combined to form in him an intellectual toiler of the
surest, though not perhaps of the highest quality. He was in harness almost
to the end. He was destined scarcely to know the miseries of enforced
idleness or of consciously failing powers. In 1842 he completed the
laborious reduction of Lalande's great catalogue, undertaken at the request
of the British Association, and was still engaged in seeing it through the
press when he was attacked with what proved his last, as it was probably his
first serious illness. He, however, recovered sufficiently to attend the
Oxford Commemoration of July 2, 1844, where an honorary degree of
D.C.L. was conferred upon him in company with Airy and Struve; but sank
rapidly after the effort, and died on the 30th of August following, at the age
of seventy, lamented and esteemed by all who knew him.

It is now time to consider his share in the promotion of solar research.
Eclipses of the sun, both ancient and modern, were a speciality with him,
and he was fortunate in those which came under his observation. Such

phenomena are of three kinds—partial, annular, and total. In a partial
eclipse, the moon, instead of passing directly between us and the sun, slips
by, as it were, a little on one side, thus cutting off from our sight only a
portion of his surface. An annular eclipse, on the other hand, takes place
when the moon is indeed centrally interposed, but falls short of the apparent
size required for the entire concealment of the solar disc, which
consequently remains visible as a bright ring or annulus, even when the
obscuration is at its height. In a total eclipse, on the contrary, the sun
completely disappears behind the dark body of the moon. The difference of
the two latter varieties is due to the fact that the apparent diameter of the
sun and moon are so nearly equal as to gain alternate preponderance one
over the other through the slight periodical changes in their respective
distances from the earth.

Now, on the 15th of May, 1836, an annular eclipse was visible in the
northern parts of Great Britain, and was observed by Baily at Inch Bonney,
near Jedburgh. It was here that he saw the phenomenon which obtained the
name of "Baily's Beads," from the notoriety conferred upon it by his vivid
description.

"When the cusps of the sun," he writes, "were about 40° asunder, a row of
lucid points, like a string of bright beads, irregular in size and distance from
each other, suddenly formed round that part of the circumference of the
moon that was about to enter on the sun's disc. Its formation, indeed, was so
rapid that it presented the appearance of having been caused by the ignition
of a fine train of gunpowder. Finally, as the moon pursued her course, the
dark intervening spaces (which, at their origin, had the appearance of lunar
mountains in high relief, and which still continued attached to the sun's
border) were stretched out into long, black, thick, parallel lines, joining the
limbs of the sun and moon; when all at once they suddenly gave way, and
left the circumference of the sun and moon in those points, as in the rest,
comparatively smooth and circular, and the moon perceptibly advanced on
the face of the sun."[152]

These curious appearances were not an absolute novelty. Weber in 1791,
and Von Zach in 1820, had seen the "beads"; Van Swinden had described
the "belts" or "threads."[153] These last were, moreover (as Baily clearly
perceived), completely analogous to the "black ligament" which formed so
troublesome a feature in the transits of Venus in 1764 and 1769, and which,
to the regret and confusion, though no longer to the surprise of observers,
was renewed in that of 1874. The phenomenon is largely an effect of what
is called irradiation, by which a bright object seems to encroach upon a
dark one; but under good atmospheric and instrumental conditions it
becomes inconspicuous. The "Beads" must always appear when the
projected lunar edge is serrated with mountains. In Baily's observation, they
were exaggerated and distorted by an irradiative clinging together of the
limbs of sun and moon.

The immediate result, however, was powerfully to stimulate attention to
solar eclipses in their physical aspect. Never before had an occurrence of
the kind been expected so eagerly or prepared for so actively as that which
was total over Central and Southern Europe on the 8th of July, 1842.
Astronomers hastened from all quarters to the favoured region. The
Astronomer Royal (Airy) repaired to Turin; Baily to Pavia; Otto Struve
threw aside his work amidst the stars at Pulkowa, and went south as far as
Lipeszk; Schumacher travelled from Altona to Vienna; Arago from Paris to
Perpignan. Nor did their trouble go unrewarded. The expectations of the
most sanguine were outdone by the wonders disclosed.

Baily (to whose narrative we again have recourse) had set up his Dollond's
achromatic in an upper room of the University of Pavia, and was eagerly
engaged in noting a partial repetition of the singular appearances seen by
him in 1836, when he was "astounded by a tremendous burst of applause
from the streets below, and at the same moment was electrified at the sight
of one of the most brilliant and splendid phenomena that can well be
imagined. For at that instant the dark body of the moon was suddenly
surrounded with a corona, or kind of bright glory similar in shape and
relative magnitude to that which painters draw round the heads of saints,

and which by the French is designated an auréole. Pavia contains many
thousand inhabitants, the major part of whom were, at this early hour,
walking about the streets and squares or looking out of windows, in order to
witness this long-talked-of phenomenon; and when the total obscuration
took place, which was instantaneous, there was a universal shout from
every observer, which 'made the welkin ring,' and, for the moment,
withdrew my attention from the object with which I was immediately
occupied. I had indeed anticipated the appearance of a luminous circle
round the moon during the time of total obscurity; but I did not expect, from
any of the accounts of preceding eclipses that I had read, to witness so
magnificent an exhibition as that which took place.... The breadth of the
corona, measured from the circumference of the moon, appeared to me to
be nearly equal to half the moon's diameter. It had the appearance of
brilliant rays. The light was most dense close to the border of the moon, and
became gradually and uniformly more attenuate as its distance therefrom
increased, assuming the form of diverging rays in a rectilinear line, which at
the extremity were more divided, and of an unequal length; so that in no
part of the corona could I discover the regular and well-defined shape of a
ring at its outer margin. It appeared to me to have the sun for its centre, but
I had no means of taking any accurate measures for determining this point.
Its colour was quite white, not pearl-colour, nor yellow, nor red, and the
rays had a vivid and flickering appearance, somewhat like that which a
gaslight illumination might be supposed to assume if formed into a similar
shape.... Splendid and astonishing, however, as this remarkable
phenomenon really was, and although it could not fail to call forth the
admiration and applause of every beholder, yet I must confess that there
was at the same time something in its singular and wonderful appearance
that was appalling; and I can readily imagine that uncivilised nations may
occasionally have become alarmed and terrified at such an object, more
especially at times when the true cause of the occurrence may have been but
faintly understood, and the phenomenon itself wholly unexpected.

"But the most remarkable circumstance attending the phenomenon was the
appearance of three large protuberances apparently emanating from the
circumference of the moon, but evidently forming a portion of the corona.
They had the appearance of mountains of a prodigious elevation; their
colour was red, tinged with lilac or purple; perhaps the colour of the peach-
blossom would more nearly represent it. They somewhat resembled the
snowy tops of the Alpine mountains when coloured by the rising or setting
sun. They resembled the Alpine mountains also in another respect,
inasmuch as their light was perfectly steady, and had none of that flickering
or sparkling motion so visible in other parts of the corona. All the three
projections were of the same roseate cast of colour, and very different from
the brilliant vivid white light that formed the corona; but they differed from
each other in magnitude.... The whole of these three protuberances were
visible even to the last moment of total obscuration; at least, I never lost
sight of them when looking in that direction; and when the first ray of light
was admitted from the sun, they vanished, with the corona, altogether, and
daylight was instantaneously restored."[154]

Notwithstanding unfavourable weather, the "red flames" were perceived
with little less clearness and no less amazement from the Superga than at
Pavia, and were even discerned by Mr. Airy with the naked eye. "Their
form" (the Astronomer Royal wrote) "was nearly that of saw-teeth in the
position proper for a circular saw turned round in the same direction in
which the hands of a watch turn.... Their colour was a full lake-red, and
their brilliancy greater than that of any other part of the ring."[155]

The height of these extraordinary objects was estimated by Arago at two
minutes of arc, representing, at the sun's distance, an actual elevation of
54,000 miles. When carefully watched, the rose-flush of their illumination
was perceived to fade through violet to white as the light returned, the same
changes in a reversed order having accompanied their first appearance.
Their forms, however, during about three minutes of visibility, showed no
change, although of so apparently unstable a character as to suggest to

Arago "mountains on the point of crumbling into ruins" through
topheaviness.[156]

The corona, both as to figure and extent, presented very different
appearances at different stations. This was no doubt due to varieties in
atmospheric conditions. At the Superga, for instance, all details of structure
seem to have been effaced by the murky air, only a comparatively feeble
ring of light being seen to encircle the moon. Elsewhere, a brilliant radiated
formation was conspicuous, spreading at four opposite points into four vast
luminous expansions, compared to feather-plumes or aigrettes.[157] Arago at
Perpignan noticed considerable irregularities in the divergent rays. Some
appeared curved and twisted, a few lay across the others, in a direction
almost tangential to the moon's limb, the general effect being described as
that of a "hank of thread in disorder."[158] At Lipeszk, where the sun stood
much higher above the horizon than in Italy or France, the corona showed
with surprising splendour. Its apparent extent was judged by Struve to be no
less than twenty-five minutes (more than six times Airy's estimate), while
the great plumes spread their radiance to three or four degrees from the dark
lunar edge. So dazzling was the light that many well-instructed persons
denied the totality of the eclipse. Nor was the error without precedent,
although the appearances attending respectively a total and an annular
eclipse are in reality wholly dissimilar. In the latter case, the surviving ring
of sunlight becomes so much enlarged by irradiation, that the interposed
dark lunar body is reduced to comparative insignificance, or even
invisibility. Maclaurin tells us[159] that during an eclipse of this character
which he observed at Edinburgh in 1737, "gentlemen by no means
shortsighted declared themselves unable to discern the moon upon the sun
without the aid of a smoked glass;" and Baily (who, however, was
shortsighted) could distinguish, in 1836, with the naked eye, no trace of
"the globe of purple velvet" which the telescope revealed as projected upon
the face of the sun.[160] Moreover, the diminution of light is described by
him as "little more than might be caused by a temporary cloud passing over

the sun"; the birds continued in full song, and "one cock in particular was
crowing with all his might while the annulus was forming."

Very different were the effects of the eclipse of 1842, as to which some
interesting particulars were collected by Arago.[161] Beasts of burthen, he
tells us, paused in their labour, and could by no amount of punishment be
induced to move until the sun reappeared. Birds and beasts abandoned their
food; linnets were found dead in their cages; even ants suspended their toil.
Diligence-horses, on the other hand, seemed as insensible to the
phenomenon as locomotives. The convolvulus and some other plants closed
their leaves, but those of the mimosa remained open. The little light that
remained was of a livid hue. One observer described the general coloration
as resembling the lees of wine, but human faces showed pale olive or
greenish. We may, then, rest assured that none of the remarkable
obscurations recorded in history were due to eclipses of the annular kind.

The existence of the corona is no modern discovery. Indeed, it is too
conspicuous an apparition to escape notice from the least attentive or least
practised observer of a total eclipse. Nevertheless, explicit references to it
are rare in early times. Plutarch, however, speaks of a "certain splendour"
compassing round the hidden edge of the sun, as a regular feature of total
eclipses;[162] and the corona is expressly mentioned in a description of an
eclipse visible at Corfu in 968 A.D.[163] The first to take the phenomenon
into scientific consideration was Kepler. He showed, from the orbital
positions at the time of the sun and moon, that an eclipse observed by
Clavius at Rome in 1567 could not have been annular,[164] as the dazzling
coronal radiance visible during the obscuration had caused it to be believed.
Although he himself never witnessed a total eclipse of the sun, he carefully
collected and compared the remarks of those more fortunate, and concluded
that the ring of "flame-like splendour" seen on such occasions was caused
by the reflection of the solar rays from matter condensed in the
neighbourhood either of the sun or moon.[165] To the solar explanation he
gave his own decided preference; but, with one of those curious flashes of

half-prophetic insight characteristic of his genius, declared that "it should
be laid by ready for use, not brought into immediate requisition."[166] So
literally was his advice acted upon, that the theory, which we now know to
be (broadly speaking) the correct one, only emerged from the repository of
anticipated truths after 236 years of almost complete retirement, and even
then timorously and with hesitation.

The first eclipse of which the attendant phenomena were observed with
tolerable exactness was that which was central in the South of France, May
12, 1706. Cassini then put forward the view that the "crown of pale light"
seen round the lunar disc was caused by the illumination of the zodiacal
light;[167] but it failed to receive the attention which, as a step in the right
direction, it undoubtedly merited. Nine years later we meet with Halley's
comments on a similar event, the first which had occurred in London since
March 20, 1140. By nine in the morning of May 3, 1715, the obscuration,
he tells us, "was about ten digits,[168] when the face and colour of the sky
began to change from perfect serene azure blue to a more dusky livid
colour, having an eye of purple intermixt.... A few seconds before the sun
was all hid, there discovered itself round the moon a luminous ring, about a
digit or perhaps a tenth part of the moon's diameter in breadth. It was of a
pale whiteness, or rather pearl colour, seeming to be a little tinged with the
colours of the iris, and to be concentric with the moon, whence I concluded
it the moon's atmosphere. But the great height thereof, far exceeding our
earth's atmosphere, and the observation of some, who found the breadth of
the ring to increase on the west side of the moon as emersion approached,
together with the contrary sentiments of those whose judgment I shall
always revere" (Newton is most probably referred to), "makes me less
confident, especially in a matter whereto I confess I gave not all the
attention requisite." He concludes by declining to decide whether the
"enlightened atmosphere," which the appearance "in all respects
resembled," "belonged to sun or moon."[169]

A French Academician, who happened to be in London at the time, was less
guarded in expressing an opinion. The Chevalier de Louville declared
emphatically for the lunar atmospheric theory of the corona,[170] and his
authority carried great weight. It was, however, much discredited by an
observation made by Maraldi in 1724, to the effect that the luminous ring,
instead of travelling with the moon, was traversed by it.[171] This was in
reality decisive, though, as usual, belief lagged far behind demonstration. In
1715 a novel explanation had been offered by Delisle and Lahire,[172]
supported by experiments regarded at the time as perfectly satisfactory. The
aureola round the eclipsed sun, they argued, is simply a result of the
diffraction, or apparent bending of the sunbeams that graze the surface of
the lunar globe—an effect of the same kind as the coloured fringes of
shadows. And this view prevailed amongst men of science until (and even
after) Brewster showed, with clear and simple decisiveness, that such an
effect could by no possibility be appreciable at our distance from the moon.
[173] Don José Joaquim de Ferrer, however, who observed a total eclipse of
the sun at Kinderhook, in the State of New York, on June 16, 1806, ignoring
this refined optical rationale, considered two alternative explanations of the
phenomenon as alone possible. The bright ring round the moon must be due
to the illumination either of a lunar or of a solar atmosphere. If the former,
he calculated that it should have a height fifty times that of the earth's
gaseous envelope. "Such an atmosphere," he rightly concluded, "cannot
belong to the moon, but must without any doubt belong to the sun."[174] But
he stood alone in this unhesitating assertion.

The importance of the problem was first brought fully home to astronomers
by the eclipse of 1842. The brilliant and complex appearance which on that
occasion challenged the attention of so many observers, demanded and
received, no longer the casual attention hitherto bestowed upon it, but the
most earnest study of those interested in the progress of science.
Nevertheless, it was only by degrees, and through a process of "exclusions"
(to use a Baconian phrase) that the corona was put in its right place as a
solar appendage. As every other available explanation proved inadmissible

and dropped out of sight, the broad presentation of fact remained, which,
though of sufficiently obvious interpretation, was long and persistently
misconstrued. Nor was it until 1869 that absolutely decisive evidence on the
subject was forthcoming, as we shall see further on.

Sir John Herschel, writing to his venerable aunt, relates that when the
brilliant red flames burst into view behind the dark moon on the morning of
the 8th of July, 1842, the populace of Milan, with the usual inconsequence
of a crowd, raised the shout, "Es leben die Astronomen!"[175] In reality, none
were less prepared for their apparition than the class to whom the applause
due to the magnificent spectacle was thus adjudged. And in some measure
through their own fault, for many partial hints and some distinct statements
from earlier observers had given unheeded notice that some such
phenomenon might be expected to attend a solar eclipse.

What we now call the "chromosphere" is an envelope of glowing gases, by
which the sun is completely covered, and from which the "prominences" are
emanations, eruptive or flame-like. Now, continual indications of the
presence of this fire-ocean had been detected during eclipses in the
eighteenth and nineteenth centuries. Captain Stannyan, describing in a letter
to Flamsteed an occurrence of the kind witnessed by him at Berne on May 1
(o.s.), 1706, says that the sun's "getting out of the eclipse was preceded by a
blood-red streak of light from its left limb."[176] A precisely similar
appearance was noted by both Halley and De Louville in 1715; during
annular eclipses by Lord Aberdour in 1737,[177] and by Short in 1748,[178]
the tint of the ruby border being, however, subdued to "brown" or "dusky
red" by the surviving sunlight; while observations identical in character
were made at Amsterdam in 1820,[179] at Edinburgh by Henderson in 1836,
and at New York in 1838.[180]

"Flames" or "prominences," if more conspicuous, are less constant in their
presence than the glowing stratum from which they spring. The first to
describe them was a Swedish professor named Vassenius, who observed a
total eclipse at Gothenburg, May 2 (o.s.), 1733.[181] His astonishment

equalled his admiration when he perceived, just outside the edge of the
lunar disc, and suspended, as it seemed, in the coronal atmosphere, three or
four reddish spots or clouds, one of which was so large as to be detected
with the naked eye. As to their nature, he did not even offer a speculation,
further than by tacitly referring them to the moon. The observation was
repeated in 1778 by a Spanish Admiral, but with no better success in
directing efficacious attention to the phenomenon. Don Antonio Ulloa was
on board his ship the Espagne in passage from the Azores to Cape St.
Vincent on the 24th of June in that year, when a total eclipse of the sun
occurred, of which he has left a valuable description. His notices of the
corona are full of interest; but what just now concerns us is the appearance
of "a red luminous point" "near the edge of the moon," which gradually
increased in size as the moon moved away from it, and was visible during
about a minute and a quarter.[182] He was satisfied that it belonged to the
sun because of its fiery colour and growth in magnitude, and supposed that
it was occasioned by some crevice or inequality in the moon's limb, through
which the solar light penetrated.

Allusions less precise, both prior and subsequent, which it is now easy to
refer to similar objects (such as the "slender columns of smoke" seen by
Ferrer)[183] might be detailed; but the evidence already adduced suffices to
show that the prominences viewed with such amazement in 1842 were no
unprecedented or even unusual phenomenon.

It was more important, however, to decide what was their nature than
whether their appearance might have been anticipated. They were generally,
and not very incorrectly, set down as solar clouds. Arago believed them to
shine by reflected light,[184] but the Abbé Peytal rightly considered them to
be self-luminous. Writing in a Montpellier paper of July 16, 1842, he
declared that we had now become assured of the existence of a third or
outer solar envelope, composed of a glowing substance of a bright rose tint,
forming mountains of prodigious elevation, analogous in character to the
clouds piled above our horizons.[185] This first distinct recognition of a very

important feature of our great luminary was probably founded on an
observation made by Bérard at Toulon during the then recent eclipse, "of a
very fine red band, irregularly dentelated, or, as it were, crevassed here and
there,"[186] encircling a large arc of the moon's circumference. It can hardly,
however, be said to have attracted general notice until July 28, 1851. On
that day a total eclipse took place, which was observed with considerable
success in various parts of Sweden and Norway by a number of English
astronomers. Mr. Hind saw, on the south limb of the moon, "a long range of
rose-coloured flames,"[187] described by Dawes as "a low ridge of red
prominences, resembling in outline the tops of a very irregular range of
hills."[188] Airy termed the portion of this "rugged lines of projections"
visible to him the sierra, and was struck with its brilliant light and "nearly
scarlet" colour.[189] Its true character of a continuous solar envelope was
inferred from these data by Grant, Swan, and Littrow, and was by Father
Secchi, after the great eclipse of 1860,[190] formally accepted as established.

Several prominences of remarkable forms, especially one variously
compared to a Turkish scimitar, a sickle, and a boomerang, were seen in
1851. In connection with them two highly significant circumstances were
pointed out. First, that of the approximate coincidence between their
positions and those of sun-spots previously observed.[191] Next, that "the
moon passed over them, leaving them behind, and revealing successive
portions as she advanced."[192] This latter perfectly well-attested fact was
justly considered by the Astronomer Royal and others as affording absolute
certainty of the solar dependence of these singular objects. Nevertheless
sceptics were still found. M. Faye, of the French Academy, inclined to a
lunar origin for them;[193] Feilitsch of Greifswald published in 1852 a
treatise for the express purpose of proving all the luminous phenomena
attendant on solar eclipses—corona, prominences and "sierra"—to be
purely optical appearances.[194] Happily, however, the unanswerable
arguments of the photographic camera were soon to be made available
against such hardy incredulity.

Thus, the virtual discovery of the solar appendages, both coronal and
chromospheric, may be said to have been begun in 1842, and completed in
1851. The current Herschelian theory of the solar constitution remained,
however, for the time, intact. Difficulties, indeed, were thickening around it;
but their discussion was perhaps felt to be premature, and they were
permitted to accumulate without debate, until fortified by fresh testimony
into unexpected and overwhelming preponderance.

FOOTNOTES:
[131] Kosmos, Bd. iii., p. 409; Lalande, Bibliographie Astronomique, pp. 179,
202.

[132] R. Wolf, Die Sonne und ihre Flecken, p. 9. Marius himself, however,
seems to have held the Aristotelian terrestrial-exhalation theory of cometary
origin. See his curious little tract, Astronomische und Astrologische
Beschreibung der Cometen, Nürnberg, 1619.

[133] Phil. Trans., vol. xxvii., p. 274. Umbræ (now called penumbræ) are spaces
of half-shadow which usually encircle spots. Faculæ ("little torches," so named
by Scheiner) are bright streaks or patches closely associated with spots.

[134] Mém. Ac. Sc., 1776 (pub. 1779), p. 507. D. Cassini, however, first put
forward about 1671 the hypothesis alluded to in the text. See Delambre, Hist. de
l'Astr. Mod., t. ii., p. 694; and Kosmos, Bd. iii., p. 410.

[135] Phil. Trans., vol. lxiv., part i., pp. 7-11.

[136] Rosa Ursina, lib. iv., p. 507.

[137] R. Wolf, Die Sonne und ihre Flecken, p. 12.

[138] Schellen, Die Spectralanalyse, Bd. ii., p. 56 (3rd ed.).

[139] Phil. Trans., vol. lxiv., p. 20.

[140] Ibid., vol. lxxxv., 1795, p. 63.

[141] Phil. Trans., vol. xci., 1801, p. 303.

[142] The supposed opaque or protective stratum beneath the photosphere was
named by him "planetary," from the analogy of terrestrial clouds.

[143] Ibid., p. 305.

[144] Novum Organum, lib. ii. aph. 20.

[145] Brewster's Life of Newton, vol. ii., p. 103.

[146] Beschäftigungen d. Berl. Ges. Naturforschender Freunde, Bd. ii., p. 233.

[147] Gentleman's Magazine, 1787, vol. ii., p. 636.

[148] Results, etc., p. 432.

[149] Ibid., p. 434.

[150] See ante, p. 31.

[151] Memoir of Francis Baily, Mem. R. A. S., vol. xv., p. 524.

[152] Mem. R. A. S., vol. x., pp. 5-6.

[153] Ibid., pp. 14-17.

[154] Mem. R. A. S., vol. xv., pp. 4-6.

[155] Ibid., p. 16.

[156] Annuaire, 1846, p. 409.

[157] Ibid., p. 317.

[158] Ibid., p. 322.

[159] Phil. Trans., vol. xl., p. 192.

[160] Mem. R. A. S., vol. x., p. 17.

[161] Ann. du Bureau des Long., 1846, p. 309.

[162] De Facie in Orbe Lunæ, xix., 10. Cf. Grant, Astr. Nach., No. 1838. As to
the phenomenon mentioned by Philostratus in his Life of Apollonius (viii. 23),
see W. T. Lynn, Observatory, vol. ix., p. 128.

[163] Schmidt, Astr. Nach., No. 1832.

[164] Astronomiæ Pars Optica, Op. omnia, t. ii., p. 317.

[165] De Stellâ Novâ, Op., t. ii., pp. 696, 697.

[166] Astr. Pars Op., p. 320.

[167] Mém. de l'Ac. des Sciences, 1706, p. 119.

[168] A digit = 1/12 of the solar diameter.

[169] Phil. Trans., vol. xxix., pp. 247-249.

[170] Mém. de l'Ac. des Sciences, 1715; Histoire, p. 49; Mémoires, pp. 93-98.

[171] Ibid., 1724, p. 178.

[172] Mém. de l'Ac. des Sciences, 1715, pp. 161, 166-169.

[173] Ed. Ency., art. Astronomy, p. 635.

[174] Trans. Am. Phil. Soc., vol. vi., p. 274.

[175] Memoir of Caroline Herschel, p. 327.

[176] Phil. Trans., vol. xxv., p. 2240.

[177] Ibid., vol. xl., p. 182.

[178] Ibid., vol. xlv., p. 586.

[179] Mem. R. A. S., vol. i., pp. 145, 148.

[180] American Journal of Science, vol. xlii., p. 396.

[181] Phil. Trans., vol. xxxviii., p. 134. Father Secchi, however, adverted to a
distinct mention of a prominence observed in 1239 A.D. A description of a total
eclipse of that date includes the remark, "Et quoddam foramen erat ignitum in
circulo solis ex parte inferiore" (Muratori, Rer. It. Scriptores, t. xiv., col. 1097).
The "circulus solis" of course signifies the corona.

[182] Phil. Trans., vol. lxix., p. 114.

[183] Trans. Am. Phil. Soc., vol. vi., 1809, p. 267.

[184] Annuaire, 1846, p. 460.

[185] Ibid., p. 439, note.

[186] Ibid., p. 416.

[187] Mem. R. A. S., vol. xxi., p. 82.

[188] Ibid., p. 90.

[189] Ibid., pp. 7, 8.

[190] Le Soleil, t. i., p. 386.

[191] By Williams and Stanistreet, Mem. R. A. S., vol. xxi., pp. 54, 56. Santini
had made a similar observation at Padua in 1842. Grant, Hist. Astr., p. 401.

[192] Lassell in Month. Not., vol. xii., p. 53.

[193] Comptes Rendus, t. xxxiv., p. 155.

[194] Optische Untersuchungen, and Zeitschrift für populäre Mittheilungen, Bd.
i., 1860, p. 201.

CHAPTER IV
PLANETARY DISCOVERIES

In the course of his early gropings towards a law of the planetary distances,
Kepler tried the experiment of setting a planet, invisible by reason of its
smallness, to revolve in the vast region of seemingly desert space separating
Mars from Jupiter.[195] The disproportionate magnitude of the same interval
was explained by Kant as due to the overweening size of Jupiter. The zone
in which each planet moved was, according to the philosopher of
Königsberg, to be regarded as the empty storehouse from which its
materials had been derived. A definite relation should thus exist between the
planetary masses and the planetary intervals.[196] Lambert, on the other
hand, sportively suggested that the body or bodies (for it is noticeable that
he speaks of them in the plural) which once bridged this portentous gap in
the solar system, might, in some remote age, have been swept away by a
great comet, and forced to attend its wanderings through space.[197]

These speculations were destined before long to assume a more definite
form. Johann Daniel Titius, a professor at Wittenberg (where he died in
1796), pointed out in 1772, in a note to a translation of Bonnet's
Contemplation de la Nature,[198] the existence of a remarkable symmetry in
the disposition of the bodies constituting the solar system. By a certain
series of numbers, increasing in regular progression,[199] he showed that the
distances of the six known planets from the sun might be represented with a
close approach to accuracy. But with one striking interruption. The term of
the series succeeding that which corresponded to the orbit of Mars was
without a celestial representative. The orderly flow of the sequence was
thus singularly broken. The space where a planet should—in fulfilment of
the "Law"—have revolved, was, it appeared, untenanted. Johann Elert
Bode, then just about to begin his long career as leader of astronomical

thought and work at Berlin, marked at once the anomaly, and filled the
vacant interval with a hypothetical planet. The discovery of Uranus, at a
distance falling but slightly short of perfect conformity with the law of
Titius, lent weight to a seemingly hazardous prediction, and Von Zach was
actually at the pains, in 1785, to calculate what he termed "analogical"
elements[200] for this unseen and (by any effect or influence) unfelt body.
The search for it, through confessedly scarcely less chimerical than that of
alchemists for the philosopher's stone, he kept steadily in view for fifteen
years, and at length (September 21, 1800) succeeded in organising, in
combination with five other German astronomers assembled at Lilienthal, a
force of what he jocularly termed celestial police, for the express purpose of
tracking and intercepting the fugitive subject of the sun. The zodiac was
accordingly divided for purposes of scrutiny into twenty-four zones; their
apportionment to separate observers was in part effected, and the
association was rapidly getting into working order, when news arrived that
the missing planet had been found, through no systematic plan of search,
but by the diligent, though otherwise directed labours of a distant watcher
of the skies.

Giuseppe Piazzi was born at Ponte in the Valtelline, July 16, 1746. He
studied at various places and times under Tiraboschi, Beccaria, Jacquier,
and Le Sueur; and having entered the Theatine order of monks at the age of
eighteen, he taught philosophy, science, and theology in several of the
Italian cities, as well as in Malta, until 1780, when the chair of mathematics
in the University of Palermo was offered to and accepted by him. Prince
Caramanico, then viceroy of Sicily, had scientific leanings, and was easily
won over to the project of building an observatory, a commodious
foundation for which was afforded by one of the towers of the viceregal
palace. This architecturally incongruous addition to an ancient Saracenic
edifice—once the abode of Kelbite and Zirite Emirs—was completed in
February, 1791. Piazzi, meanwhile, had devoted nearly three years to the
assiduous study of his new profession, acquiring a practical knowledge of
Lalande's methods at the École Militaire, and of Maskelyne's at the Royal

Observatory; and returned to Palermo in 1789, bringing with him, in the
great five-foot circle which he had prevailed upon Ramsden to construct,
the most perfect measuring instrument hitherto employed by an astronomer.

He had been above nine years at work on his star-catalogue, and was still
profoundly unconscious that a place amongst the Lilienthal band[201] of
astronomical detectives was being held in reserve for him, when, on the
first evening of the nineteenth century, January 1, 1801, he noticed the
position of an eighth-magnitude star in a part of the constellation Taurus to
which an error of Wollaston's had directed his special attention.
Reobserving, according to his custom, the same set of fifty stars on four
consecutive nights, it seemed to him, on the 2nd, that the one in question
had slightly shifted its position to the west; on the 3rd he assured himself of
the fact, and believed that he had chanced upon a new kind of comet
without tail or coma. The wandering body, whatever its nature, exchanged
retrograde for direct motion on January 14,[202] and was carefully watched
by Piazzi until February 11, when a dangerous illness interrupted his
observations. He had, however, not omitted to give notice of his discovery;
but so precarious were communications in those unpeaceful times, that his
letter to Oriani of January 23 did not reach Milan until April 5, while a
missive of one day later addressed to Bode came to hand at Berlin, March
20. The delay just afforded time for the publication, by a young philosopher
of Jena named Hegel, of a "Dissertation" showing, by the clearest light of
reason, that the number of the planets could not exceed seven, and exposing
the folly of certain devotees of induction who sought a new celestial body
merely to fill a gap in a numerical series.[203]

Unabashed by speculative scorn, Bode had scarcely read Piazzi's letter
when he concluded that it referred to the precise body in question. The news
spread rapidly, and created a profound sensation, not unmixed with alarm
lest this latest addition to the solar family should have been found only to
be again lost. For by that time Piazzi's moving star was too near the sun to
be any longer visible, and in order to rediscover it after conjunction a

tolerably accurate knowledge of its path was indispensable. But a planetary
orbit had never before been calculated from such scanty data as Piazzi's
observation afforded;[204] and the attempts made by nearly every
astronomer of note in Germany to compass the problem were manifestly
inadequate, failing even to account for the positions in which the body had
been actually seen, and à fortiori serving only to mislead as to the places
where, from September, 1801, it ought once more to have become
discernible. It was in this extremity that the celebrated mathematician Gauss
came to the rescue. He was then in his twenty-fifth year, and was earning
his bread by tuition at Brunswick, with many possibilities, but no settled
career before him. The news from Palermo may be said to have converted
him from an arithmetician into an astronomer. He was already in possession
of a new and more general method of computing elliptical orbits; and the
system of "least squares," which he had devised though not published,
enabled him to extract the most probable result from a given set of
observations. Armed with these novel powers, he set to work; and the
communication in November of his elements and ephemeris for the lost
object revived the drooping hopes of the little band of eager searchers.
Their patience, however, was to be still further tried. Clouds, mist, and sleet
seemed to have conspired to cover the retreat of the fugitive; but on the last
night of the year the sky cleared unexpectedly with the setting in of a hard
frost, and there, in the north-western part of Virgo, nearly in the position
assigned by Gauss to the runaway planet, a strange star was discerned by
Von Zach[205] at Gotha, and on a subsequent evening—the anniversary of
the original discovery—by Olbers at Bremen. The name of Ceres (as the
tutelary goddess of Sicily) was, by Piazzi's request, bestowed upon this first
known of the numerous, and probably all but innumerable family of the
minor planets.

The recognition of the second followed as the immediate consequence of
the detection of the first. Olbers had made himself so familiar with the
positions of the small stars along the track of the long-missing body, that he
was at once struck (March 28, 1802) with the presence of an intruder near

the spot where he had recently identified Ceres. He at first believed the
new-comer to be a variable star usually inconspicuous, but just then at its
maximum of brightness; but within two hours he had convinced himself
that it was no fixed star, but a rapidly moving object. The aid of Gauss was
again invoked, and his prompt calculations showed that this fresh celestial
acquaintance (named "Pallas" by Olbers), revolved round the sun at nearly
the same mean distance as Ceres, and was beyond question of a strictly
analogous character.

This result was perplexing in the extreme. The symmetry and simplicity of
the planetary scheme appeared fatally compromised by the admission of
many, where room could, according to old-fashioned rules, only be found
for one. A daring hypothesis of Olbers's invention provided an exit from the
difficulty. He supposed that both Ceres and Pallas were fragments of a
primitive trans-Martian planet, blown to pieces in the remote past, either by
the action of internal forces or by the impact of a comet; and predicted that
many more such fragments would be found to circulate in the same region.
He, moreover, pointed out that these numerous orbits, however much they
might differ in other respects, must all have a common line of intersection,
[206] and that the bodies moving in them must consequently pass, at each
revolution, through two opposite points of the heavens, one situated in the
Whale, the other in the constellation of the Virgin, where already Pallas had
been found and Ceres recaptured. The intimation that fresh discoveries
might be expected in those particular regions was singularly justified by the
detection of two bodies now known respectively as Juno and Vesta. The
first was found near the predicted spot in Cetus by Harding, Schröter's
assistant at Lilienthal, September 2, 1804; the second by Olbers himself in
Virgo, after three years of persistent scrutiny, March 29, 1807.

The theory of an exploded planet now seemed to have everything in its
favour. It required that the mean or average distances of the newly-
discovered bodies should be nearly the same, but admitted a wide range of
variety in the shapes and positions of their orbits, provided always that they
preserved common points of intersection. These conditions were fulfilled

with a striking approach to exactness. Three of the four "asteroids" (a
designation introduced by Sir. W. Herschel[207]) conformed with very
approximate precision to "Bode's law" of distances; they all traversed, in
their circuits round the sun, nearly the same parts of Cetus and Virgo; while
the eccentricities and inclinations of their paths departed widely from the
planetary type—that of Pallas, to take an extreme instance, making with the
ecliptic an angle of nearly 35°. The minuteness of these bodies appeared
further to strengthen the imputation of a fragmentary character. Herschel
estimated the diameter of Ceres at 162, that of Pallas at 147 miles.[208] But
these values are now known to be considerably too small. A suspected
variability of brightness in some of the asteroids, somewhat hazardously
explained as due to the irregularities of figure to be expected in cosmical
potsherds (so to speak), was added to the confirmatory evidence.[209] The
strong point of the theory, however, lay not in what it explained, but in what
it had predicted. It had been twice confirmed by actual exploration of the
skies, and had produced, in the recognition of Vesta, the first recorded
instance of the premeditated discovery of a heavenly body.

The view not only commended itself to the facile imagination of the
unlearned, but received the sanction of the highest scientific authority. The
great Lagrange bestowed upon it his analytical imprimatur, showing that
the explosive forces required to produce the supposed catastrophe came
well within the bounds of possibility; since a velocity of less than twenty
times that of a cannon-ball leaving the gun's mouth would have sufficed,
according to his calculation, to launch the asteroidal fragments on their
respective paths. Indeed, he was disposed to regard the hypothesis of
disruption as more generally available than its author had designed it to be,
and proposed to supplement with it, as explanatory of the eccentric orbits of
comets, the nebular theory of Laplace, thereby obtaining, as he said, "a
complete view of the origin of the planetary system more conformable to
Nature and mechanical laws than any yet proposed."[210]

Nevertheless the hypothesis of Olbers has not held its ground. It seemed as
if all the evidence available for its support had been produced at once and
spontaneously, while the unfavourable items were elicited slowly, and, as it
were, by cross-examination. A more extended acquaintance with the group
of bodies whose peculiarities it was framed to explain has shown them,
after all, as recalcitrant to any such explanation. Coincidences at the first
view significant and striking have been swamped by contrary examples;
and a hasty general conclusion has, by a not uncommon destiny, at last
perished under the accumulation of particulars. Moreover, as has been
remarked by Professor Newcomb,[211] mutual perturbations would rapidly
efface all traces of a common disruptive origin, and the catastrophe, to be
perceptible in its effects, should have been comparatively recent.

A new generation of astronomers had arisen before any additions were
made to the little family of the minor planets. Piazzi died in 1826, Harding
in 1834, Olbers in 1840; all those who had prepared or participated in the
first discoveries passed away without witnessing their resumption. In 1830,
however, a certain Hencke, ex-postmaster in the Prussian town of Driessen,
set himself to watch for new planets, and after fifteen long years his
patience was rewarded. The asteroid found by him, December 8, 1845,
received the name of Astræa, and his further prosecution of the search
resulted, July 1, 1847, in the discovery of Hebe. A few weeks later (August
13), John Russell Hind (1823-1893), after many months' exploration from
Mr. Bishop's observatory in the Regent's Park, picked up Iris, and October
18, Flora.[212] The next on the list was Metis, found by Mr. Graham, April
25, 1848, at Markree, in Ireland.[213] At the close of the period to which our
attention is at present limited, the number of these small bodies known to
astronomy was thirteen; and the course of discovery has since proceeded far
more rapidly and with less interruption.

Both in itself and in its consequences the recognition of the minor planets
was of the highest importance to science. The traditional ideas regarding the
constitution of the solar system were enlarged by the admission of a new

class of bodies, strongly contrasted, yet strictly co-ordinate with the old-
established planetary order; the profusion of resource, so conspicuous in the
living kingdoms of Nature, was seen to prevail no less in the celestial
spaces; and some faint preliminary notion was afforded of the indefinite
complexity of relations underlying the apparent simplicity of the majestic
scheme to which our world belongs. Both theoretical and practical
astronomy derived profit from the admission of these apparently
insignificant strangers to the rights of citizenship of the solar system. The
disturbance of their motions by their giant neighbours afforded a more
accurate knowledge of the Jovian mass, which Laplace had taken about
1/50 too small; the anomalous character of their orbits presented geometers
with highly stimulating problems in the theory of perturbation; while the
exigencies of the first discovery had produced the Theoria Motus, and won
Gauss over to the ranks of calculating astronomy. Moreover, the sure
prospect of further detections powerfully incited to the exploration of the
skies; observers became more numerous and more zealous in view of the
prizes held out to them; star-maps were diligently constructed, and the
sidereal multitude strewn along the great zodiacal belt acquired a fresh
interest when it was perceived that its least conspicuous member might be a
planetary shred or projectile in the dignified disguise of a distant sun.
Harding's "Celestial Atlas," designed for the special purpose of facilitating
asteroidal research, was the first systematic attempt to represent to the eye
the telescopic aspect of the heavens. It was while engaged on its
construction that the Lilienthal observer successfully intercepted Juno on
her passage through the Whale in 1804; whereupon promoted to Göttingen,
he there completed, in 1822, the arduous task so opportunely entered upon a
score of years previously. Still more important were the great star-maps of
the Berlin Academy, undertaken at Bessel's suggestion, with the same
object of distinguishing errant from fixed stars, and executed, under Encke's
supervision, during the years 1830-59. They have played a noteworthy part
in the history of planetary discovery, nor of the minor kind alone.

We have now to recount an event unique in scientific history. The discovery
of Neptune has been characterised as the result of a "movement of the age,"
[214] and with some justice. It had become necessary to the integrity of
planetary theory. Until it was accomplished, the phantom of an unexplained
anomaly in the orderly movements of the solar system must have continued
to haunt astronomical consciousness. Moreover, it was prepared by many,
suggested as possible by not a few, and actually achieved, simultaneously,
independently, and completely, by two investigators.

The position of the planet Uranus was recorded as that of a fixed star no
less than twenty times between 1690 and the epoch of its final detection by
Herschel. But these early observations, far from affording the expected
facilities for the calculation of its orbit, proved a source of grievous
perplexity. The utmost ingenuity of geometers failed to combine them
satisfactorily with the later Uranian places, and it became evident, either
that they were widely erroneous, or that the revolving body was wandering
from its ancient track. The simplest course was to reject them altogether,
and this was done in the new Tables published in 1821 by Alexis Bouvard,
the indefatigable computating partner of Laplace. But the trouble was not
thus to be got rid of. After a few years fresh irregularities began to appear,
and continued to increase until absolutely "intolerable." It may be stated as
illustrative of the perfection to which astronomy had been brought, that
divergencies regarded as menacing the very foundation of its theories never
entered the range of unaided vision. In other words, if the theoretical and
the real Uranus had been placed side by side in the sky, they would have
seemed, to the sharpest eye, to form a single body.[215]

The idea that these enigmatical disturbances were due to the attraction of an
unknown exterior body was a tolerably obvious one; and we accordingly
find it suggested in many different quarters. Bouvard himself was perhaps
the first to conceive it. He kept the possibility continually in view, and
bequeathed to his nephew's diligence the inquiry into its reality when he felt
that his own span was drawing to a close; but before any progress had been

made with it, he had already (June 7, 1843) "ceased to breathe and to
calculate." The Rev. T. J. Hussey actually entertained in 1834 the notion,
but found his powers inadequate to the task, of assigning an approximate
place to the disturbing body; and Bessel, in 1840, laid his plans for an
assault in form upon the Uranian difficulty, the triumphant exit from which
fatal illness frustrated his hopes of effecting or even witnessing.

The problem was practically untouched when, in 1841, an undergraduate of
St. John's College, Cambridge, formed the resolution of grappling with it.
The projected task was an arduous one. There were no guiding precedents
for its conduct. Analytical obstacles had to be encountered so formidable as
to appear invincible even to such a mathematician as Airy. John Couch
Adams, however, had no sooner taken his degree, which he did as senior
wrangler in January, 1843, than he set resolutely to work, and on October
21, 1845, was able to communicate to the Astronomer Royal numerical
estimates of the elements and mass of the unknown planet, together with an
indication of its actual place in the heavens. These results, it has been well
said,[216] gave "the final and inexorable proof" of the validity of Newton's
Law. The date October 21, 1845, "may therefore be regarded as marking a
distinct epoch in the history of gravitational astronomy."

Sir George Biddell Airy had begun in 1835 his long and energetic
administration of the Royal Observatory, and was already in possession of
data vitally important to the momentous inquiry then on foot. At his
suggestion, and under his superintendence, the reduction of all the planetary
observations made at Greenwich from 1750 onwards had been undertaken
in 1833. The results, published in 1846, constituted a permanent and
universal stock of materials for the correction of planetary theory. But in the
meantime, investigators, both native and foreign, were freely supplied with
the "places and errors," which, clearly exhibiting the discrepancies between
observation and calculation—between what was and what was expected—
formed the very groundwork of future improvements.

Mr. Adams had no reason to complain of official discourtesy. His labours
received due and indispensable aid; but their purpose was regarded as
chimerical. "I have always," Sir George Airy wrote,[217] "considered the
correctness of a distant mathematical result to be a subject rather of moral
than of mathematical evidence." And that actually before him seemed, from
its very novelty, to incur a suspicion of unlikelihood. No problem in
planetary disturbance had heretofore been attacked, so to speak, from the
rear. The inverse method was untried, and might well be deemed
impracticable. For the difficulty of determining the perturbations produced
by a given planet is small compared with the difficulty of finding a planet
by its resulting perturbations. Laplace might have quailed before it; yet it
was now grappled with as a first essay in celestial dynamics. Moreover,
Adams unaccountably neglected to answer until too late a question regarded
by Airy in the light of an experimentum crucis as to the soundness of the
new theory. Nor did he himself take any steps to obtain a publicity which he
was more anxious to merit than to secure. The investigation consequently
remained buried in obscurity. It is now known that had a search been
instituted in the autumn of 1845 for the remote body whose existence had
been so marvellously foretold, it would have been found within three and a
half lunar diameters (1° 49′) of the spot assigned to it by Adams.

A competitor, however, equally daring and more fortunate—audax fortunâ
adjutus, as Gauss said of him—was even then entering the field. Urbain
Jean Joseph Leverrier, the son of a small Government employé in
Normandy, was born at Saint-Lô, March 11, 1811. He studied with brilliant
success at the École Polytechnique, accepted the post of astronomical
teacher there in 1837, and, "docile to circumstance," immediately
concentrated the whole of his vast, though as yet undeveloped powers upon
the formidable problems, of celestial mechanics. He lost no time in proving
to the mathematical world that the race of giants was not extinct. Two
papers on the stability of the solar system, presented to the Academy of
Sciences, September 16 and October 14, 1839, showed him to be the
worthy successor of Lagrange and Laplace, and encouraged hopes destined

to be abundantly realised. His attention was directed by Arago to the
Uranian difficulty in 1845, when he cheerfully put aside certain intricate
cometary researches upon which he happened to be engaged, in order to
obey with dutiful promptitude the summons of the astronomical chief of
France. In his first memoir on the subject (communicated to the Academy,
November 10, 1845), he proved the inadequacy of all known causes of
disturbance to account for the vagaries of Uranus; in a second (June 1,
1848), he demonstrated that only an exterior body, occupying at a certain
date a determinate position in the zodiac, could produce the observed
effects; in a third (August 31, 1846), he assigned the orbit of the disturbing
body, and announced its visibility as an object with a sensible disc about as
bright as a star of the eighth magnitude.

The question was now visibly approaching an issue. On September 10, Sir
John Herschel declared to the British Association respecting the
hypothetical new planet: "We see it as Columbus saw America from the
coast of Spain. Its movements have been felt, trembling along the far-
reaching line of our analysis with a certainty hardly inferior to that of ocular
demonstration." Less than a fortnight later, September 23, Professor Galle,
of the Berlin Observatory, received a letter from Leverrier requesting his aid
in the telescopic part of the inquiry already analytically completed. He
directed his refractor to the heavens that same night, and perceived, within
less than a degree of the spot indicated, an object with a measurable disc
nearly three seconds in diameter. Its absence from Bremiker's recently-
completed map of that region of the sky showed it to be no star, and its
movement in the predicted direction confirmed without delay the strong
persuasion of its planetary nature.[218]

In this remarkable manner the existence of the remote member of our
system known as "Neptune" was ascertained. But the discovery, which
faithfully reflected the duplicate character of the investigation which led to
it, had been already secured at Cambridge before it was announced from
Berlin. Sir George Airy's incredulity vanished in the face of the striking
coincidence between the position assigned by Leverrier to the unknown

planet in June, and that laid down by Adams in the previous October; and
on the 9th of July he wrote to Professor Challis, director of the Cambridge
Observatory, recommending a search with the great Northumberland
equatoreal. Had a good star-map been at hand, the process would have been
a simple one; but of Bremiker's "Hora XXI." no news had yet reached
England, and there was no other sufficiently comprehensive to be available
for an inquiry which, in the absence of such aid, promised to be both long
and laborious. As the event proved, it might have been neither. "After four
days of observing," Challis wrote, October 12, 1846, to Airy, "the planet
was in my grasp if only I had examined or mapped the observations."[219]
Had he done so, the first honours in the discovery, both theoretical and
optical, would have fallen to the University of Cambridge. But Professor
Challis had other astronomical avocations to attend to, and, moreover, his
faith in the precision of the indications furnished to him was, by his own
confession, a very feeble one. For both reasons he postponed to a later stage
of the proceedings the discussion and comparison of the data nightly
furnished to him by his telescope, and thus allowed to lie, as it were, latent
in his observations the momentous result which his diligence had insured,
but which his delay suffered to be anticipated.[220]

Nevertheless, it should not be forgotten that the Berlin astronomer had two
circumstances in his favour apart from which his swift success could hardly
have been achieved. The first was the possession of a good star-map; the
second was the clear and confident nature of Leverrier's instructions. "Look
where I tell you," he seemed authoritatively to say, "and you will see an
object such as I describe."[221] And in fact, not only Galle on the 23rd of
September, but also Challis on the 29th, immediately after reading the
French geometer's lucid and impressive treatise, picked out from among the
stellar points strewing the zodiac, a small planetary disc, which eventually
proved to be that of the precise body he had been in search of during two
months.

The controversy that ensued had its ignominious side; but it was entered
into by neither of the parties principally concerned. Adams bore the
disappointment, which the dilatory proceedings at Greenwich and
Cambridge had inflicted upon him, with quiet heroism. His silence on the
subject of what another man would have called his wrongs remained
unbroken to the end of his life;[222] and he took every opportunity of
testifying his admiration for the genius of Leverrier.

Personal questions, however, vanish in the magnitude of the event they
relate to. By it the last lingering doubts as to the absolute exactness of the
Newtonian Law were dissipated. Recondite analytical methods received a
confirmation brilliant and intelligible even to the minds of the vulgar, and
emerged from the patient solitude of the study to enjoy an hour of
clamorous triumph. For ever invisible to the unaided eye of man, a sister-
globe to our earth was shown to circulate, in perpetual frozen exile, at thirty
times its distance from the sun. Nay, the possibility was made apparent that
the limits of our system were not even thus reached, but that yet profounder
abysses of space might shelter obedient, though little favoured, members of
the solar family, by future astronomers to be recognised through the
sympathetic thrillings of Neptune, even as Neptune himself was recognised
through the tell-tale deviations of Uranus.

It is curious to find that the fruit of Adams's and Leverrier's laborious
investigations had been accidentally all but snatched half a century before it
was ripe to be gathered. On the 8th, and again on the 10th of May, 1795,
Lalande noted the position of Neptune as that of a fixed star, but perceiving
that the two observations did not agree, he suppressed the first as erroneous,
and pursued the inquiry no further. An immortality which he would have
been the last to despise hung in the balance; the feather-weight of his
carelessness, however, kicked the beam, and the discovery was reserved to
be more hardly won by later comers.

Bode's Law did good service in the quest for a trans-Uranian planet by
affording ground for a probable assumption as to its distance. A starting-

point for approximation was provided by it; but it was soon found to be
considerably at fault. Even Uranus is about 36 millions of miles nearer to
the sun than the order of progression requires; and Neptune's vast distance
of 2,800 million should be increased by no less than 800 million miles, and
its period of 165 lengthened out to 225 years,[223] in order to bring it into
conformity with the curious and unexplained rule which planetary
discoveries have alternately tended to confirm and to invalidate.

Within seventeen days of its identification with the Berlin achromatic,
Neptune was found to be attended by a satellite. This discovery was the first
notable performance of the celebrated two-foot reflector[224] erected by Mr.
Lassell at his suggestively named residence of Starfield, near Liverpool.
William Lassell was a brewer by profession, but by inclination an
astronomer. Born at Bolton in Lancashire, June 18, 1799, he closed a life of
eminent usefulness to science, October 5, 1818, thus spanning with his
well-spent years four-fifths of the momentous period which we have
undertaken to traverse. At the age of twenty-one, being without the means
to purchase, he undertook to construct telescopes, and naturally turned his
attention to the reflecting sort, as favouring amateur efforts by the
comparative simplicity of its structure. His native ingenuity was
remarkable, and was developed by the hourly exigencies of his successive
enterprises. Their uniform success encouraged him to enlarge his aims, and
in 1844 he visited Birr Castle for the purpose of inspecting the machine
used in polishing the giant speculum of Parsonstown. In the construction of
his new instrument, however, he eventually discarded the model there
obtained, and worked on a method of his own, assisted by the supreme
mechanical skill of James Nasmyth. The result was a Newtonian of
exquisite definition, with an aperture of two, and a focal length of twenty
feet, provided by a novel artifice with the equatoreal mounting, previously
regarded as available only for refractors.

This beautiful instrument afforded to its maker, October 10, 1846, a cursory
view of a Neptunian attendant. But the planet was then approaching the sun,

and it was not until the following July that the observation could be
verified, which it was completely, first by Lassell himself, and somewhat
later by Otto Stuve and Bond of Cambridge (U.S.). When it is considered
that this remote object shines by reflecting sunlight reduced by distance to
1/900th of the intensity with which it illuminates our moon, the fact of its
visibility, even in the most perfect telescopes, is a somewhat surprising one.
It can only, indeed, be accounted for by attributing to it dimensions very
considerable for a body of the secondary order. It shares with the moons of
Uranus the peculiarity of retrograde motion; that is to say, its revolutions,
running counter to the grand current of movement in the solar system, are
performed from east to west, in a plane inclined at an angle of 35° to that of
the ecliptic. Their swiftness serves to measure the mass of the globe round
which they are performed. For while our moon takes twenty-seven days and
nearly eight hours to complete its circuit of the earth, the satellite of
Neptune, at a distance not greatly inferior, sweeps round its primary in five
days and twenty-one hours, showing (according to a very simple principle
of computation) that it is urged by a force seventeen times greater than the
terrestrial pull upon the lunar orb. Combining this result with those of
Professor Barnard's[225] and Dr. See's[226] recent measurements of the small
telescopic disc of this farthest known planet, it is found that while in mass
Neptune equals seventeen, in bulk it is equivalent to forty-nine earths. This
is as much as to say that it is composed of relatively very light materials, or
more probably of materials distended by internal heat, as yet unwasted by
radiation into space, to about five times the volume they would occupy in
the interior of our globe. The fact, at any rate, is fairly well ascertained, that
the average density of Neptune is about twice that of water.

We must now turn from this late-recognised member of our system to
bestow some brief attention upon the still fruitful field of discovery offered
by one of the immemorial five. The family of Saturn, unlike that of its
brilliant neighbour, has been gradually introduced to the notice of
astronomers. Titan, the sixth Saturnian moon in order of distance, led the
way, being detected by Huygens, March 25, 1655; Cassini made the

acquaintance of four more between 1671 and 1684; while Mimas and
Enceladus, the two innermost, were caught by Herschel in 1789, as they
threaded their lucid way along the edge of the almost vanished ring. In the
distances of these seven revolving bodies from their primary, an order of
progression analogous to that pointed out by Titius in the planetary intervals
was found to prevail; but with one conspicuous interruption, similar to that
which had first suggested the search for new members of the solar system.
Between Titan and Japetus—the sixth and seventh reckoning outwards—
there was obviously room for another satellite. It was discovered on both
sides of the Atlantic simultaneously, on the 19th of September, 1848. Mr. W.
C. Bond, employing the splendid 15-inch refractor of the Harvard
Observatory, noticed, September 16, a minute star situated in the plane of
Saturn's rings. The same object was discerned by Mr. Lassell on the 18th.
On the following evening, both observers perceived that the problematical
speck of light kept up with, instead of being left behind by the planet as it
moved, and hence inferred its true character.[227] Hyperion, the seventh by
distance and eighth by recognition of Saturn's attendant train, is of so
insignificant a size when compared with some of its fellow-moons (Titan is
but little inferior to the planet Mars), as to have suggested to Sir John
Herschel[228] the idea that it might be only one of several bodies revolving
very close together—in fact, an asteroidal satellite; but the conjecture has,
so far, not been verified.

The coincidence of its duplicate discovery was singularly paralleled two
years later. Galileo's amazement when his "optic glass" revealed to him the
"triple" form of Saturn—planeta tergeminus—has proved to be, like the
laughter of the gods, "inextinguishable." It must revive in every one who
contemplates anew the unique arrangements of that world apart known to us
as the Saturnian system. The resolution of the so-called ansæ, or "handles,"
into one encircling ring by Huygens in 1655, the discovery by Cassini in
1675 of the division of that ring into two concentric ones, together with
Laplace's investigation of the conditions of stability of such a formation,
constituted, with some minor observations, the sum of the knowledge

obtained, up to the middle of the last century, on the subject of this
remarkable formation. The first place in the discovery now about to be
related belongs to an American astronomer.

William Cranch Bond, born in 1789 at Portland, in the State of Maine, was
a watchmaker, whom the solar eclipse of 1806 attracted to study the
wonders of the heavens. When, in 1815, the erection of an observatory in
connection with Harvard College, Cambridge, was first contemplated, he
undertook a mission to England for the purpose of studying the working of
similar institutions there, and on his return erected a private observatory at
Dorchester, where he worked diligently for many years. Then at last, in
1843, the long-postponed design of the Harvard authorities was resumed,
and on the completion of the new establishment, Bond, who had been from
1838 officially connected with the College and had carried on his scientific
labours within its precincts, was offered and accepted the post of its
director. Placed in 1847 in possession of one of the finest instruments in the
world—a masterpiece of Merz and Mahler—he headed the now long list of
distinguished Transatlantic observers. Like the elder Struve, he left an heir
to his office and to his eminence, but George Bond unfortunately died in
1865, at the early age of thirty-nine, having survived his father but six
years.

On the night of November 15, 1850—the air, remarkably enough, being so
hazy that only the brightest stars could be perceived with the naked eye—
William Bond discerned a dusky ring, extending about halfway between the
inner brighter one and the globe of Saturn. A fortnight later, but before the
observation had been announced in England, the same appearance was seen
by the Rev. W. R. Dawes with the comparatively small refractor of his
observatory at Wateringbury, and on December 3 was described by Mr.
Lassell (then on a visit to him) as "something like a crape veil covering a
part of the sky within the inner ring."[229] Next morning the Times
containing the report of Bond's discovery reached Wateringbury. The most
surprising circumstance in the matter was that the novel appendage had
remained so long unrecognised. As the rings opened out to their full extent,

it became obvious with very moderate optical assistance; yet some of the
most acute observers who have ever lived, using instruments of vast power,
had heretofore failed to detect its presence. It soon appeared, however, that
Galle of Berlin[230] had noticed, June 10, 1838, a veil-like extension of the
lucid ring across half the dark space separating it from the planet; but the
observation, although communicated at the time to the Berlin Academy of
Sciences, had remained barren. Traces of the dark ring, moreover, were
found in drawings executed by Campani in 1664[231] and by Hooke in 1666;
[232] while Picard (June 15, 1673),[233] Hadley (spring of 1720),[234] and
Herschel,[235] had all undoubtedly seen it under the aspect of a dark bar or
belt crossing the Saturnian globe. It was, then, of no recent origin; but there
seemed reason to think that it had lately gained considerably in brightness.
The full meaning of this suspected change it was reserved for later
investigations to develop.

What we may, in a certain sense, call the closing result of the race for
discovery, in which several observers seemed at that time to be engaged,
was the establishment, on a satisfactory footing, of our acquaintance with
the dependent system of Uranus. Sir William Herschel, whose researches
formed, in so many distinct lines of astronomical inquiry, the starting-points
of future knowledge, detected, January 11, 1787,[236] two Uranian moons,
since called Oberon and Titania, and ascertained the curious circumstance
of their motion in a plane almost at right angles to the ecliptic, in a direction
contrary to that of all previously known denizens (other than cometary) of
the solar kingdom. He believed that he caught occasional glimpses of four
more, but never succeeded in assuring himself of their substantial existence.
Even the two first remained unseen save by himself until 1828, when his
son re-observed them with a 20-foot reflector, similar to that with which
they had been originally discovered. Thenceforward they were kept fairly
within view, but their four questionable companions, in spite of some false
alarms of detection, remained in the dubious condition in which Herschel
had left them. At last, on October 24, 1851,[237] after some years of fruitless
watching, Lassell espied "Ariel" and "Umbriel," two Uranian attendants,

interior to Oberon and Titania, and of about half their brightness; so that
their disclosure is still reckoned amongst the very highest proofs of
instrumental power and perfection. In all probability they were then for the
first time seen; for although Professor Holden[238] made out a plausible case
in favour of the fitful visibility to Herschel of each of them in turn, Lassell's
argument[239] that the glare of the planet in Herschel's great specula must
have rendered almost impossible the perception of objects so minute and so
close to its disc, appears tolerably decisive to the contrary. Uranus is thus
attended by four moons, and, so far as present knowledge extends, by no
more. Among the most important of the "negative results"[240] secured by
Lassell's observations at Malta during the years 1852-53 and 1861-65, were
the convincing evidence afforded by them that, without great increase of
optical power, no further Neptunian or Uranian satellites can be perceived,
and the consequent relegation of Herschel's baffling quartette,
notwithstanding the unquestioned place long assigned to them in
astronomical text-books, to the Nirvana of non-existence.

FOOTNOTES:
[195] Op., t. i., p. 107. He interposed, but tentatively only, another similar body
between Mercury and Venus.
[196] Allgemeine Naturgeschichte (ed. 1798), pp. 118, 119.

[197] Cosmologische Briefe, No. 1 (quoted by Von Zach, Monat. Corr., vol. iii.,
p. 592).

[198] Second ed., p. 7. See Bode, Von dem neuen Hauptplaneten, p. 43, note.

[199] The representative numbers are obtained by adding 1 to the following
series (irregular, it will be observed, in its first member, which should be 1/2
instead of 0); 0, 3, 6, 12, 24, 48, etc. The formula is a purely empirical one, and
is, moreover, completely at fault as regards the distance of Neptune.
[200] Monat. Corr., vol. iii., p. 596.

[201] Wolf, Geschichte der Astronomie, p. 648.

[202] Such reversals of direction in the apparent movements of the planets are a
consequence of the earth's revolution in its orbit.

[203] Dissertatio Philosophica de Orbitis Planetarum, 1801. See Wolf, Gesch.
d. Astr., p. 685.

[204] Observations on Uranus, as a supposed fixed star, went back to 1690.

[205] He had caught a glimpse of it on December 7, but was prevented by bad
weather from verifying his suspicion. Monat. Corr., vol. v., p. 171.

[206] Planetary fragments, hurled in any direction, and with any velocity short of
that which would for ever release them from the solar sway, would continue to
describe elliptic orbits round the sun, all passing through the scene of the
explosion, and thus possessing a common line of intersection.

[207] Phil. Trans., vol. xcii., part ii., p. 228.
[208] Ibid., p. 218. In a letter to Von Zach of June 24, 1802, he speaks of Pallas
as "almost incredibly small," and makes it only seventy English miles in
diameter. Monat. Corr., vol. vi., pp. 89, 90.

[209] Olbers, Monat. Corr., vol. vi., p. 88.

[210] Conn. d. Tems for 1814, p. 218.

[211] Popular Astronomy, p. 327.

[212] Month. Not., vol. vii., p. 299; vol. viii., p. 1.

[213] Ibid., p. 146.

[214] Airy, Mem. R. A. S., vol. xvi., p. 386.

[215] See Newcomb's Pop. Astr., p. 359. The error of Uranus amounted, in 1844,
to 2′; but even the tailor of Breslau, whose extraordinary powers of vision
Humboldt commemorates (Kosmos, Bd. ii., p. 112), could only see Jupiter's first
satellite at its greatest elongation, 2′ 15′. He might, however, possibly have
distinguished two objects of equal lustre at a lesser interval.

[216] J. W. L. Glaisher, Observatory, vol. xv., p. 177.

[217] Mem. R. A. S., vol. xvi., p. 399.

[218] For an account of D'Arrest's share in the detection see Copernicus, vol. ii.,
pp. 63, 96.

[219] Mem. R. A. S., vol. xvi., p. 412.

[220] He had recorded the places of 3,150 stars (three of which were different
positions of the planet), and was preparing to map them, when, October 1, news
of the discovery arrived from Berlin. Prof. Challis's Report, quoted in Obituary
Notice, Month. Not., Feb., 1883, p. 170.

[221] See Airy in Mem. R. A. S., vol. xvi., p. 411.

[222] He died January 21, 1892, in his 71st year.

[223] Ledger, The Sun, its Planets and their Satellites, p. 414.

[224] Presented by the Misses Lassell, after their father's death, to the
Greenwich Observatory.

[225] Astr. Jour., No. 508.

[226] Report of U.S. Naval Observatory for 1900, p. 15.

[227] Grant, Hist. of Astr., p. 271.

[228] Month. Not., vol. ix., p. 91.

[229] Month. Not., vol. xi., p. 21.

[230] Astr. Nach., No. 756 (May 2, 1851).

[231] Phil. Trans., vol. i., p. 246. See H. T. Vivian, Engl. Mech., April 20, 1894.

[232] Secchi, Month. Not., vol. xiii., p. 248.
[233] Hind, ibid., vol. xv., p. 32.

[234] Lynn, Observatory, Oct. 1, 1883; Hadley, Phil. Trans., vol. xxxii., p. 385.

[235] Proctor, Saturn and its System, p. 64.

[236] Phil. Trans., vol. lxxvii., p. 125.

[237] Month. Not., vol. xi., p. 248.

[238] Ibid., vol. xxxv., pp. 16-22.

[239] Ibid., p. 26.

[240] Ibid., vol. xli., p. 190.

CHAPTER V
COMETS

Newton showed that the bodies known as "comets," or hirsute stars, obey
the law of gravitation; but it was by no means certain that the individual of
the species observed by him in 1680 formed a permanent member of the
solar system. The velocity, in fact, of its rush round the sun was quite
possibly sufficient to carry it off for ever into the depths of space, there to
wander, a celestial casual, from star to star. With another comet, however,
which appeared two years later, the case was different. Edmund Halley, who
afterwards succeeded Flamsteed as Astronomer Royal, calculated the
elements of its orbit on Newton's principles, and found them to resemble so
closely those similarly arrived at for comets observed by Peter Apian in
1531, and by Kepler in 1607, as almost to compel the inference that all
three were apparitions of a single body. This implied its revolution in a
period of about seventy-six years, and Halley accordingly fixed its return
for 1758-9. So fully alive was he to the importance of the announcement
that he appealed to a "candid posterity," in the event of its verification, to
acknowledge that the discovery was due to an Englishman. The prediction
was one of the test-questions put by Science to Nature, on the replies to
which largely depend both the development of knowledge and the
conviction of its reality. In the present instance, the answer afforded may be
said to have laid the foundation of this branch of astronomy. Halley's comet
punctually reappeared on Christmas Day, 1758, and effected its perihelion
passage on the 12th of March following, thus proving beyond dispute that
some at least of these erratic bodies are domesticated within our system,
and strictly conform, if not to its unwritten customs (so to speak), at any
rate to its fundamental laws. Their movements, in short, were demonstrated
by the most unanswerable of all arguments—that of verified calculation—to

be calculable, and their investigation was erected into a legitimate
department of astronomical science.

This notable advance was the chief result obtained in the field of inquiry
just now under consideration during the eighteenth century. But before it
closed, its cultivation had received a powerful stimulus through the
invention of an improved method. The name of Olbers has already been
brought prominently before our readers in connection with asteroidal
discoveries; these, however, were but chance excursions from the path of
cometary research which he steadily pursued through life. An early
predilection for the heavens was fixed in this particular direction by one of
the happy inspirations of genius. As he was watching, one night in the year
1779, by the sick-bed of a fellow-student in medicine at Göttingen, an
important simplification in the mode of computing the paths of comets
occurred to him. Although not made public until 1797, "Olbers's method"
was then universally adopted, and is still regarded as the most expeditious
and convenient in cases where absolute rigour is not required. By its
introduction, not only many a toilsome and thankless hour was spared, but
workers were multiplied, and encouraged in the prosecution of labours
more useful than attractive.

The career of Heinrich Olbers is a brilliant example of what may be done
by an amateur in astronomy. He at no time did regular work in an
observatory; he was never the possessor of a transit or any other fixed
instrument; moreover, all the best years of his life were absorbed in the
assiduous exercise of a toilsome profession. Born in 1758 at the village of
Arbergen, where his father was pastor, he settled in 1781 as a physician in
the neighbouring town of Bremen, and continued in active practice there for
over forty years. It was thus only the hours which his robust constitution
enabled him to spare from sleep that were available for his intellectual
pleasures. Yet his recreation was, as Von Zach remarked,[241] no less prolific
of useful results than the severest work of other men. The upper part of his
house in the Sandgasse was fitted up with such instruments and appliances
as restrictions of space permitted, and there, night after night during half a

century and upwards, he discovered, calculated, or observed the cometary
visitants of northern skies. Almost as effective in promoting the interests of
science as the valuable work actually done by him, was the influence of his
genial personality. He engaged confidence by his ready and discerning
sympathy; he inspired affection by his benevolent disinterestedness; he
quickened thought and awakened zeal by the suggestions of a lively and
inventive spirit, animated with the warmest enthusiasm for the advancement
of knowledge. Nearly every astronomer in Germany enjoyed the benefits of
a frequently active correspondence with him, and his communications to the
scientific periodicals of the time were numerous and striking. The motive
power of his mind was thus widely felt and continually in action. Nor did it
wholly cease to be exerted even when the advance of age and the progress
of infirmity rendered him incapable of active occupation. He was, in fact,
alive even to the last day of his long life of eighty-one years; and his death,
which occurred March 2, 1840, left vacant a position which a rare
combination of moral and intellectual qualities had conspired to render
unique.

Amongst the many younger men who were attracted and stimulated by
intercourse with him was Johann Franz Encke. But while Olbers became a
mathematician because he was an astronomer, Encke became an astronomer
because he was a mathematician. A born geometer, he was naturally sent to
Göttingen and placed under the tuition of Gauss. But geometers are men;
and the contagion of patriotic fervour which swept over Germany after the
battle of Leipsic did not spare Gauss's promising pupil. He took up arms in
the Hanseatic Legion, and marched and fought until the oppressor of his
country was safely ensconced behind the ocean-walls of St. Helena. In the
course of his campaigning he met Lindenau, the militant director of the
Seeberg Observatory, and by his influence was appointed his assistant, and
eventually, in 1822, became his successor. Thence he was promoted in 1825
to Berlin, where he superintended the building of the new observatory, so
actively promoted by Humboldt, and remained at its head until within some
eighteen months of his death in August, 1865.

On the 26th of November, 1818, Pons of Marseilles discovered a comet,
whose inconspicuous appearance gave little promise of its becoming one of
the most interesting objects in our system. Encke at once took the
calculation of its elements in hand, and brought out the unexpected result
that it revolved round the sun in a period of about 3-1/3 years.[242] He,
moreover, detected its identity with comets seen by Méchain in 1786, by
Caroline Herschel in 1795, by Pons, Huth, and Bouvard in 1805, and after
six laborious weeks of research into the disturbances experienced by it from
the planets during the entire interval since its first ascertained appearance,
he fixed May 24, 1822, as the date of its next return to perihelion. Although
on that occasion, owing to the position of the earth, invisible in the northern
hemisphere, Sir Thomas Brisbane's observatory at Paramatta was
fortunately ready equipped for its recapture, which Rümker effected quite
close to the spot indicated by Encke's ephemeris.

The importance of this event can be better understood when it is
remembered that it was only the second instance of the recognised return of
a comet (that of Halley's, sixty-three years previously, having, as already
stated, been the first); and that it, moreover, established the existence of a
new class of celestial objects, somewhat loosely distinguished as "comets of
short period." These bodies (of which about thirty have been found to
circulate within the orbit of Saturn) are remarkable as showing certain
planetary affinities in the manners of their motions not at all perceptible in
the wider travelling members of their order. They revolve, without
exception, in the same direction as the planets—from west to east; they
exhibit a marked tendency to conform to the zodiacal track which limits
planetary excursions north and south; and their paths round the sun,
although much more eccentric than the approximately circular planetary
orbits, are far less so than the extravagantly long ellipses in which comets
comparatively untrained (as it were) in the habits of the solar system
ordinarily perform their revolutions.

No great comet is of the "planetary" kind. These are, indeed, only by
exception visible to the naked eye; they possess extremely feeble tail-

producing powers, and give small signs of central condensation. Thin wisps
of cosmical cloud, they flit across the telescopic field of view without
sensibly obscuring the smallest star. Their appearance, in short, suggests—
what some notable facts in their history will presently be shown to confirm
—that they are bodies already effete, and verging towards dissolution. If it
be asked what possible connection can be shown to exist between the
shortness of period by which they are essentially characterised, and what
we may call their superannuated condition, we are not altogether at a loss
for an answer. Kepler's remark,[243] that comets are consumed by their own
emissions, has undoubtedly a measure of truth in it. The substance ejected
into the tail must, in overwhelmingly large proportion, be for ever lost to
the central mass from which it issues. True, it is of a nature inconceivably
tenuous; but unrepaired waste, however small in amount, cannot be
persisted in with impunity. The incitement to such self-spoliation proceeds
from the sun; it accordingly progresses more rapidly the more numerous are
the returns to the solar vicinity. Comets of short period may thus reasonably
be expected to wear out quickly.

They are, moreover, subject to many adventures and vicissitudes. Their
aphelia—or the farthest points of their orbits from the sun—are usually, if
not invariably, situated so near to the path either of Jupiter or of Saturn, as
to permit these giant planets to act as secondary rulers of their destinies. By
their influence they were, in all likelihood, originally fixed in their present
tracks; and by their influence, exerted in an opposite sense, they may, in
some cases, be eventually ejected from them. Careers so varied, as can
easily be imagined, are apt to prove instructive, and astronomers have not
been backward in extracting from them the lessons they are fitted to convey.
Encke's comet, above all, has served as an index to much curious
information, and it may be hoped that its function in that respect is by no
means at an end. The great extent of the solar system traversed by its
eccentric path makes it peculiarly useful for the determination of the
planetary masses. At perihelion it penetrates within the orbit of Mercury; it
considerably transcends at aphelion the farthest excursion of Pallas. Its

vicinity to the former planet in August, 1835, offered the first convenient
opportunity of placing that body in the astronomical balance. Its weight or
mass had previously been assumed, not ascertained; and the comparatively
slight deviation from its regular course impressed upon the comet by its
attractive power showed that it had been assumed nearly twice too great.
[244] That fundamental datum of planetary astronomy—the mass of Jupiter
—was corrected by similar means; and it was reassuring to find the
correction in satisfactory accord with that already introduced from
observations of the asteroidal movements.

The fact that comets contract in approaching the sun had been noticed by
Hevelius; Pingré admitted it with hesitating perplexity;[245] the example of
Encke's comet rendered it conspicuous and undeniable. On the 28th of
October, 1828, the diameter of the nebulous matter composing this body
was estimated at 312,000 miles. It was then about one and a half times
further from the sun than the earth is at the time of the equinox. On the 24th
of December following, its distance being reduced by nearly two-thirds, it
was found to be only 14,000 miles across.[246] That is to say, it had shrunk
during those two months of approach to 1/11000th part of its original
volume! Yet it had still seventeen days' journey to make before reaching
perihelion. The same curious circumstance was even more markedly
apparent at its return in 1838. Its bulk, or the actual space occupied by it,
appeared to be reduced, as it drew near the hearth of our system, in the
enormous proportion of 800,000 to 1. A corresponding expansion
accompanied on each occasion its retirement from the sphere of
observation. Similar changes of volume, though rarely to the same
astounding extent, have been perceived in other comets. They still remain
unexplained; but it can scarcely be doubted that they are due to the action of
the same energetic internal forces which reveal themselves in so many
splendid and surprising cometary phenomena.

Another question of singular interest was raised by Encke's acute inquiries
into the movements and disturbances of the first known "comet of short

period." He found from the first that its revolutions were subject to some
influence besides that of gravity. After every possible allowance had been
made for the pulls, now backward, now forward, exerted upon it by the
several planets, there was still a surplus of acceleration left unaccounted for.
Each return to perihelion took place about two and a half hours sooner than
received theories warranted. Here, then, was a "residual phenomenon" of
the utmost promise for the disclosure of novel truths. Encke (in accordance
with the opinion of Olbers) explained it as due to the presence in space of
some such "subtle matter" as was long ago invoked by Euler[247] to be the
agent of eventual destruction for the fair scheme of planetary creation. The
apparent anomaly of accounting for an accelerative effect by a retarding
cause disappears when it is considered that any check to the motion of
bodies revolving round a centre of attraction causes them to draw closer to
it, thus shortening their periods and quickening their circulation. If space
were filled with a resisting medium capable of impeding, even in the most
infinitesimal degree, the swift course of the planets, their orbits should
necessarily be, not ellipses, but very close elliptical spirals along which they
would slowly, but inevitably, descend into the burning lap of the sun. The
circumstance that no such tendency can be traced in their revolutions by no
means sets the question at rest. For it might well be that an effect totally
imperceptible until after the lapse of countless ages, as regards the solid
orbs of our system, might be obvious in the movements of bodies like
comets of small mass and great bulk; just as a feather or a gauze veil at
once yields its motion to the resistance of the air, while a cannon-ball cuts
its way through with comparatively slight loss of velocity.

It will thus be seen that issues of the most momentous character hang on the
time-keeping of comets; for plainly all must in some degree suffer the same
kind of hindrance as Encke's, if the cause of that hindrance be the one
suggested. None of its congeners, however, show any trace of similar
symptoms. True, the late Professor Oppolzer announced,[248] in 1880, that a
comet, first seen by Pons in 1819, and rediscovered by Winnecke in 1858,
having a period of 2,052 days (5·6 years), was accelerated at each

revolution precisely in the manner required by Encke's theory. But M. von
Haerdtl's subsequent investigation, the materials for which included
numerous observations of the body in question at its return to the sun in
1886, decisively negatived the presence of any such effect.[249] Moreover,
the researches of Von Asten and Backlund[250] into the movements of
Encke's comet revealed a perplexing circumstance. They confirmed Encke's
results for the period covered by them, but exhibited the acceleration as
having suddenly diminished by nearly one-half in 1868. The reality and
permanence of this change were fully established by observations of the
ensuing return in March, 1885. Some physical alteration of the retarded
body seems indicated; but visual evidence countenances no such
assumption. In aspect the comet is no less thin and diffuse than in 1795 or
in 1848.

The character of the supposed resistance in inter-planetary space has, it may
be remarked, been often misapprehended. What Encke stipulated for was
not a medium equally diffused throughout the visible universe, such as the
ethereal vehicle of the vibrations of light, but a rare fluid, rapidly increasing
in density towards the sun.[251] This cannot be a solar atmosphere, since it is
mathematically certain, as Laplace has shown,[252] that no envelope
partaking of the sun's axial rotation can extend farther from his surface than
nine-tenths of the mean distance of Mercury; while physical evidence
assures us that the actual depth of the solar atmosphere bears a very minute
proportion to the possible depth theoretically assigned to it. That matter,
however, not atmospheric in its nature—that is, neither forming one body
with the sun nor altogether aëriform—exists in its neighbourhood, can
admit of no reasonable doubt. The great lens-shaped mass of the zodiacal
light, stretching out at times far beyond the earth's orbit, may indeed be
regarded as an extension of the corona, the streamers of which themselves
mark the wide diffusion, all round the solar globe, of granular or gaseous
materials. Yet comets have been known to penetrate the sphere occupied by
them without perceptible loss of velocity. The hypothesis, then, of a

resisting medium receives at present no countenance from the movements
of comets, whether of short or of long periods.

Although Encke's comet has made thirty-five complete rounds of its orbit
since its first detection in 1786, it shows no certain signs of decay.
Variations in its brightness are, it is true, conspicuous, but they do not
proceed continuously.[253]

The history of the next known planet-like comet has proved of even more
curious interest than that of the first. It was discovered by an Austrian
officer named Wilhelm von Biela at Josephstadt in Bohemia, February 27,
1826, and ten days later by the French astronomer Gambart at Marseilles.
Both observers computed its orbit, showed its remarkable similarity to that
traversed by comets visible in 1772 and 1805, and connected them together
as previous appearances of the body just detected by assigning to its
revolutions a period of between six and seven years. The two brief letters
conveying these strikingly similar inferences were printed side by side in
the same number of the Astronomische Nachrichten (No. 94); but Biela's
priority in the discovery of the comet was justly recognised by the bestowal
upon it of his name.

The object in question was at no time, subsequently to 1805, visible to the
naked eye. Its aspect in Sir John Herschel's great reflector on the 23rd of
September, 1832, was described by him as that of a "conspicuous nebula,"
nearly 3 minutes in diameter. No trace of a tail was discernible. While he
was engaged in watching it, a small knot of minute stars was directly
traversed by it, "and when on the cluster," he tells us,[254] it "presented the
appearance of a nebula resolvable and partly resolved into stars, the stars of
the cluster being visible through the comet." Yet the depth of cometary
matter through which such faint stellar rays penetrated undimmed, was,
near the central parts of the globe, not less than 50,000 miles.

It is curious to find that this seemingly harmless, and we may perhaps add
effete body, gave occasion to the first (and not the last) cometary "scare" of

an enlightened century. Its orbit, at the descending node, may be said to
have intersected that of the earth; since, according as it bulged in or out
under the disturbing influence of the planets, the passage of the comet was
affected inside or outside the terrestrial track. Now, certain calculations
published by Olbers in 1828[255] showed that, on October 29, 1832, a
considerable portion of its nebulous surroundings would actually sweep
over the spot which, a month later, would be occupied by our planet. It
needed no more to set the popular imagination in a ferment. Astronomers,
after all, could not, by an alarmed public, be held to be infallible. Their
computations, it was averred, which a trifling oversight would suffice to
vitiate, exhibited clearly enough the danger, but afforded no guarantee of
safety from a collision, with all the terrific consequences frigidly
enumerated by Laplace. Nor did the panic subside until Arago formally
demonstrated that the earth and the comet could by no possibility approach
within less than fifty millions of miles.[256]

The return of the same body in 1845-46 was marked by an extraordinary
circumstance. When first seen, November 28, it wore its usual aspect of a
faint round patch of cosmical fog; but on December 19, Mr. Hind noticed
that it had become distorted somewhat into the form of a pear; and ten days
later, it had divided into two separate objects. This singular duplication was
first perceived at New Haven in America, December 29,[257] by Messrs.
Herrick and Bradley, and by Lieutenant Maury at Washington, January 13,
1846. The earliest British observer of the phenomenon (noticed by
Wichmann the same evening at Königsberg) was Professor Challis. "I see
two comets!" he exclaimed, putting his eye to the great equatoreal of the
Cambridge Observatory on the night of January 15; then, distrustful of what
his senses had told him, he called in his judgment to correct their
improbable report by resolving one of the dubious objects into a hazy star.
[258] On the 23rd, however, both were again seen by him in unmistakable
cometary shape, and until far on in March (Otto Struve caught a final
glimpse of the pair on the 16th of April),[259] continued to be watched with
equal curiosity and amazement by astronomers in every part of the northern

hemisphere. What Seneca reproved Ephorus for supposing to have taken
place in 373 b.c.—what Pingré blamed Kepler for conjecturing in 1618—
had then actually occurred under the attentive eyes of science in the middle
of the nineteenth century!

At a distance from each other of about two-thirds the distance of the moon
from the earth, the twin comets meantime moved on tranquilly, so far, at
least, as their course through the heaven was concerned. Their extreme
lightness, or the small amount of matter contained in each, could not have
received a more signal illustration than by the fact that their revolutions
round the sun were performed independently; that is to say, they travelled
side by side without experiencing any appreciable mutual disturbance, thus
plainly showing that at an interval of only 157,250 miles their attractive
power was virtually inoperative. Signs of internal agitation, however, were
not wanting. Each fragment threw out a short tail in a direction
perpendicular to the line joining their centres, and each developed a bright
nucleus, although the original comet had exhibited neither of these signs of
cometary vitality. A singular interchange of brilliancy was, besides,
observed to take place between the coupled objects, each of which
alternately outshone and was outshone by the other, while an arc of light,
apparently proceeding from the more lustrous, at times bridged the
intervening space. Obviously, the gravitational tie, rendered powerless by
exiguity of matter, was here replaced by some other form of mutual action,
the nature of which can as yet be dealt with only by conjecture.

Once more, in August, 1852, the double comet returned to the
neighbourhood of the sun, but under circumstances not the most
advantageous for observation. Indeed, the companion was not detected until
September 16, when Father Secchi at Rome perceived it to have increased
its distance from the originating body to a million and a quarter of miles, or
about eight times the average interval at the former appearance. Both
vanished shortly afterwards, and have never since been seen,
notwithstanding the eager watch kept for objects of such singular interest,
and the accurate knowledge of their track supplied by Santini's

investigations. A dangerously near approach to Jupiter in 1841 is believed
to have occasioned their disruption, and the disaggregating process thus
started was likely to continue. We can scarcely doubt that the fate has
overtaken them which Newton assigned as the end of all cometary
existence. Diffundi tandem et spargi per cœlos universos.[260]

Biela's is not the only vanished comet. Brorsen's, discovered at Kiel in
1846, and observed at four subsequent returns, failed unaccountably to
become visible in 1890.[261] Yet numerous sentinels were on the alert to
surprise its approach along a well-ascertained track, traversed in five and a
half years. The object presented from the first a somewhat time-worn
aspect. It was devoid of tail, or any other kind of appendage; and the rapid
loss of the light acquired during perihelion passage was accompanied by
inordinate expansion of an already tenuous globular mass. Another lost or
mislaid comet is one found by De Vico at Rome, August 22, 1844. It was
expected to return early in 1850, but did not, and has never since been seen;
unless its re-appearance as E. Swift's comet of 1894 should be ratified by
closer inquiry.[262]

A telescopic comet with a period of 7-1/2 years, discovered November 22,
1843, by M. Faye of the Paris Observatory, formed the subject of a
characteristically patient and profound inquiry on the part of Leverrier,
designed to test its suggested identity with Lexell's comet of 1770. The
result was decisive against the hypothesis of Valz, the divergences between
the orbits of the two bodies being found to increase instead of to diminish,
as the history of the new-comer was traced backward into the previous
century.[263] Faye's comet pursues the most nearly circular path of any
similar known object; even at its nearest approach to the sun it remains
farther off than Mars when he is most distant from it; and it was proved by
the admirable researches of Professor Axel Möller,[264] director of the
Swedish observatory of Lund, to exhibit no trace of the action of a resisting
medium.

Periodical comets are evidently bodies which have each lived through a
chapter of accidents, and a significant hint as to the nature of their
adventures can be gathered from the fact that their aphelia are pretty closely
grouped about the tracks of the major planets. Halley's, and five other
comets are thus related to Neptune; three connect themselves with Uranus,
two with Saturn, above a score with Jupiter. Some form of dependence is
plainly indicated, and the researches of Tisserand,[265] Callandreau,[266] and
Newton[267] of Yale College, leave scarcely a doubt that the "capture-
theory" represents the essential truth in the matter. The original parabolic
paths of these comets were then changed into ellipses by the backward pull
of a planet, whose sphere of attraction they chanced to enter when
approaching the sun from outer space. Moreover, since a body thus affected
should necessarily return at each revolution to the scene of encounter, the
same process of retardation may, in some cases, have been repeated many
times, until the more restricted cometary orbits were reduced to their
present dimensions. The prevalence, too, among periodical comets, of direct
motion, is shown to be inevitable by M. Callandreau's demonstration that
those travelling in a retrograde direction would, by planetary action, be
thrown outside the probable range of terrestrial observation. The scarcity of
hyperbolic comets can be similarly explained. They would be created
whenever the attractive influence of the disturbing planet was exerted in a
forward or accelerative sense, but could come only by a rare exception to
our notice. The inner planets, including the earth, have also unquestionably
played their parts in modifying cometary orbits; and Mr. Plummer suggests,
with some show of reason, that the capture of Encke's comet may be a feat
due to Mercury.[268]

No great comet appeared between the "star" which presided at the birth of
Napoleon and the "vintage" comet of 1811. The latter was first described by
Flaugergues at Viviers, March 26, 1811; Wisniewski, at Neu-Tscherkask in
Southern Russia, caught a final glimpse of it, August 17, 1812. Two
disappearances in the solar rays as the earth moved round in its orbit, and
two reappearances after conjunction, were included in this unprecedentedly

long period of visibility of 510 days. This relative permanence (so far as the
inhabitants of Europe were concerned) was due to the high northern latitude
attained near perihelion, combined with a certain leisureliness of movement
along a path everywhere external to that of the earth. The magnificent
luminous train of this body, on October 15, the day of its nearest terrestrial
approach, covered an arc of the heavens 23-1/2 degrees in length,
corresponding to a real extension of one hundred millions of miles. Its form
was described by Sir William Herschel as that of "an inverted hollow cone,"
and its colour as yellowish, strongly contrasted with the bluish-green tint of
the "head," round which it was flung like a transparent veil. The planetary
disc of the head, 127,000 miles across, appeared to be composed of
strongly-condensed nebulous matter; but somewhat eccentrically situated
within it was a star-like nucleus of a reddish tinge, which Herschel
presumed to be solid, and ascertained, with his usual care, to have a
diameter of 428 miles. From the total absence of phases, as well as from the
vivacity of its radiance, he confidently inferred that its light was not
borrowed, but inherent.[269]

This remarkable apparition formed the subject of a memoir by Olbers,[270]
the striking yet steadily reasoned out suggestions contained in which there
was at that time no means of following up with profit. Only of late has the
"electrical theory," of which Zöllner[271] regarded Olbers as the founder,
assumed a definite and measurable form, capable of being tested by the
touchstone of fact, as knowledge makes its slow inroads on the fundamental
mystery of the physical universe.

The paraboloidal shape of the bright envelope separated by a dark interval
from the head of the great comet of 1811, and constituting, as it were, the
root of its tail, seemed to the astronomer of Bremen to reveal the presence
of a double repulsion; the expelled vapours accumulating where the two
forces, solar and cometary, balanced each other, and being then swept
backwards in a huge train. He accordingly distinguished three classes of
these bodies:—First, comets which develop no matter subject to solar

repulsion. These have no tails, and are probably mere nebulosities, without
solid nuclei. Secondly, comets which are acted upon by solar repulsion
only, and consequently throw out no emanations towards the sun. Of this
kind was a bright comet visible in 1807.[272] Thirdly, comets like that of
1811, giving evidence of action of both kinds. These are distinguished by a
dark hoop encompassing the head and dividing it from the luminous
envelope, as well as by an obscure caudal axis, resulting from the hollow,
cone-like structure of the tail.

Again, the ingenious view subsequently propounded by M. Brédikhine as to
the connection between the form of these appendages and the kind of matter
composing them, was very clearly anticipated by Olbers. The amount of
tail-curvature, he pointed out, depends in each case upon the proportion
borne by the velocity of the ascending particles to that of the comet in its
orbit; the swifter the outrush, the straighter the resulting tail. But the
velocity of the ascending particles varies with the energy of their repulsion
by the sun, and this again, it may be presumed, with their quality. Thus
multiple tails are developed when the same comet throws off, as it
approaches perihelion, specifically distinct substances. The long, straight
ray which proceeded from the comet of 1807, for example, was doubtless
made up of particles subject to a much more vigorous solar repulsion than
those formed into the shorter curved emanation issuing from it nearly in the
same direction. In the comet of 1811 he calculated that the particles
expelled from the head travelled to the remote extremity of the tail in eleven
minutes, indicating by this enormous rapidity of movement (comparable to
that of the transmission of light) the action of a force much more powerful
than the opposing one of gravity. The not uncommon phenomena of
multiple envelopes, on the other hand, he explained as due to the varying
amounts of repulsion exercised by the nucleus itself on the different kinds
of matter developed from it.

The movements and perturbations of the comet of 1811 were no less
profoundly studied by Argelander than its physical constitution by Olbers.
The orbit which he assigned to it is of such vast dimensions as to require no

less that 3,065 years for the completion of its circuit; and to carry the body
describing it at each revolution to fourteen times the distance from the sun
of the frigid Neptune. Thus, when it last visited our neighbourhood,
Achilles may have gazed on its imposing train as he lay on the sands all
night bewailing the loss of Patroclus; and when it returns, it will perhaps be
to shine upon the ruins of empires and civilizations still deep buried among
the secrets of the coming time.[273]

On the 26th of June, 1819, while the head of a comet passed across the face
of the sun, the earth was in all probability involved in its tail. But of this
remarkable double event nothing was known until more than a month later,
when the fact of its past occurrence emerged from the calculations of
Olbers.[274] Nor had the comet itself been generally visible previous to the
first days of July. Several observers, however, on the publication of these
results, brought forward accounts of singular spots perceived by them upon
the sun at the time of the transit, and an original drawing of one of them, by
Pastorff of Buchholtz, has been preserved. This undoubtedly authentic
delineation[275] represents a round nebulous object with a bright spot in the
centre, of decidedly cometary aspect, and not in the least like an ordinary
solar "macula." Mr. Hind,[276] nevertheless, showed its position on the sun
to be irreconcilable with that which the comet must have occupied; and Mr.
Ranyard's discovery of a similar smaller drawing by the same author, dated
May 26, 1828,[277] reduces to evanescence the probability of its connection
with that body. Indeed, recent experience renders very doubtful the
possibility of such an observation.

The return of Halley's comet in 1835 was looked forward to as an
opportunity for testing the truth of floating cometary theories, and did not
altogether disappoint expectation. As early as 1817, its movements and
disturbances since 1759 were proposed by the Turin Academy of Sciences
as the subject of a prize ultimately awarded to Baron Damoiseau.
Pontécoulant was adjudged a similar distinction by the Paris Academy in

1829; while Rosenberger's calculations were rewarded with the gold medal
of the Royal Astronomical Society.[278]

They were verified by the detection at Rome, August 6, 1835, of a nearly
circular misty object not far from the predicted place of the comet. It was
not, however, until the middle of September that it began to throw out a tail,
which by the 15th of October had attained a length of about 24 degrees (on
the 19th, at Madras, it extended to fully 30),[279] the head showing to the
naked eye as a reddish star rather brighter than Aldebaran or Antares.[280]
Some curious phenomena accompanied the process of tail-formation. An
outrush of luminous matter, resembling in shape a partially opened fan,
issued from the nucleus towards the sun, and at a certain point, like smoke
driven before a high wind, was vehemently swept backwards in a prolonged
train. The appearance of the comet at this time was compared by Bessel,
[281] who watched it with minute attention, to that of a blazing rocket. He
made the singular observation that this fan of light, which seemed the
source of supply for the tail, oscillated like a pendulum to and fro across a
line joining the sun and nucleus, in a period of 4-3/5 days; and he was
unable to escape from the conclusion[282] that a repulsive force, about twice
as powerful as the attractive force of gravity, was concerned in the
production of these remarkable effects. Nor did he hesitate to recur to the
analogy of magnetic polarity, or to declare, still more emphatically than
Olbers, "the emission of the tail to be a purely electrical phenomenon."[283]

The transformations undergone by this body were almost as strange and
complete as those which affected the brigands in Dante's Inferno. When
first seen, it wore the aspect of a nebula; later it put on the distinctive garb
of a comet; it next appeared as a star; finally, it dilated, first in a spherical,
then in a paraboloidal form, until May 5, 1836, when it vanished from
Herschel's observation at Feldhausen as if by melting into adjacent space
from the excessive diffusion of its light. A very uncommon circumstance in
its development was that it lost all trace of tail previous to its arrival at
perihelion on the 16th of November. Nor did it begin to recover its

elongated shape for more than two months afterwards. On the 23rd of
January, Boguslawski perceived it as a star of the sixth magnitude, without
measurable disc.[284] Only two nights later, Maclear, director of the Cape
Observatory, found the head to be 131 seconds across.[285] And so rapidly
did the augmentation of size progress, that Sir John Herschel estimated the
actual bulk of this singular object to have increased forty-fold in the
ensuing week. "I can hardly doubt," he remarks, "that the comet was fairly
evaporated in perihelio by the heat, and resolved into transparent vapour,
and is now in process of rapid condensation and re-precipitation on the
nucleus."[286] A plausible, but no longer admissible, interpretation of this
still unexplained phenomenon. The next return of this body, which will be
considerably accelerated by Jupiter's influence, is expected to take place in
1910.[287]

By means of an instrument devised to test the quality of light, Arago
obtained decisive evidence that some at least of the radiance proceeding
from Halley's comet was derived by reflection from the sun.[288] Indications
of the same kind had been afforded[289] by the comet which suddenly
appeared above the north-western horizon of Paris, July 3, 1819, after
having enveloped (as already stated) our terrestrial abode in its filmy
appendages; but the "polariscope" had not then reached the perfection
subsequently given to it, and its testimony was accordingly far less reliable
than in 1835. Such experiments, however, are in reality more beautiful and
ingenious than instructive, since ignited as well as obscure bodies possess
the power of throwing back light incident upon them, and will consequently
transmit to us from the neighbourhood of the sun rays partly direct, partly
reflected, of which a certain proportion will exhibit the peculiarity known
as polarisation.

The most brilliant comets of the century were suddenly rivalled if not
surpassed by the extraordinary object which blazed out beside the sun,
February 28, 1843. It was simultaneously perceived in Mexico and the
United States, in Southern Europe, and at sea off the Cape of Good Hope,

where the passengers on board the Owen Glendower were amazed by the
sight of a "short, dagger-like object," closely following the sun towards the
western horizon.[290] At Florence, Amici found its distance from the sun's
centre at noon to be only 1° 23′; and spectators at Parma were able, when
sheltered from the direct glare of mid-day, to trace the tail to a length of
four or five degrees. The full dimensions of this astonishing appurtenance
began to be disclosed a few days later. On the 3rd of March it measured
25°, and on the 11th, at Calcutta, Mr. Clerihew observed a second streamer,
nearly twice as long as the first, and making an angle with it of 18°, to have
been emitted in a single day. This rapidity of projection, Sir John Herschel
remarked, "conveys an astounding impression of the intensity of the forces
at work." "It is clear," he continued, "that if we have to deal here with
matter, such as we conceive it—viz., possessing inertia—at all, it must be
under the dominion of forces incomparably more energetic than gravitation,
and quite of a different nature."[291]

On the 17th of March a silvery ray, some 40° long and slightly curved at its
extremity, shone out above the sunset clouds in this country. No previous
intimation had been received of the possibility of such an apparition, and
even astronomers—no lightning messages across the seas being as yet
possible—were perplexed. The nature of the phenomenon, indeed, soon
became evident, but the wonder of it did not diminish with the study of its
attendant circumstances. Never before, within astronomical memory, had
our system been traversed by a body pursuing such an adventurous career.
The closest analogy was offered by the great comet of 1680 (Newton's),
which rushed past the sun at a distance of only 144,000 miles; but even this
—on the cosmical scale—scarcely perceptible interval was reduced nearly
one-half in the case we are now concerned with. The centre of the comet of
1843 approached the formidable luminary within 78,000 miles, leaving, it is
estimated, a clear space of not more than 32,000 between the surfaces of the
bodies brought into such perilous proximity. The escape of the wanderer
was, however, secured by the extraordinary rapidity of its flight. It swept
past perihelion at a rate—366 miles a second—which, if continued, would

have carried it right round the sun in two hours; and in only eleven minutes
more than that short period it actually described half the curvature of its
orbit—an arc of 180°—although in travelling over the remaining half many
hundreds of sluggish years will doubtless be consumed.

The behaviour of this comet may be regarded as an experimentum crucis as
to the nature of tails. For clearly no fixed appendage many millions of miles
in length could be whirled like a brandished sabre from one side of the sun
to the other in 131 minutes. Cometary trains are then, as Olbers rightly
conceived them to be, emanations, not appendages—inconceivably rapid
outflows of highly rarefied matter, the greater part, if not all, of which
becomes permanently detached from the nucleus.

That of the comet of 1843 reached, about the time that it became visible in
this country, the extravagant length of 200 millions of miles.[292] It was
narrow, and bounded by nearly parallel and nearly rectilinear lines,
resembling—to borrow a comparison of Aristotle's—a "road" through the
constellations; and after the 3rd of March showed no trace of hollowness,
the axis being, in fact, rather brighter than the edges. Distinctly perceptible
in it were those singular aurora-like coruscations which gave to the
"tresses" of Charles V.'s comet the appearance—as Cardan described it—of
"a torch agitated by the wind," and have not unfrequently been observed to
characterise other similar objects. A consideration first adverted to by
Olbers proves these to originate in our own atmosphere. For owing to the
great difference in the distances from the earth of the origin and extremity
of such vast effluxes, the light proceeding from their various parts is
transmitted to our eyes in notably different intervals of time. Consequently
a luminous undulation, even though propagated instantaneously from end to
end of a comet's tail, would appear to us to occupy many minutes in its
progress. But the coruscations in question pass as swiftly as a falling star.
They are, then, of terrestrial production.

Periods of the utmost variety were by different computators assigned to the
body, which arrived at perihelion, February 27, 1843, at 9.47 p.m. Professor

Hubbard of Washington found that it required 533 years to complete a
revolution; MM. Laugier and Mauvais of Paris considered the true term to
be 35;[293] Clausen looked for its return at the end of between six and seven.
A recent discussion[294] by Professor Kreutz of all the available data gives a
probable period of 512 years for this body, and precludes its hypothetical
identity with the comet of 1668, known as the "Spina" of Cassini.

It may now be asked, what were the conclusions regarding the nature of
comets drawn by astronomers from the considerable amount of novel
experience accumulated during the first half of this century? The first and
best assured was that the matter composing them is in a state of extreme
tenuity. Numerous and trustworthy observations showed that the feeblest
rays of light might traverse some hundreds of thousands of miles of their
substance, even where it was apparently most condensed, without being
perceptibly weakened. Nay, instances were recorded in which stars were
said to have gained in brightness from the process![295] On the 24th of June,
1825, Olbers[296] saw the comet then visible all but obliterated by the
central passage of a star too small to be distinguished with the naked eye, its
own light remaining wholly unchanged. A similar effect was noted
December 1, 1811, when the great comet of that year approached so close to
Altair, the lucida of the Eagle, that the star seemed to be transformed into
the nucleus of the comet.[297] Even the central blaze of Halley's comet in
1835 was powerless to impede the passage of stellar rays. Struve[298]
observed at Dorpat, on September 17, an all but central occultation;
Glaisher[299] one (so far as he could ascertain) absolutely so eight days later
at Cambridge. In neither case was there any appreciable diminution of the
star's light. Again, on the 11th of October, 1847, Mr. Dawes,[300] an
exceptionally keen observer, distinctly saw a star of the tenth magnitude
through the exact centre of a comet discovered on the first of that month by
Maria Mitchell of Nantucket.

Examples, on the other hand, are not wanting of the diminution of stellar
light under similar circumstances;[301] and we meet two alleged instances of

the vanishing of a star behind a comet. Wartmann of Geneva observed the
first, November 28, 1828;[302] but his instrument was defective, and the
eclipsing body, Encke's comet, has shown itself otherwise perfectly
translucent. The second case of occultation occurred September 13, 1890,
when an eleventh magnitude star was stated to have completely disappeared
during the transit over it of Denning's comet.[303]

From the failure to detect any effects of refraction in the light of stars
occulted by comets, it was inferred (though, as we know now, erroneously)
that their composition is rather that of dust than that of vapour; that they
consist not of any continuous substance, but of discrete solid particles, very
finely divided and widely scattered. In conformity with this view was the
known smallness of their masses. Laplace had shown that if the amount of
matter forming Lexell's comet had been as much as 1/5000 of that
contained in our globe, the effect of its attraction, on the occasion of its
approach within 1,438,000 miles of the earth, July 1, 1770, must have been
apparent in the lengthening of the year. And that some comets, at any rate,
possess masses immeasurably below this maximum value was clearly
proved by the undisturbed parallel march of the two fragments of Biela's in
1846.

But the discovery in this branch most distinctive of the period under review
is that of "short period" comets, of which four[304] were known in 1850.
These, by the character of their movements, serve as a link between the
planetary and cometary worlds, and by the nature of their construction,
seem to mark a stage in cometary decay. For that comets are rather
transitory agglomerations, than permanent products of cosmical
manufacture, appeared to be demonstrated by the division and
disappearance of one amongst their number, as well as by the singular and
rapid changes in appearance undergone by many, and the seemingly
irrevocable diffusion of their substance visible in nearly all. They might
then be defined, according to the ideas respecting them prevalent fifty years
ago, as bodies unconnected by origin with the solar system, but

encountered, and to some extent appropriated, by it in its progress through
space, owing their visibility in great part, if not altogether, to light reflected
from the sun, and their singular and striking forms to the action of repulsive
forces emanating from him, the penalty of their evanescent splendour being
paid in gradual waste and final dissipation and extinction.

FOOTNOTES:
[241] Allgemeine Geographische Ephemeriden, vol. iv., p. 287.

[242] Astr. Jahrbuch, 1823, p. 217. The period (1,208 days) of this body is
considerably shorter than that of any other known comet.

[243] "Sicut bombyces filo fundendo, sic cometas cauda exspiranda consumi et
denique mori."—De Cometis, Op., vol. vii., p. 110.

[244] Considerable uncertainty, however, still prevails on the point. The inverse
relation assumed by Lagrange to exist between distance from the sun and density
brought out the Mercurian mass 1/2025810 that of the sun (Laplace, Exposition
du Syst. du Monde, t. ii., p. 50, ed. 1824). Von Asten deduced from the
movements of Encke's comet, 1818-48, a value of 1/7636440; while Backlund
from its seven returns, 1871-1891, derived 1/9647000 (Comptes Rendus, Oct. 1,
1894).

[245] Arago, Annuaire (1832), p. 218.

[246] Hind, The Comets, p. 20.

[247] Phil. Trans., vol. xlvi., p. 204.

[248] Astr. Nach., No. 2,134.

[249] Comptes Rendus, t. cvii., p. 588.

[250] Mém. de St. Pétersbourg, t. xxxii., No. 3, 1884; Astr. Nach., No. 2,727.

[251] Month. Not., vol. xix., p. 72.
[252] Mécanique Céleste, t. ii., p. 197.

[253] See Berberich, Astr. Nach., Nos. 2,836-7, 3,125; Deichmüller, Ibid., No.
3,123.

[254] Month. Not., vol. ii., p. 117.

[255] Astr. Nach., No. 128.

[256] Annuaire, 1832, p. 186.

[257] Am. Journ. of Science, vol. i. (2nd series), p. 293. Prof. Hubbard's
calculations indicated a probability that the definitive separation of the two
nuclei occurred as early as September 30, 1884. Astronomical Journal (Gould's),
vol. iv., p. 5. See also, on the subject of this comet, W. T. Lynn, Intellectual
Observer, vol. xi., p. 208; E. Ledger, Observatory, August, 1883, p. 244; and H.
A. Newton, Am. Journ. of Science, vol. xxxi., p. 81, February, 1886.

[258] Month. Not., vol. vii., p. 73.

[259] Bulletin Ac. Imp. de St. Pétersbourg, t. vi., col. 77. The latest observation
of the parent nucleus was that of Argelander, April 27, at Bonn.

[260] D'Arrest, Astr. Nach., No. 1,624.

[261] Der Brorsen'sche Comet. Von Dr. E. Lamp, Kiel, 1892; Plummer,
Knowledge, vol. xix., p. 41.

[262] Schulhof, Astr. Nach., No. 3,267; Observatory, vol. xviii., p. 64; F. H.
Seares, Astr. Nach., Nos. 3,606-7; Plummer, Knowledge, vol. xix., p. 156.

[263] Comptes Rendus, t. xxv., p. 570.

[264] Month. Not., vol. xii., p. 248.

[265] Bull. Astr., t. vi., pp. 241, 289.

[266] Étude sur la Théorie des Comètes périodiques. Annales de l'Observatoíre,
t. xx., Paris, 1891.

[267] Amer. Journ. of Science, vol. xlii., pp. 183, 482, 1891.

[268] Observatory, vol. xiv., p. 194.

[269] Phil. Trans., vol. cii., pp. 118-124.

[270] Ueber den Schweif des grossen Cometen von 1811, Monat. Corr., vol. xxv.,
pp. 3-22. Reprinted by Zöllner. Ueber die Natur der Cometen, pp. 3-15.

[271] Natur der Cometen, p. 148.

[272] The subject of a classical memoir by Bessel, published in 1810.

[273] A fresh investigation of its orbit has been published by N. Herz of Vienna.
See Bull. Astr., t. ix., p. 427.

[274] Astr. Jahrbuch (Bode's), 1823, p. 134.

[275] Reproduced in Webb's Celestial Objects, 4th ed.

[276] Month. Not., vol. xxxvi., p. 309.

[277] Celestial Objects, p. 40, note.

[278] See Airy's Address, Mem. R. A. S., vol. x., p. 376. Rosenberger calculated
no more, though he lived until 1890. W. T. Lynn, Observatory, vol. xiii., p. 125.

[279] Hind, The Comets, p. 47.

[280] Arago, Annuaire, 1836, p. 228.

[281] Astr. Nach., No. 300.

[282] It deserves to be recorded that Robert Hooke drew a very similar inference
from his observations of the comets of 1680 and 1682. Month. Not., vol. xiv., pp.
77-83.

[283] Briefwechsel zwischen Olbers und Bessel, Bd. ii., p. 390.

[284] Herschel, Results, p. 405.

[285] Mem. R. A. S., vol. x., p. 92,

[286] Results, p. 401.

[287] Pontécoulant, Comptes Rendus, t. lviii., p. 825.

[288] Annuaire, 1836, p. 233.

[289] Cosmos, vol. i., p. 90, note (Otté's trans.).

[290] Herschel, Outlines of Astronomy, p. 399, 9th ed.

[291] Outlines, p. 398.

[292] Boguslawski calculated that it extended on the 21st of March to 581
millions.—Report. Brit. Ass., 1845, p. 89.

[293] Comptes Rendus, t. xvi., p. 919.

[294] Observatory, vol. xxiv., p. 167; Astr. Nach., No. 3,320.

[295] Piazzi noticed a considerable increase of lustre in a very faint star of the
twelfth magnitude viewed through a comet. Mädler, Reden, etc., p. 248, note.

[296] Astr. Jahrbuch, 1828, p. 151.

[297] Mädler, Gesch. d. Astr., Bd. ii., p. 412.

[298] Recueil de l'Ac. Imp. de St. Pétersbourg, 1835, p. 143.

[299] Guillemin's World of Comets, trans, by J. Glaisher, p. 294, note.

[300] Month. Not., vol. viii., p. 9.

[301] A real, though only partial stoppage of light seems indicated by Herschel's
observations on the comet of 1807. Stars seen through the tail, October 18, lost
much of their lustre. One near the head was only faintly visible by glimpses.
Phil. Trans., vol. xcvii., p. 153.

[302] Arago, Annuaire, 1832, p. 205.

[303] Ibid., 1891, p. 290.

[304] Viz., Encke's, Biela's, Faye's, and Brorsen's.

CHAPTER VI
INSTRUMENTAL ADVANCES

It is impossible to follow with intelligent interest the course of astronomical
discovery without feeling some curiosity as to the means by which such
surpassing results have been secured. Indeed, the bare acquaintance with
what has been achieved, without any corresponding knowledge of how it
has been achieved, supplies food for barren wonder rather than for fruitful
and profitable thought. Ideas advance most readily along the solid ground
of practical reality, and often find true sublimity while laying aside empty
marvels. Progress is the result, not so much of sudden flights of genius, as
of sustained, patient, often commonplace endeavour; and the true lesson of
scientific history lies in the close connection which it discloses between the
most brilliant developments of knowledge and the faithful accomplishment
of his daily task by each individual thinker and worker.

It would be easy to fill a volume with the detailed account of the long
succession of optical and mechanical improvements by means of which the
observation of the heavens has been brought to its present degree of
perfection; but we must here content ourselves with a summary sketch of
the chief amongst them. The first place in our consideration is naturally
claimed by the telescope.

This marvellous instrument, we need hardly remind our readers, is of two
distinct kinds—that in which light is gathered together into a focus by
refraction, and that in which the same end is attained by reflection. The
image formed is in each case viewed through a magnifying lens, or
combination of lenses, called the eye-piece. Not for above a century after
the "optic glasses" invented or stumbled upon by the spectacle-maker of
Middelburg (1608) had become diffused over Europe, did the reflecting
telescope come, even in England, the place of its birth, into general use. Its

principle (a sufficiently obvious one) had indeed been suggested by
Mersenne as early as 1639;[305] James Gregory in 1663[306] described in
detail a mode of embodying that principle in a practical shape; and Newton,
adopting an original system of construction, actually produced in 1668 a
tiny speculum, one inch across, by means of which the apparent distance of
objects was reduced thirty-nine times. Nevertheless, the exorbitantly long
tubeless refractors, introduced by Huygens, maintained their reputation
until Hadley exhibited to the Royal Society, January 12, 1721,[307] a
reflector of six inches aperture, and sixty-two in focal length, which rivalled
in performance, and of course indefinitely surpassed in manageability, one
of the "aerial" kind of 123 feet.

The concave-mirror system now gained a decided ascendant, and was
brought to unexampled perfection by James Short of Edinburgh during the
years 1732-68. Its resources were, however, first fully developed by
William Herschel. The energy and inventiveness of this extraordinary man
marked an epoch wherever they were applied. His ardent desire to measure
and gauge the stupendous array of worlds which his specula revealed to
him, made him continually intent upon adding to their "space-penetrating
power" by increasing their light-gathering surface. These, as he was the first
to explain,[308] are in a constant proportion one to the other. For a telescope
with twice the linear aperture of another will collect four times as much
light, and will consequently disclose an object four times as faint as could
be seen with the first, or, what comes to the same, an object equally bright
at twice the distance. In other words, it will possess double the space-
penetrating power of the smaller instrument. Herschel's great mirrors—the
first examples of the giant telescopes of modern times—were then primarily
engines for extending the bounds of the visible universe; and from the
sublimity of this "final cause" was derived the vivid enthusiasm which
animated his efforts to success.

It seems probable that the seven-foot telescope constructed by him in 1775
—that is within little more than a year after his experiments in shaping and

polishing metal had begun—already exceeded in effective power any work
by an earlier optician; and both his skill and his ambition rapidly developed.
His efforts culminated, after mirrors of ten, twenty, and thirty feet focal
length had successively left his hands, in the gigantic forty-foot, completed
August 28, 1789. It was the first reflector in which only a single mirror was
employed. In the "Gregorian" form, the focussed rays are, by a second
reflection from a small concave[309] mirror, thrown straight back through a
central aperture in the larger one, behind which the eye-piece is fixed. The
object under examination is thus seen in the natural direction. The
"Newtonian," on the other hand, shows the object in a line of sight at right
angles to the true one, the light collected by the speculum being diverted to
one side of the tube by the interposition of a small plane mirror, situated at
an angle of 45° to the axis of the instrument. Upon these two systems
Herschel worked until 1787, when, becoming convinced of the supreme
importance of economising light (necessarily wasted by the second
reflection), he laid aside the small mirror of his forty-foot then in course of
construction, and turned it into a "front-view" reflector. This was done—
according to the plan proposed by Lemaire in 1732—by slightly inclining
the speculum so as to enable the image formed by it to be viewed with an
eye-glass fixed at the upper margin of the tube. The observer thus stood
with his back turned to the object he was engaged in scrutinising.

The advantages of the increased brilliancy afforded by this modification
were strikingly illustrated by the discovery, August 28 and September 17,
1789, of the two Saturnian satellites nearest the ring. Nevertheless, the
monster telescope of Slough cannot be said to have realised the sanguine
expectations of its constructor. The occasions on which it could be usefully
employed were found to be extremely rare. It was injuriously affected by
every change of temperature. The great weight (25 cwt.) of a speculum four
feet in diameter rendered it peculiarly liable to distortion. With all
imaginable care, the delicate lustre of its surface could not be preserved
longer than two years,[310] when the difficult process of repolishing had to
be undertaken. It was accordingly never used after 1811, when, having gone

blind from damp, it lapsed by degrees into the condition of a museum
inmate.

The exceedingly high magnifying powers employed by Herschel
constituted a novelty in optical astronomy, to which he attached great
importance. The work of ordinary observation would, however, be hindered
rather than helped by them. The attempt to increase in this manner the
efficacy of the telescope is speedily checked by atmospheric, to say nothing
of other difficulties. Precisely in the same proportion as an object is
magnified, the disturbances of the medium through which it is seen are
magnified also. Even on the clearest and most tranquil nights, the air is
never for a moment really still. The rays of light traversing it are continually
broken by minute fluctuations of refractive power caused by changes of
temperature and pressure, and the currents which these engender. With such
luminous quiverings and waverings the astronomer has always more or less
to reckon; their absence is simply a question of degree; if sufficiently
magnified, they are at all times capable of rendering observation
impossible.

Thus, such powers as 3,000, 4,000, 5,000, even 6,652,[311] which Herschel
now and again applied to his great telescopes, must, save on the rarest
occasions, prove an impediment rather than an aid to vision. They were,
however, used by him only for special purposes, experimentally, not
systematically, and with the clearest discrimination of their advantages and
drawbacks. It is obvious that perfectly different ends are subserved by
increasing the aperture and by increasing the power of a telescope. In the
one case, a larger quantity of light is captured and concentrated; in the
other, the same amount is distributed over a wider area. A diminution of
brilliancy in the image accordingly attends, cœteris paribus, upon each
augmentation of its apparent size. For this reason, such faint objects as
nebulæ are most successfully observed with moderate powers applied to
instruments of a great capacity for light, the details of their structure
actually disappearing when highly magnified. With stellar groups the
reverse is the case. Stars cannot be magnified, simply because they are too

remote to have any sensible dimensions; but the space between them can. It
was thus for the purpose of dividing very close double stars that Herschel
increased to such an unprecedented extent the magnifying capabilities of his
instruments; and to this improvement incidentally the discovery of Uranus,
March 13, 1781,[312] was due. For by the examination with strong lenses of
an object which, even with a power of 227, presented a suspicious
appearance, he was able at once to pronounce its disc to be real, not merely
"spurious," and so to distinguish it unerringly from the crowd of stars
amidst which it was moving.

While the reflecting telescope was astonishing the world by its rapid
development in the hands of Herschel, its unpretending rival was slowly
making its way towards the position which the future had in store for it. The
great obstacle which long stood in the way of the improvement of refractors
was the defect known as "chromatic aberration." This is due to no other
cause than that which produces the rainbow and the spectrum—the
separation, or "dispersion" in their passage through a refracting medium, of
the variously coloured rays composing a beam of white light. In an ordinary
lens there is no common point of concentration; each colour has its own
separate focus; and the resulting image, formed by the superposition of as
many images as there are hues in the spectrum, is indefinitely terminated
with a tinted border, eminently baffling to exactness of observation.

The extravagantly long telescopes of the seventeenth century were designed
to avoid this evil (as well as another source of indistinct vision in the
spherical shape of lenses); but no attempt to remedy it was made until an
Essex gentleman succeeded, in 1733, in so combining lenses of flint and
crown glass as to produce refraction without colour.[313] Mr. Chester More
Hall was, however, equally indifferent to fame and profit, and took no pains
to make his invention public. The effective discovery of the achromatic
telescope was, accordingly, reserved for John Dollond, whose method of
correcting at the same time chromatic and spherical aberration was laid
before the Royal Society in 1758. Modern astronomy may be said to have

been thereby rendered possible. Refractors have always been found better
suited than reflectors to the ordinary work of observatories. They are, so to
speak, of a more robust, as well as of a more plastic nature. They suffer less
from vicissitudes of temperature and climate. They retain their efficiency
with fewer precautions and under more trying circumstances. Above all,
they co-operate more readily with mechanical appliances, and lend
themselves with far greater facility to purposes of exact measurement.

A practical difficulty, however, impeded the realisation of the brilliant
prospects held out by Dollond's invention. It was found impossible to
procure flint-glass, such as was needed for optical use—that is, of perfectly
homogeneous quality—except in fragments of insignificant size. Discs of
more than two or three inches in diameter were of extreme rarity; and the
crushing excise duty imposed upon the article by the financial unwisdom of
the Government, both limited its production, and, by rendering experiments
too costly for repetition, barred its improvement.

Up to this time, Great Britain had left foreign competitors far behind in the
instrumental department of astronomy. The quadrants and circles of Bird,
Cary and Ramsden were unapproached abroad. The reflecting telescope
came into existence and reached maturity on British soil. The refracting
telescope was cured of its inherent vices by British ingenuity. But with the
opening of the nineteenth century, the almost unbroken monopoly of skill
and contrivance which our countrymen had succeeded in establishing was
invaded, and British workmen had to be content to exchange a position of
supremacy for one of at least partial temporary inferiority.

Somewhat about the time that Herschel set about polishing his first
speculum, Pierre Louis Guinand, a Swiss artisan, living near Chaux-de-
Fonds, in the canton of Neuchâtel, began to grind spectacles for his own
use, and was thence led on to the rude construction of telescopes by fixing
lenses in pasteboard tubes. The sight of an England achromatic stirred a
higher ambition, and he took the first opportunity of procuring some flint
glass from England (then the only source of supply), with the design of

imitating an instrument the full capabilities of which he was destined to be
the humble means of developing. The English glass proving of inferior
quality, he conceived the possibility, unaided and ignorant of the art as he
was, of himself making better, and spent seven years (1784-90) in fruitless
experiments directed to that end. Failure only stimulated him to enlarge
their scale. He bought some land near Les Brenets, constructed upon it a
furnace capable of melting two quintals of glass, and reducing himself and
his family to the barest necessaries of life, he poured his earnings (he at this
time made bells for repeaters) unstintingly into his crucibles.[314] His
undaunted resolution triumphed. In 1799 he carried to Paris and there
showed to Lalande several discs of flawless crystal four to six inches in
diameter. Lalande advised him to keep his secret, but in 1805 he was
induced to remove to Munich, where he became the instructor of the
immortal Fraunhofer. His return to Les Brenets in 1814 was signalised by
the discovery of an ingenious mode of removing striated portions of glass
by breaking and re-soldering the product of each melting, and he eventually
attained to the manufacture of perfect discs up to 18 inches in diameter. An
object-glass for which he had furnished the material to Cauchoix, procured
him, in 1823, a royal invitation to settle in Paris; but he was no longer equal
to the change, and died at the scene of his labours, February 13 following.

This same lens (12 inches across) was afterwards purchased by Sir James
South, and the first observation made with it, February 13, 1830, disclosed
to Sir John Herschel the sixth minute star in the central group of the Orion
nebula, known as the "trapezium."[315] Bequeathed by South to Trinity
College, Dublin, it was employed at the Dunsink Observatory by Brünnow
and Ball in their investigations of stellar parallax. A still larger objective (of
nearly 14 inches) made of Guinand's glass was secured in Paris, about the
same time, by Mr. Edward Cooper of Markree Castle, Ireland. The
peculiarity of the method discovered at Les Brenets resided in the
manipulation, not in the quality of the ingredients; the secret, that is to say,
was not chemical, but mechanical.[316] It was communicated by Henry
Guinand (a son of the inventor) to Bontemps, one of the directors of the

glassworks at Choisy-le-Roi, and by him transmitted to Messrs. Chance of
Birmingham, with whom he entered into partnership when the
revolutionary troubles of 1848 obliged him to quit his native country. The
celebrated American opticians, Alvan Clark & Sons, derived from the
Birmingham firm the materials for some of their early telescopes, notably
the 19-inch Chicago and 26-inch Washington equatoreals; but the discs for
the great Lick refractor, and others shaped by them in recent years, have
been supplied by Feil of Paris.

Two distinguished amateurs, meanwhile, were preparing to reassert on
behalf of reflecting instruments their claim to the place of honour in the van
of astronomical discovery. Of Mr. Lassell's specula something has already
been said.[317] They were composed of an alloy of copper and tin, with a
minute proportion of arsenic (after the example of Newton[318]), and were
remarkable for perfection of figure and brilliancy of surface.

The capabilities of the Newtonian plan were developed still more fully—it
might almost be said to the uttermost—by the enterprise of an Irish
nobleman. William Parsons, known as Lord Oxmantown until 1841, when,
on his father's death, he succeeded to the title of Earl of Rosse, was born at
York, June 17, 1800. His public duties began before his education was
completed. He was returned to Parliament as member for King's County
while still an undergraduate at Oxford, and continued to represent the same
constituency for thirteen years (1821-34). From 1845 until his death, which
took place, October 31, 1867, he sat, silent but assiduous, in the House of
Lords as an Irish representative peer; he held the not unlaborious post of
President of the Royal Society from 1849 to 1854; presided over the
meeting of the British Association at Cork in 1843, and was elected Vice-
Chancellor of Dublin University in 1862. In addition to these extensive
demands upon his time and thoughts, were those derived from his position
as practically the feudal chief of a large body of tenantry in times of great
and anxious responsibility, to say nothing of the more genial claims of an
unstinted hospitality. Yet, while neglecting no public or private duty, this

model nobleman found leisure to render to science services so conspicuous
as to entitle his name to a lasting place in its annals. He early formed the
design of reaching the limits of the attainable in enlarging the powers of the
telescope, and the qualities of his mind conspired with the circumstances of
his fortune to render the design a feasible one. From refractors it was
obvious that no such vast and rapid advance could be expected. English
glass-manufacture was still in a backward state. So late as 1839, Simms
(successor to the distinguished instrumentalist Edward Troughton) reported
a specimen of crystal scarcely 7-1/2 inches in diameter, and perfect only
over six, to be unique in the history of English glass-making.[319] Yet at that
time the fifteen-inch achromatic of Pulkowa had already left the workshop
of Fraunhofer's successors at Munich. It was not indeed until 1845, when
the impost which had so long hampered their efforts was removed, that the
optical artists of these islands were able to compete on equal terms with
their rivals on the Continent. In the case of reflectors, however, there
seemed no insurmountable obstacle to an almost unlimited increase of light-
gathering capacity; and it was here, after some unproductive experiments
with fluid lenses, that Lord Oxmantown concentrated his energies.

He had to rely entirely on his own invention, and to earn his own
experience. James Short had solved the problem of giving to metallic
surfaces a perfect parabolic figure (the only one by which parallel incident
rays can be brought to an exact focus); but so jealous was he of his secret,
that he caused all his tools to be burnt before his death;[320] nor was
anything known of the processes by which Herschel had achieved his
astonishing results. Moreover, Lord Oxmantown had no skilled workmen to
assist him. His implements, both animate and inanimate, had to be formed
by himself. Peasants taken from the plough were educated by him into
efficient mechanics and engineers. The delicate and complex machinery
needed in operations of such hairbreadth nicety as his enterprise involved,
the steam-engine which was to set it in motion, at times the very crucibles
in which his specula were cast, issued from his own workshops.

In 1827 experiments on the composition of speculum-metal were set on
foot, and the first polishing-machine ever driven by steam-power was
contrived in 1828. But twelve arduous years of struggle with recurring
difficulties passed before success began to dawn. A material less tractable
than the alloy selected, of four chemical equivalents of copper to one of tin,
[321] can scarcely be conceived. It is harder than steel, yet brittle as glass,
crumbling into fragments with the slightest inadvertence of handling or
treatment;[322] and the precision of figure requisite to secure good definition
is almost beyond the power of language to convey. The quantities involved
are so small as not alone to elude sight, but to confound imagination. Sir
John Herschel tells us that "the total thickness to be abraded from the edge
of a spherical speculum 48 inches in diameter and 40 feet focus, to convert
it into a paraboloid, is only 1/21333 of an inch;"[323] yet upon this minute
difference of form depends the clearness of the image, and, as a
consequence, the entire efficiency of the instrument. "Almost infinite,"
indeed (in the phrase of the late Dr. Robinson), must be the exactitude of
the operation adapted to bring about so delicate a result.

At length, in 1839, two specula, each three feet in diameter, were turned out
in such perfection as to prompt a still bolder experiment. The various
processes needed to insure success were now ascertained and under control;
all that was necessary was to repeat them on a larger scale. A gigantic
mirror, six feet across and fifty-four in focal length, was accordingly cast on
the 13th of April, 1842; in two months it was ground down to figure by
abrasion with emery and water, and daintily polished with rouge; and by the
month of February, 1845, the "leviathan of Parsonstown" was available for
the examination of the heavens.

The suitable mounting of this vast machine was a problem scarcely less
difficult than its construction. The shape of a speculum needs to be
maintained with an elaborate care equal to that used in imparting it. In fact,
one of the most formidable obstacles to increasing the size of such
reflecting surfaces consists in their liability to bend under their own weight.

That of the great Rosse speculum was no less than four tons. Yet, although
six inches in thickness, and composed of a material only a degree inferior in
rigidity to wrought iron, the strong pressure of a man's hand at its back
produced sufficient flexure to distort perceptibly the image of a star
reflected in it.[324] Thus the delicacy of its form was perishable equally by
the stress of its own gravity, and by the slightest irregularity in the means
taken to counteract that stress. The problem of affording a perfectly equable
support in all possible positions was solved by resting the speculum upon
twenty-seven platforms of cast iron, felt-covered, and carefully fitted to the
shape of the areas they were to carry, which platforms were themselves
borne by a complex system of triangles and levers, ingeniously adapted to
distribute the weight with complete uniformity.[325]

A tube which resembled, when erect, one of the ancient round towers of
Ireland,[326] served as the habitation of the great mirror. It was constructed
of deal staves bound together with iron hoops, was fifty-eight feet long
(including the speculum-box), and seven in diameter. A reasonably tall man
may walk through it (as Dean Peacock once did) with umbrella uplifted.
Two piers of solid masonry, about fifty feet high, seventy long, and twenty-
three apart, flanked the huge engine on either side. Its lower extremity
rested on a universal joint of cast iron; above, it was slung in chains, and
even in a gale of wind remained perfectly steady. The weight of the entire,
although amounting to fifteen tons, was so skilfully counterpoised, that the
tube could with ease be raised or depressed by two men working a
windlass. Its horizontal range was limited by the lofty walls erected for its
support to about ten degrees on each side of the meridian; but it moved
vertically from near the horizon through the zenith as far as the pole. Its
construction was of the Newtonian kind, the observer looking into the side
of the tube near its upper end, which a series of galleries and sliding stages
enabled him to reach in any position. It has also, though rarely, been used
without a second mirror, as a "Herschelian" reflector.

The splendour of the celestial objects as viewed with this vast "light-
grasper" surpassed all expectation. "Never in my life," exclaimed Sir James
South, "did I see such glorious sidereal pictures."[327] The orb of Jupiter
produced an effect compared to that of the introduction of a coach-lamp
into the telescope;[328] and certain star-clusters exhibited an appearance (we
again quote Sir James South) "such as man before had never seen, and
which for its magnificence baffles all description." But it was in the
examination of the nebulæ that the superiority of the new instrument was
most strikingly displayed. A large number of these misty objects, which the
utmost powers of Herschel's specula had failed to resolve into stars, yielded
at once to the Parsonstown reflector; while many others showed under
entirely changed forms through the disclosure of previously unseen details
of structure.

One extremely curious result of the increase of light was the abolition of
any sharp distinction between the two classes of "annular" and "planetary"
nebulæ. Up to that time, only four ring-shaped systems—two in the
northern and two in the southern hemisphere—were known to astronomers;
they were now reinforced by five of the planetary kind, the discs of which
were observed to be centrally perforated; while the definite margins visible
in weaker instruments were replaced by ragged edges or filamentous
fringes.

Still more striking was the discovery of an entirely new and most
remarkable species of nebulæ. These were termed "spiral," from the more
or less regular convolutions, resembling the whorls of a shell, in which the
matter composing them appeared to be distributed. The first and most
conspicuous specimen of this class was met with in April, 1845; it is
situated in Canes Venatici, close to the tail of the Great Bear, and wore, in
Sir J. Herschel's instruments, the aspect of a split ring encompassing a
bright nucleus, thus presenting, as he supposed, a complete analogue to the
system of the Milky Way. In the Rosse mirror it shone out as a vast
whirlpool of light—a stupendous witness to the presence of cosmical

activities on the grandest scale, yet regulated by laws as to the nature of
which we are profoundly ignorant. Professor Stephen Alexander of New
Jersey, however, concluded, from an investigation (necessarily founded on
highly precarious data) of the mechanical condition of these extraordinary
agglomerations, that we see in them "the partially scattered fragments of
enormous masses once rotating in a state of dynamical equilibrium." He
further suggested "that the separation of these fragments may still be in
progress,"[329] and traced back their origin to the disruption, through its own
continually accelerated rotation, of a "primitive spheroid" of inconceivably
vast dimensions. Such also, it was added (the curvilinear form of certain
outliers of the Milky Way giving evidence of a spiral structure), is probably
the history of our own cluster; the stars composing which, no longer held
together in a delicately adjusted system like that of the sun and planets, are
advancing through a period of seeming confusion towards an appointed
goal of higher order and more perfect and harmonious adaptation.[330]

The class of spiral nebulæ included, in 1850, fourteen members, besides
several in which the characteristic arrangement seemed partial or dubious.
[331] A tendency in the exterior stars of other clusters to gather into curved
branches (as in our Galaxy) was likewise noted; and the existence of
unsuspected analogies was proclaimed by the significant combination in the
"Owl" nebula (a large planetary in Ursa Major)[332] of the twisted forms of
a spiral with the perforated effect distinctive of an annular nebula. Once
more, by the achievements of the Parsonstown reflector, the supposition of
a "shining fluid" filling vast regions of space was brought into (as it has
since proved) undeserved discredit. Although Lord Rosse himself rejected
the inference, that because many nebulæ had been resolved, all were
resolvable, very few imitated his truly scientific caution; and the results of
Bond's investigations[333] with the Harvard College refractor quickened and
strengthened the current of prevalent opinion. It is now certain that the
evidence furnished on both sides of the Atlantic as to the stellar
composition of some conspicuous objects of this class (notably the Orion
and "Dumb-bell" nebulæ) was delusive; but the spectroscope alone was

capable of meeting it with a categorical denial. Meanwhile there seemed
good ground for the persuasion, which now, for the last time, gained the
upper hand, that nebulæ are, without exception, true "island-universes," or
assemblages of distant suns.

Lord Rosse's telescope possesses a nominal power of 6,000—that is, it
shows the moon as if viewed with the naked eye at a distance of forty miles.
But this seeming advantage is neutralised by the weakening of the available
light through excessive diffusion, as well as by the troubles of the surging
sea of air through which the observation must necessarily be made.
Professor Newcomb, in fact, doubts whether with any telescope our satellite
has ever been seen to such advantage as it would be if brought within 500
miles of the unarmed eye.[334]

The French opticians' rule of doubling the number of millimetres contained
in the aperture of an instrument to find the highest magnifying power
usually applicable to it, would give 3,600 as the maximum for the leviathan
of Birr Castle; but in a climate like that of Ireland the occasions must be
rare when even that limit can be reached. Indeed, the experience acquired
by its use plainly shows that atmospheric rather than mechanical difficulties
impede a still further increase of telescopic power. Its construction may
accordingly be said to mark the ne plus ultra of effort in one direction, and
the beginning of its conversion towards another. It became thenceforward
more and more obvious that the conditions of observation must be
ameliorated before any added efficacy could be given to it. The full effect
of an uncertain climate in nullifying optical improvements was recognised,
and the attention of astronomers began to be turned towards the advantages
offered by more tranquil and more translucent skies.

Scarcely less important for the practical uses of astronomy than the optical
qualities of the telescope is the manner of its mounting. The most admirable
performance of the optician can render but unsatisfactory service if its
mechanical accessories are ill-arranged or inconvenient. Thus the
astronomer is ultimately dependent upon the mechanician; and so

excellently have his needs been served, that the history of the ingenious
contrivances by which discoveries have been prepared would supply a
subject (here barely glanced at) not far inferior in extent and instruction to
the history of those discoveries themselves.

There are two chief modes of using the telescope, to which all others may
be considered subordinate.[335] Either it may be invariably directed towards
the south, with no motion save in the plane of the meridian, so as to
intercept the heavenly bodies at the moment of transit across that plain; or it
may be arranged so as to follow the daily revolution of the sky, thus
keeping the object viewed permanently in sight instead of simply noting the
instant of its flitting across the telescopic field. The first plan is that of the
"transit instrument," the second that of the "equatoreal." Both were, by a
remarkable coincidence, introduced about 1690[336] by Olaus Römer, the
brilliant Danish astronomer who first measured the velocity of light.

The uses of each are entirely different. With the transit, the really
fundamental task of astronomy—the determination of the movements of the
heavenly bodies—is mainly accomplished; while the investigation of their
nature and peculiarities is best conducted with the equatoreal. One is the
instrument of mathematical, the other of descriptive astronomy. One
furnishes the materials with which theories are constructed and the tests by
which they are corrected; the other registers new facts, takes note of new
appearances, sounds the depths and peers into every nook of the heavens.

The great improvement of giving to a telescope equatoreally mounted an
automatic movement by connecting it with clockwork, was proposed in
1674 by Robert Hooke. Bradley in 1721 actually observed Mars with a
telescope "moved by a machine that made it keep pace with the stars;"[337]
and Von Zach relates[338] that he had once followed Sirius for twelve hours
with a "heliostat" of Ramsden's construction. But these eighteenth-century
attempts were of no practical effect. Movement by clockwork was virtually
a complete novelty when it was adopted by Fraunhofer in 1824 to the
Dorpat refractor. By simply giving to an axis unvaryingly directed towards

the celestial pole an equable rotation with a period of twenty-four hours, a
telescope attached to it, and pointed in any direction, will trace out on the
sky a parallel of declination, thus necessarily accompanying the movement
of any star upon which it may be fixed. It accordingly forms part of the
large sum of Fraunhofer's merits to have secured this inestimable advantage
to observers.

Sir John Herschel considered that Lassell's application of equatoreal
mounting to a nine-inch Newtonian in 1840 made an epoch in the history of
"that eminently British instrument, the reflecting telescope."[339] Nearly a
century earlier,[340] it is true, Short had fitted one of his Gregorians to a
complicated system of circles in such a manner that, by moving a handle, it
could be made to follow the revolution of the sky; but the arrangement did
not obtain, nor did it deserve, general adoption. Lassell's plan was a totally
different one; he employed the crossed axes of the true equatoreal, and his
success removed, to a great extent, the fatal objection of inconvenience in
use, until then unanswerably urged against reflectors. The very largest of
these can now be mounted equatoreally; even the Rosse, within its limited
range, has been for some years provided with a movement by clockwork
along declination-parallels.

The art of accurately dividing circular arcs into the minute equal parts
which serve as the units of astronomical measurement, remained, during the
whole of the eighteenth century, almost exclusively in English hands. It was
brought to a high degree of perfection by Graham, Bird and Ramsden, all of
whom, however, gave the preference to the old-fashioned mural quadrant
and zenith-sector over the entire circle, which Römer had already found the
advantage of employing. The five-foot vertical circle, which Piazzi with
some difficulty induced Ramsden to complete for him in 1789, was the first
divided instrument constructed in what may be called the modern style. It
was provided with magnifiers for reading off the divisions (one of the
neglected improvements of Römer), and was set up above a smaller
horizontal circle, forming an "altitude and azimuth" combination (again

Römer's invention), by which both the elevation of a celestial object above
the horizon and its position as referred to the horizon could be measured. In
the same year, Borda invented the "repeating circle" (the principle of which
had been suggested by Tobias Mayer in 1756[341]), a device for
exterminating, so far as possible, errors of graduation by repeating an
observation with different parts of the limb. This was perhaps the earliest
systematic effort to correct the imperfections of instruments by the manner
of their use.

The manufacture of astronomical circles was brought to a very refined state
of excellence early in the nineteenth century by Reichenbach at Munich,
and after 1818 by Repsold at Hamburg. Bessel states[342] that the "reading-
off" on an instrument of the kind by the latter artist was accurate to about
1/80th of a human hair. Meanwhile the traditional reputation of the English
school was fully sustained; and Sir George Airy did not hesitate to express
his opinion that the new method of graduating circles, published by
Troughton in 1809,[343] was the "greatest improvement ever made in the art
of instrument-making."[344] But a more secure road to improvement than
that of mere mechanical exactness was pointed out by Bessel. His
introduction of a regular theory of instrumental errors might almost be said
to have created a new art of observation. Every instrument, he declared in
memorable words,[345] must be twice made—once by the artist, and again
by the observer. Knowledge is power. Defects that are ascertained and can
be allowed for are as good as non-existent. Thus the truism that the best
instrument is worthless in the hands of a careless or clumsy observer,
became supplemented by the converse maxim, that defective appliances
may, through skilful use, be made to yield valuable results. The Königsberg
observations—of which the first instalment was published in 1815—set the
example of regular "reduction" for instrumental errors. Since then, it has
become an elementary part of an astronomer's duty to study the
idiosyncrasy of each one of the mechanical contrivances at his disposal, in
order that its inevitable, but now certified deviations from ideal accuracy
may be included amongst the numerous corrections by which the pure

essence of even approximate truth is distilled from the rude impressions of
sense.

Nor is this enough; for the casual circumstances attending each observation
have to be taken into account with no less care than the inherent or
constitutional peculiarities of the instrument with which it is made. There is
no "once for all" in astronomy. Vigilance can never sleep; patience can
never tire. Variable as well as constant sources of error must be anxiously
heeded; one infinitesimal inaccuracy must be weighed against another; all
the forces and vicissitudes of nature—frosts, dews, winds, the interchanges
of heat, the disturbing effects of gravity, the shiverings of the air, the
tremors of the earth, the weight and vital warmth of the observer's own
body, nay, the rate at which his brain receives and transmits its impressions,
must all enter into his calculations, and be sifted out from his results.

It was in 1823 that Bessel drew attention to discrepancies in the times of
transits given by different astronomers.[346] The quantities involved were far
from insignificant. He was himself nearly a second in advance of all his
contemporaries, Argelander lagging behind him as much as a second and a
quarter. Each individual, in fact, was found to have a certain definite rate of
perception, which, under the name of "personal equation," now forms so
important an element in the correction of observations that a special
instrument for accurately determining its amount in each case is in actual
use at Greenwich.

Such are the refinements upon which modern astronomy depends for its
progress. It is a science of hairbreadths and fractions of a second. It exists
only by the rigid enforcement of arduous accuracy and unwearying
diligence. Whatever secrets the universe still has in store for man will only
be communicated on these terms. They are, it must be acknowledged,
difficult to comply with. They involve an unceasing struggle against the
infirmities of his nature and the instabilities of his position. But the end is
not unworthy the sacrifices demanded. One additional ray of light thrown
on the marvels of creation—a single, minutest encroachment upon the

strongholds of ignorance—is recompense enough for a lifetime of toil. Or
rather, the toil is its own reward, if pursued in the lofty spirit which alone
becomes it. For it leads through the abysses of space and the unending
vistas of time to the very threshold of that infinity and eternity of which the
disclosure is reserved for a life to come.

FOOTNOTES:
[305] Grant, Hist. Astr., p. 527.

[306] Optica Promota, p. 93.

[307] Phil. Trans., vol. xxxii., p. 383.

[308] Ibid., vol. xc., p. 65.

[309] Cassegrain, a Frenchman, substituted in 1672 a convex for a concave
secondary speculum. The tube was thereby enabled to be shortened by twice the
focal length of the mirror in question. The great Melbourne reflector (four feet
aperture, by Grubb) is constructed upon this plan.

[310] Phil. Trans., vol. civ., p. 275, note.

[311] Phil. Trans., vol. xc., p. 70. With the forty-foot, however, only very
moderate powers seemed to have been employed, whence Dr. Robinson argued a
deficiency of defining power. Proc. Roy. Irish Ac., vol. ii., p. 11.

[312] Phil. Trans., vol. lxxi., p. 492.

[313] It is remarkable that, as early as 1695, the possibility of an achromatic
combination was inferred by David Gregory from the structure of the human
eye. See his Catoptricæ et Dioptricæ Sphericæ Elementa, p. 98.

[314] Wolf, Biographien, Bd. ii., p. 301.

[315] Month. Not., vol. i., p. 153. note.

[316] Henrivaux, Encyclopédie Chimique, t. v., fasc. 5, p. 363.

[317] See ante, p. 83.

[318] Phil. Trans., vol. vii., p. 4007.

[319] J. Herschel, The Telescope, p. 39.

[320] Month. Not., vol. xxix., p. 125.

[321] A slight excess of copper renders the metal easier to work, but liable to
tarnish. Robinson, Proc. Roy. Irish Ac., vol. ii., p. 4.

[322] Brit. Ass., 1843, Dr. Robinson's closing Address. Athenæum, Sept. 23, p.
866.

[323] The Telescope, p. 82.

[324] Lord Rosse in Phil. Trans., vol. cxl., p. 302.

[325] This method is the same in principle with that applied by Grubb in 1834 to
a 15-inch speculum for the observatory of Armagh. Phil. Trans., vol. clix., p.
145.

[326] Robinson, Proc. Roy. Ir. Ac., vol. iii., p. 120.

[327] Astr. Nach., No. 536.

[328] Airy, Month. Not., vol. ix., p. 120.

[329] Astronomical Journal (Gould's), vol. ii., p. 97.

[330] Ibid., p. 160.

[331] Lord Rosse in Phil. Trans., vol. cxl., p. 505.

[332] No. 2343 of Herschel's (1864) Catalogue. Before 1850 a star was visible in
each of the two larger openings by which it is pierced; since then, one only.
Webb, Celestial Objects (4th ed.), p. 409.

[333] Mem. Am. Ac., vol. iii., p. 87; Astr. Nach., No. 611.

[334] Pop. Astr., p. 145.

[335] This statement must be taken in the most general sense. Supplementary
observations of great value are now made at Greenwich with the altitude and
azimuth instrument, which likewise served Piazzi to determine the places of his
stars; while a "prime vertical instrument" is prominent at Pulkowa.

[336] As early as 1620, according to R. Wolf (Ges. der Astr., p. 587), Father
Scheiner made the experiment of connecting a telescope with an axis directed to
the pole, while Chinese "equatoreal armillæ," dating from the thirteenth century,
existed at Pekin until 1900, when they were carried off as "loot" to Berlin. J. L.
E. Dreyer, Copernicus, vol. i., p. 134.

[337] Miscellaneous Works, p. 350.

[338] Astr. Jahrbuch, 1799 (published 1796), p. 115.

[339] Month. Not., vol. xli., p. 189.

[340] Phil. Trans., vol. xlvi., p. 242.

[341] Grant, Hist. of Astr., p. 487.

[342] Pop. Vorl., p. 546.

[343] Phil. Trans., vol. xcix., p. 105.

[344] Report Brit. Ass., 1832, p. 132.

[345] Pop. Vorl., p. 432.

[346] C. T. Anger, Grundzüge der neucren astronomischen Beobachtungs-Kunst,
p. 3.

PART II
RECENT PROGRESS OF ASTRONOMY

CHAPTER I
FOUNDATION OF ASTRONOMICAL PHYSICS

In the year 1826, Heinrich Schwabe of Dessau, elated with the hope of
speedily delivering himself from the hereditary incubus of an apothecary's
shop,[347] obtained from Munich a small telescope and began to observe the
sun. His choice of an object for his researches was instigated by his friend
Harding of Göttingen. It was a peculiarly happy one. The changes visible in
the solar surface were then generally regarded as no less capricious than the
changes in the skies of our temperate regions. Consequently, the reckoning
and registering of sun-spots was a task hardly more inviting to an
astronomer than the reckoning and registering of summer clouds. Cassini,
Keill, Lemonnier, Lalande, were unanimous in declaring that no trace of
regularity could be detected in their appearances or effacements.[348]
Delambre pronounced them "more curious than really useful."[349] Even
Herschel, profoundly as he studied them, and intimately as he was
convinced of their importance as symptoms of solar activity, saw no reason
to suspect that their abundance and scarcity were subject to orderly
alternation. One man alone in the eighteenth century, Christian Horrebow of
Copenhagen, divined their periodical character, and foresaw the time when
the effects of the sun's vicissitudes upon the globes revolving round him
might be investigated with success; but this prophetic utterance was of the
nature of a soliloquy rather than of a communication, and remained hidden
away in an unpublished journal until 1859, when it was brought to light in a
general ransacking of archives.[350]

Indeed, Schwabe himself was far from anticipating the discovery which fell
to his share. He compared his fortune to that of Saul, who, seeking his
father's asses, found a kingdom.[351] For the hope which inspired his early
resolution lay in quite another direction. His patient ambush was laid for a

possible intramercurial planet, which, he thought, must sooner or later
betray its existence in crossing the face of the sun. He took, however, the
most effectual measures to secure whatever new knowledge might be
accessible. During forty-three years his "imperturbable telescope"[352] never
failed, weather and health permitting, to bring in its daily report as to how
many, or if any, spots were visible on the sun's disc, the information
obtained being day by day recorded on a simple and unvarying system. In
1843 he made his first announcement of a probable decennial period,[353]
but it met with no general attention; although Julius Schmidt of Bonn
(afterwards director of the Athens Observatory) and Gautier of Geneva
were impressed with his figures, and Littrow had himself, in 1836,[354]
hinted at the likelihood of some kind of regular recurrence. Schwabe,
however, worked on, gathering each year fresh evidence of a law such as he
had indicated; and when Humboldt published in 1851, in the third volume
of his Kosmos,[355] a table of the sun-spot statistics collected by him from
1826 downwards, the strength of his case was perceived with, so to speak, a
start of surprise; the reality and importance of the discovery were
simultaneously recognised, and the persevering Hofrath of Dessau found
himself famous among astronomers. His merit—recognised by the bestowal
of the Astronomical Society's Gold Medal in 1857—consisted in his choice
of an original and appropriate line of work, and in the admirable tenacity of
purpose with which he pursued it. His resources and acquirements were
those of an ordinary amateur; he was distinguished solely by the
unfortunately rare power of turning both to the best account. He died where
he was born and had lived, April 11, 1875, at the ripe age of eighty-six.

Meanwhile an investigation of a totally different character, and conducted
by totally different means, had been prosecuted to a very similar
conclusion. Two years after Schwabe began his solitary observations,
Humboldt gave the first impulse, at the Scientific Congress of Berlin in
1828, to a great international movement for attacking simultaneously, in
various parts of the globe, the complex problem of terrestrial magnetism.
Through the genius and energy of Gauss, Göttingen became its centre.

Thence new apparatus, and a new system for its employment, issued; there,
in 1833, the first regular magnetic observatory was founded, whilst at
Göttingen was fixed the universal time-standard for magnetic observations.
A letter addressed by Humboldt in April, 1836, to the Duke of Sussex as
President of the Royal Society, enlisted the co-operation of England. A
network of magnetic stations was spread all over the British dominions,
from Canada to Van Diemen's Land; measures were concerted with foreign
authorities, and an expedition was fitted out, under the able command of
Captain (afterwards Sir James) Clark Ross, for the special purpose of
bringing intelligence on the subject from the dismal neighbourhood of the
South Pole. In 1841, the elaborate organisation created by the disinterested
efforts of scientific "agitators" was complete; Gauss's "magnetometers"
were vibrating under the view of attentive observers in five continents, and
simultaneous results began to be recorded.

Ten years later, in September, 1851, Dr. John Lamont, the Scotch director of
the Munich Observatory, in reviewing the magnetic observations made at
Göttingen and Munich from 1835 to 1850, perceived with some surprise
that they gave unmistakable indications of a period which he estimated at
10-1/3 years.[356] The manner in which this periodicity manifested itself
requires a word of explanation. The observations in question referred to
what is called the "declination" of the magnetic needle—that is, to the
position assumed by it with reference to the points of the compass when
moving freely in a horizontal plane. Now this position—as was discovered
by Graham in 1722—is subject to a small daily fluctuation, attaining its
maximum towards the east about 8 A.M., and its maximum towards the
west shortly before 2 P.M. In other words, the direction of the needle
approaches (in these countries at the present time) nearest to the true north
some four hours before noon, and departs farthest from it between one and
two hours after noon. It was the range of this daily variation that Lamont
found to increase and diminish once in every 10-1/3 years.

In the following winter, Sir Edward Sabine, ignorant as yet of Lamont's
conclusion, undertook to examine a totally different set of observations. The

materials in his hands had been collected at the British colonial stations of
Toronto and Hobarton from 1843 to 1848, and had reference, not to the
regular diurnal swing of the needle, but to those curious spasmodic
vibrations, the inquiry into the laws of which was the primary object of the
vast organisation set on foot by Humboldt and Gauss. Yet the upshot was
practically the same. Once in about ten years, magnetic disturbances
(termed by Humboldt "storms") were perceived to reach a maximum of
violence and frequency. Sabine was the first to note the coincidence
between this unlooked-for result and Schwabe's sun-spot period. He showed
that, so far as observation had yet gone, the two cycles of change agreed
perfectly both in duration and phase, maximum corresponding to maximum,
minimum to minimum. What the nature of the connection could be that
bound together by a common law effects so dissimilar as the rents in the
luminous garment of the sun, and the swayings to and fro of the magnetic
needle, was and still remains beyond the reach of well-founded theory; but
the fact was from the first undeniable.

The memoir containing this remarkable disclosure was presented to the
Royal Society, March 18, and read May 6, 1852.[357] On the 31st of July
following, Rudolf Wolf at Berne,[358] and on the 18th of August, Alfred
Gautier at Sion,[359] announced, separately and independently, perfectly
similar conclusions. This triple event is perhaps the most striking instance
of the successful employment of the Baconian method of co-operation in
discovery, by which "particulars" are amassed by one set of investigators—
corresponding to the "Depredators" and "Inoculators" of Solomon's House
—while inductions are drawn from them by another and a higher class—the
"Interpreters of Nature." Yet even here the convergence of two distinct lines
of research was wholly fortuitous, and skilful combination owed the most
brilliant part of its success to the unsought bounty of what we call Fortune.

The exactness of the coincidence thus brought to light was fully confirmed
by further inquiries. A diligent search through the scattered records of sun-
spot observations, from the time of Galileo and Scheiner onwards, put

Wolf[360] in possession of materials by which he was enabled to correct
Schwabe's loosely-indicated decennial period to one of slightly over eleven
(11.11) years; and he further showed that this fell in with the ebb and flow
of magnetic change even better than Lamont's 10-1/3 year cycle. The
analogy was also pointed out between the "light-curve," or zig-zagged line
representing on paper the varying intensity in the lustre of certain stars, and
the similar delineation of spot-frequency; the ascent from minimum to
maximum being, in both cases, usually steeper than the descent from
maximum to minimum; while an additional point of resemblance was
furnished by the irregularities in height of the various maxima. In other
words, both the number of spots on the sun and the brightness of variable
stars increase, as a rule, more rapidly than they decrease; nor does the
amount of that increase, in either instance, show any approach to
uniformity.

The endeavour, suggested by the very nature of the phenomenon, to connect
sun-spots with weather was less successful. The first attempt of the kind
was made by Sir William Herschel in 1801, and a very notable one it was.
Meteorological statistics, save of the scantiest and most casual kind, did not
then exist; but the price of corn from year to year was on record, and this,
with full recognition of its inadequacy, he adopted as his criterion. Nor was
he much better off for information respecting the solar condition. What little
he could obtain, however, served, as he believed, to confirm his surmise
that a copious emission of light and heat accompanies an abundant
formation of "openings" in the dazzling substance whence our supply of
those indispensable commodities is derived.[361] He gathered, in short, from
his inquiries very much what he had expected to gather, namely, that the
price of wheat was high when the sun showed an unsullied surface, and that
food and spots became plentiful together.[362]

Yet this plausible inference was scarcely borne out by a more exact
collocation of facts. Schwabe failed to detect any reflection of the sun-spot
period in his meteorological register. Gautier[363] reached a provisional

conclusion the reverse—though not markedly the reverse—of Herschel's.
Wolf, in 1852, derived from an examination of Vogel's collection of Zürich
Chronicles (1000-1800 A.D.) evidence showing (as he thought) that
minimum years were usually wet and stormy, maximum years dry and
genial;[364] but a subsequent review of the subject in 1859 convinced him
that no relation of any kind between the two kinds of effects was traceable.
[365] With the singular affection of our atmosphere known as the Aurora
Borealis (more properly Aurora Polaris) the case was different. Here the
Zürich Chronicles set Wolf on the right track in leading him to associate
such luminous manifestations with a disturbed condition of the sun; since
subsequent detailed observation has exhibited the curve of auroral
frequency as following with such fidelity the jagged lines figuring to the
eye the fluctuations of solar and magnetic activity, as to leave no reasonable
doubt that all three rise and sink together under the influence of a common
cause. As long ago as 1716,[366] Halley had conjectured that the Northern
Lights were due to magnetic "effluvia," but there was no evidence on the
subject forthcoming until Hiorter observed at Upsala in 1741 their agitating
influence upon the magnetic needle. That the effect was no casual one was
made superabundantly clear by Arago's researches in 1819 and subsequent
years. Now both were perceived to be swayed by the same obscure power
of cosmical disturbance.

The sun is not the only one of the heavenly bodies by which the magnetism
of the earth is affected. Proofs of a similar kind of lunar action were laid by
Kreil in 1841 before the Bohemian Society of Sciences, and with minor
corrections were fully substantiated by Sabine's more extended researches.
It was thus ascertained that each lunar day, or the interval of twenty-four
hours and about fifty-four minutes between two successive meridian
passages of our satellite, is marked by a perceptible, though very small,
double oscillation of the needle—two progressive movements from east to
west, and two returns from west to east.[367] Moreover, the lunar, like the
solar influence (as was proved in each case by Sabine's analysis of the
Hobarton and Toronto observations), extends to all three "magnetic

elements," affecting not only the position of the horizontal or declination
needle, but also the dip and intensity. It seems not unreasonable to attribute
some portion of the same subtle power to the planets and even to the stars,
though with effects rendered imperceptible by distance.

We have now to speak of the discovery and application to the heavenly
bodies of a totally new method of investigation. Spectrum analysis may be
shortly described as a mode of distinguishing the various species of matter
by the kind of light proceeding from each. This definition at once explains
how it is that, unlike every other system of chemical analysis, it has proved
available in astronomy. Light, so far as quality is concerned, ignores
distance. No intrinsic change, that we yet know of, is produced in it by a
journey from the farthest bounds of the visible universe; so that, provided
only that in quantity it remain sufficient for the purpose, its peculiarities can
be equally well studied whether the source of its vibrations be one foot or a
hundred billion miles distant. Now the most obvious distinction between
one kind of light and another resides in colour. But of this distinction the
eye takes cognisance in an æsthetic, not in a scientific sense. It finds
gladness in the "thousand tints" of nature, but can neither analyse nor define
them. Here the refracting prism—or the combination of prisms known as
the "spectroscope"—comes to its aid, teaching it to measure as well as to
perceive. It furnishes, in a word, an accurate scale of colour. The various
rays which, entering the eye together in a confused crowd, produce a
compound impression made up of undistinguishable elements, are, by the
mere passage through a triangular piece of glass, separated one from the
other, and ranged side by side in orderly succession, so that it becomes
possible to tell at a glance what kinds of light are present, and what absent.
Thus, if we could only be assured that the various chemical substances
when made to glow by heat, emit characteristic rays—rays, that is,
occupying a place in the spectrum reserved for them, and for them only—
we should at once be in possession of a mode of identifying such substances
with the utmost readiness and certainty. This assurance, which forms the
solid basis of spectrum analysis, was obtained slowly and with difficulty.

The first to employ the prism in the examination of various flames (for it is
only in a state of vapour that matter emits distinctive light) was a young
Scotchman named Thomas Melvill, who died in 1753, at the age of twenty-
seven. He studied the spectrum of burning spirits, into which were
successively introduced sal ammoniac, potash, alum, nitre, and sea-salt, and
observed the singular predominance, under almost all circumstances, of a
particular shade of yellow light, perfectly definite in its degree of
refrangibility[368]—in other words, taking up a perfectly definite position in
the spectrum. His experiments were repeated by Morgan,[369] Wollaston,
and—with far superior precision and diligence—by Fraunhofer.[370] The
great Munich optician, whose work was completely original, rediscovered
Melvill's deep yellow ray and measured its place in the colour-scale. It has
since become well known as the "sodium line," and has played a very
important part in the history of spectrum analysis. Nevertheless, its ubiquity
and conspicuousness long impeded progress. It was elicited by the
combustion of a surprising variety of substances—sulphur, alcohol, ivory,
wood, paper; its persistent visibility suggesting the accomplishment of
some universal process of nature rather than the presence of one individual
kind of matter. But if spectrum analysis were to exist as a science at all, it
could only be by attaining certainty as to the unvarying association of one
special substance with each special quality of light.

Thus perplexed, Fox Talbot[371] hesitated in 1826 to enounce this
fundamental principle. He was inclined to believe that the presence in the
spectrum of any individual ray told unerringly of the volatilisation in the
flame under scrutiny of some body as whose badge or distinctive symbol
that ray might be regarded; but the continual prominence of the yellow
beam staggered him. It appeared, indeed, without fail where sodium was;
but it also appeared where it might be thought only reasonable to conclude
that sodium was not. Nor was it until thirty years later that William Swan,
[372] by pointing out the extreme delicacy of the spectral test, and the
singularly wide dispersion of sodium, made it appear probable (but even
then only probable) that the questionable yellow line was really due

invariably to that substance. Common salt (chloride of sodium) is, in fact,
the most diffusive of solids. It floats in the air; it flows with water; every
grain of dust has its attendant particle; its absolute exclusion approaches the
impossible. And withal, the light that it gives in burning is so intense and
concentrated, that if a single grain be divided into 180 million parts, and
one alone of such inconceivably minute fragments be present in a source of
light, the spectroscope will show unmistakably its characteristic beam.

Amongst the pioneers of knowledge in this direction were Sir John
Herschel[373]—who, however, applied himself to the subject in the interests
of optics, not of chemistry—W. A. Miller,[374] and Wheatstone. The last
especially made a notable advance when, in the course of his studies on the
"prismatic decomposition" of the electric light, he reached the significant
conclusion that the rays visible in its spectrum were different for each kind
of metal employed as "electrodes."[375] Thus indications of a wider
principle were to be found in several quarters, but no positive certainty on
any single point was obtained, until, in 1859, Gustav Kirchhoff, professor
of physics in the University of Heidelberg, and his colleague, the eminent
chemist Robert Bunsen, took the matter in hand. By them the general
question as to the necessary and invariable connection of certain rays in the
spectrum with certain kinds of matter, was first resolutely confronted, and
first definitely answered. It was answered affirmatively—else there could
have been no science of spectrum analysis—as the result of experiments
more numerous, more stringent, and more precise than had previously been
undertaken.[376] And the assurance of their conclusion was rendered doubly
sure by the discovery, through the peculiarities of their light alone, of two
new metals, named from the blue and red rays by which they were
respectively distinguished, "cæsium," and "rubidium."[377] Both were
immediately afterwards actually obtained in small quantities by evaporation
of the Durckheim mineral waters.

The link connecting this important result with astronomy may now be
indicated. In the year 1802 it occurred to William Hyde Wollaston to

substitute for the round hole used by Newton and his successors for the
admittance of light to be examined with the prism, an elongated "crevice"
1/20th of an inch in width. He thereupon perceived that the spectrum, thus
formed of light, as it were, purified by the abolition of overlapping images,
was traversed by seven dark lines. These he took to be natural boundaries of
the various colours,[378] and satisfied with this quasi-explanation, allowed
the subject to drop. It was independently taken up after twelve years by a
man of higher genius. In the course of experiments on light, directed
towards the perfecting of his achromatic lenses, Fraunhofer, by means of a
slit and a telescope, made the surprising discovery that the solar spectrum is
crossed, not by seven, but by thousands of obscure transverse streaks.[379]
Of these he counted some 600, and carefully mapped 324, while a few of
the most conspicuous he set up (if we may be permitted the expression) as
landmarks, measuring their distances apart with a theodolite, and affixing to
them the letters of the alphabet, by which they are still universally known.
Nor did he stop here. The same system of examination applied to the rest of
the heavenly bodies showed the mild effulgence of the moon and planets to
be deficient in precisely the same rays as sunlight; while in the stars it
disclosed the differences in likeness which are always an earnest of
increased knowledge. The spectra of Sirius and Castor, instead of being
delicately ruled crosswise throughout, like that of the sun, were seen to be
interrupted by three massive bars of darkness—two in the blue and one in
the green;[380] the light of Pollux, on the other hand, seemed precisely
similar to sunlight attenuated by distance or reflection, and that of Capella,
Betelgeux, and Procyon to share some of its peculiarities. One solar line
especially—that marked in his map with the letter D—proved common to
all the four last-mentioned stars; and it was remarkable that it exactly
coincided in position with the conspicuous yellow beam (afterwards, as we
have said, identified with the light of glowing sodium) which he had
already found to accompany most kinds of combustion. Moreover, both the
dark solar and the bright terrestrial "D lines" were displayed by the refined
Munich appliances as double.

In this striking correspondence, discovered by Fraunhofer in 1815, was
contained the very essence of solar chemistry; but its true significance did
not become apparent until long afterwards. Fraunhofer was by profession,
not a physicist, but a practical optician. Time pressed; he could not and
would not deviate from his appointed track; all that was possible to him was
to indicate the road to discovery, and exhort others to follow it.[381]

Partially and inconclusively at first this was done. The "fixed lines" (as they
were called) of the solar spectrum took up the position of a standing
problem, to the solution of which no approach seemed possible.
Conjectures as to their origin were indeed rife. An explanation put forward
by Zantedeschi[382] and others, and dubiously favoured by Sir David
Brewster and Dr. J. H. Gladstone,[383] was that they resulted from
"interference"—that is, a destruction of the motion producing in our eyes
the sensation of light, by the superposition of two light-waves in such a
manner that the crests of one exactly fill up the hollows of the other. This
effect was supposed to be brought about by imperfections in the optical
apparatus employed.

A more plausible view was that the atmosphere of the earth was the agent
by which sunlight was deprived of its missing beams. For a few of them this
is actually the case. Brewster found in 1832 that certain dark lines, which
were invisible when the sun stood high in the heavens, became increasingly
conspicuous as he approached the horizon.[384] These are the well-known
"atmospheric lines;" but the immense majority of their companions in the
spectrum remain quite unaffected by the thickness of the stratum of air
traversed by the sunlight containing them. They are then obviously due to
another cause.

There remained the true interpretation—absorption in the sun's atmosphere;
and this, too, was extensively canvassed. But a remarkable observation
made by Professor Forbes of Edinburgh[385] on the occasion of the annular
eclipse of May 15, 1836, appeared to throw discredit upon it. If the
problematical dark lines were really occasioned by the stoppage of certain

rays through the action of a vaporous envelope surrounding the sun, they
ought, it seemed, to be strongest in light proceeding from his edges, which,
cutting that envelope obliquely, passed through a much greater depth of it.
But the circle of light left by the interposing moon, and of course derived
entirely from the rim of the solar disc, yielded to Forbes's examination
precisely the same spectrum as light coming from its central parts. This
circumstance helped to baffle inquirers, already sufficiently perplexed. It
still remains an anomaly, of which no satisfactory explanation has been
offered.

Convincing evidence as to the true nature of the solar lines was however at
length, in the autumn of 1859, brought forward at Heidelburg. Kirchhoff's
experimentum crucis in the matter was a very simple one. He threw bright
sunshine across a space occupied by vapour of sodium, and perceived with
astonishment that the dark Fraunhofer line D, instead of being effaced by
flame giving a luminous ray of the same refrangibility, was deepened and
thickened by the superposition.

He tried the same experiment, substituting for sunbeams light from a
Drummond lamp, and with similar result. A dark furrow, corresponding in
every respect to the solar D-line, was instantly seen to interrupt the
otherwise unbroken radiance of its spectrum. The inference was irresistible,
that the effect thus produced artificially was brought about naturally in the
same way, and that sodium formed an ingredient in the glowing atmosphere
of the sun.[386] This first discovery was quickly followed up by the
identification of numerous bright rays in the spectra of other metallic bodies
with others of the hitherto mysterious Fraunhofer lines. Kirchhoff was thus
led to the conclusion that (besides sodium) iron, magnesium, calcium, and
chromium, are certainly solar constituents, and that copper, zinc, barium,
and nickel are also present, though in smaller quantities.[387] As to cobalt,
he hesitated to pronounce, but its existence in the sun has since been
established.

These memorable results were founded upon a general principle first
enunciated by Kirchhoff in a communication to the Berlin Academy,
December 15, 1859, and afterwards more fully developed by him.[388] It
may be expressed as follows: Substances of every kind are opaque to the
precise rays which they emit at the same temperature; that is to say, they
stop the kinds of light or heat which they are then actually in a condition to
radiate. But it does not follow that cool bodies absorb the rays which they
would give out if sufficiently heated. Hydrogen at ordinary temperatures,
for instance, is almost perfectly transparent, but if raised to the glowing
point—as by the passage of electricity—it then becomes capable of
arresting, and at the same time of displaying in its own spectrum light of
four distinct colours.

This principle is fundamental to solar chemistry. It gives the key to the
hieroglyphics of the Fraunhofer lines. The identical characters which are
written bright in terrestrial spectra are written dark in the unrolled sheaf of
sun-rays; the meaning remains unchanged. It must, however, be
remembered that they are only relatively dark. The substances stopping
those particular tints in the neighbourhood of the sun are at the same time
vividly glowing with the very same. Remove the dazzling solar
background, by contrast with which they show as obscure, and they will be
seen, and, at critical moments, actually have been seen, in all their native
splendour. It is because the atmosphere of the sun is cooler than the globe it
envelops that the different kinds of vapour constituting that atmosphere take
more than they give, absorb more light than they are capable of emitting;
raise them to the same temperature as the sun itself, and their powers of
emission and absorption being brought exactly to the same level, the
thousands of dusky rays in the solar spectrum will be at once obliterated.

The establishment of the terrestrial science of spectrum analysis was due, as
we have seen, equally to Kirchhoff and Bunsen, but its celestial application
to Kirchhoff alone. He effected this object of the aspirations, more or less
dim, of many other thinkers and workers, by the union of two separate,
though closely related lines of research—the study of the different kinds of

light emitted by various bodies, and the study of the different kinds of light
absorbed by them. The latter branch appears to have been first entered upon
by Dr. Thomas Young in 1803;[389] it was pursued by the younger Herschel,
[390] by William Allen Miller, Brewster, and Gladstone. Brewster indeed
made, in 1833,[391] a formal attempt to found what might be called an
inverse system of analysis with the prism based upon absorption; and his
efforts were repeated, just a quarter of a century later, by Gladstone.[392] But
no general point of view was attained; nor, it may be added, was it by this
path attainable.

Kirchhoff's map of the solar spectrum, drawn to scale with exquisite
accuracy, and printed in three shades of ink to convey the graduated
obscurity of the lines, was published in the Transactions of the Berlin
Academy for 1861 and 1862.[393] Representations of the principal lines
belonging to various elementary bodies formed, as it were, a series of
marginal notes accompanying the great solar scroll, enabling the veriest tiro
in the new science to decipher its meaning at a glance. Where the dark solar
and bright metallic rays agreed in position, it might safely be inferred that
the metal emitting them was a solar constituent; and such coincidences were
numerous. In the case of iron alone, no less than sixty occurred in one-half
of the spectral area, rendering the chances[394] absolutely overwhelming
against mere casual conjunction. The preparation of this elaborate picture
proved so trying to the eyes that Kirchhoff was compelled by failing vision
to resign the latter half of the task to his pupil Hofmann. The complete map
measured nearly eight feet in length.

The conclusions reached by Kirchhoff were no sooner announced than they
took their place, with scarcely a dissenting voice, among the established
truths of science. The broad result, that the dark lines in the spectrum of the
sun afford an index to its chemical composition no less reliable than any of
the tests used in the laboratory, was equally captivating to the imagination
of the vulgar, and authentic in the judgment of the learned; and, like all
genuine advances in the knowledge of Nature, it stimulated curiosity far

more than it gratified it. Now the history of how discoveries were missed is
often quite as instructive as the history of how they were made; it may then
be worth while to expend a few words on the thoughts and trials by which,
in the present case, the actual event was heralded.

Three times it seemed on the verge of being anticipated. The experiment,
which in Kirchhoff's hands proved decisive, of passing sunlight through
glowing vapours and examining the superposed spectra, was performed by
Professor W. A. Miller of King's College in 1845.[395] Nay, more, it was
performed with express reference to the question, then already (as has been
noted) in debate, of the possible production of Fraunhofer's lines by
absorption in a solar atmosphere. Yet it led to nothing.

Again, at Paris in 1849, with a view to testing the asserted coincidence
between the solar D-line and the bright yellow beam in the spectrum of the
electric arc (really due to the unsuspected presence of sodium), Léon
Foucault threw a ray of sunshine across the arc and observed its spectrum.
[396] He was surprised to see that the D-line was rendered more intensely
dark by the combination of lights. To assure himself still further, he
substituted a reflected image of one of the white-hot carbon-points for the
sunbeam, with an identical result. The same ray was missing. It needed but
another step to have generalised this result, and thus laid hold of a natural
truth of the highest importance; but that step was not taken. Foucault, keen
and brilliant though he was, rested satisfied with the information that the
voltaic arc had the power of stopping the kind of light emitted by it; he
asked no further question, and was consequently the bearer of no further
intelligence on the subject.

The truth conveyed by this remarkable experiment was, however, divined
by one eminent man. Professor Stokes of Cambridge stated to Sir William
Thomson (now Lord Kelvin), shortly after it had been made, his conviction
that an absorbing atmosphere of sodium surrounded the sun. And so
forcibly was his hearer impressed with the weight of the argument based
upon the absolute agreement of the D-line in the solar spectrum with the

yellow ray of burning sodium (then freshly certified by W. H. Miller),
combined with Foucault's "reversal" of that ray, that he regularly inculcated,
in his public lectures on natural philosophy at Glasgow, five or six years
before Kirchhoff's discovery, not only the fact of the presence of sodium in
the solar neighbourhood, but also the principle of the study of solar and
stellar chemistry in the spectra of flames.[397] Yet it does not appear to have
occurred to either of these two distinguished professors—themselves
among the foremost of their time in the successful search for new truths—to
verify practically a sagacious conjecture in which was contained the
possibility of a scientific revolution. It is just to add, that Kirchhoff was
unacquainted, when he undertook his investigation, either with the
experiment of Foucault or the speculation of Stokes.

For C. J. Ångström, on the other hand, perhaps somewhat too much has
been claimed in the way of anticipation. His Optical Researches appeared at
Upsala in 1853, and in their English garb two years later.[398] They were
undoubtedly pregnant with suggestion, yet made no epoch in discovery. The
old perplexities continued to prevail after, as before their publication. To
Ångström, indeed, belongs the great merit of having revived Euler's
principle of the equivalence of emission and absorption; but he revived it in
its original crude form, and without the qualifying proviso which alone
gave it value as a clue to new truths. According to his statement, a body
absorbs all the series of vibrations it is, under any circumstances, capable of
emitting, as well as those connected with them by simple harmonic
relations. This is far too wide. To render it either true or useful, it had to be
reduced to the cautious terms employed by Kirchhoff. Radiation strictly and
necessarily corresponds with absorption only when the temperature is the
same. In point of fact, Ångström was still, in 1853, divided between
adsorption and interference as the mode of origin of the Fraunhofer dark
rays. Very important, however, was his demonstration of the compound
nature of the spark-spectrum, which he showed to be made up of the
spectrum of the metallic electrodes superposed upon that of the gas or gases
across which the discharge passed.

It may here be useful—since without some clear ideas on the subject no
proper understanding of recent astronomical progress is possible—to take a
cursory view of the elementary principles of spectrum analysis. To many of
our readers they are doubtless already familiar; but it is better to appear trite
to some than obscure even to a few.

The spectrum, then, of a body is simply the light proceeding from it spread
out by refraction[399] into a brilliant variegated band, passing from
brownish-red through crimson, orange, yellow, green, and azure into dusky
violet. The reason of this spreading-out or "dispersion" is that the various
colours have different wave-lengths, and consequently meet with different
degrees of retardation in traversing the denser medium of the prism. The
shortest and quickest vibrations (producing the sensation we call "violet")
are thrown farthest away from their original path—in other words, suffer
the widest "deviation;" the longest and slowest (the red) travel much nearer
to it. Thus the sheaf of rays which would otherwise combine into a patch of
white light are separated through the divergence of their tracks after
refraction by a prism, so as to form a tinted riband. This visible spectrum is
prolonged invisibly at both ends by a long range of vibrations, either too
rapid or too sluggish to affect the eye as light, but recognisable through
their chemical and heating effects.

Now all incandescent solid or liquid substances, and even gases ignited
under great pressure, give what is called a "continuous spectrum;" that is to
say, the light derived from them is of every conceivable hue. Sorted out
with the prism, its tints merge imperceptibly one into the other,
uninterrupted by any dark spaces. No colours, in short, are missing. But
gases and vapours rendered luminous by heat emit rays of only a few tints,
which accordingly form an interrupted spectrum, usually designated as one
of lines or bands. And since these rays are perfectly definite and
characteristic—not being the same for any two substances—it is easy to tell
what kind of matter is concerned in producing them. We may suppose that
the inconceivably minute particles which by their rapid thrilling agitate the
ethereal medium so as to produce light, are free to give out their peculiar

tone of vibration only when floating apart from each other in gaseous form;
but when crowded together into a condensed mass, the clear ring of the
distinctive note is drowned, so to speak, in a universal molecular clang.
Thus prismatic analysis has no power to identify individual kinds of matter,
except when they present themselves as glowing vapours.

A spectrum is said to be "reversed" when lines previously seen bright on a
dark background appear dark on a bright background. In this form it is
equally characteristic of chemical composition with the "direct" spectrum,
being due to absorption, as the latter is to emission. And absorption and
emission are, by Kirchhoff's law, strictly correlative. This is easily
understood by the analogy of sound. For just as a tuning-fork responds to
sound-waves of its own pitch, but remains indifferent to those of any other,
so those particles of matter whose nature it is, when set swinging by heat, to
vibrate a certain number of times in a second, thus giving rise to light of a
particular shade of colour, appropriate those same vibrations, and those
only, when transmitted past them,—or, phrasing it otherwise, are opaque to
them, and transparent to all others.

It should further be explained that the shape of the bright or dark spaces in
the spectrum has nothing whatever to do with the nature of the phenomena.
The "lines" and "bands" so frequently spoken of are seen as such for no
other reason than because the light forming them is admitted through a
narrow, straight opening. Change that opening into a fine crescent or a
sinuous curve, and the "lines" will at once appear as crescents or curves.

Resuming in a sentence what has been already explained, we find that the
prismatic analysis of the heavenly bodies was founded upon three classes of
facts: First, the unmistakable character of the light given by each different
kind of glowing vapour; secondly, the identity of the light absorbed with the
light emitted by each; thirdly, the coincidence observed between rays
missing from the solar spectrum and rays absorbed by various terrestrial
substances. Thus, a realm of knowledge, pronounced by Morinus[400] in the
seventeenth century, and no less dogmatically by Auguste Comte[401] in the

nineteenth, hopelessly out of reach of the human intellect, was thrown
freely open, and the chemistry of the sun and stars took at once a leading
place among the experimental sciences.

The immediate increase of knowledge was not the chief result of
Kirchhoff's labours; still more important was the change in the scope and
methods of astronomy, which, set on foot in 1852 by the detection of a
common period affecting at once the spots on the sun and the magnetism of
the earth, was extended and accelerated by the discovery of spectrum
analysis. The nature of that change is concisely indicated by the heading of
the present chapter; we would now ask our readers to endeavour to realise
somewhat distinctly what is implied by the "foundation of astronomical
physics."

Just three centuries ago, Kepler drew a forecast of what he called a
"physical astronomy"—a science treating of the efficient causes of
planetary motion, and holding the "key to the inner astronomy."[402] What
Kepler dreamed of and groped after, Newton realized. He showed the
beautiful and symmetrical revolutions of the solar system to be governed by
a uniformly acting cause, and that cause no other than the familiar force of
gravity, which gives stability to all our terrestrial surroundings. The world
under our feet was thus for the first time brought into physical connection
with the worlds peopling space, and a very tangible relationship was
demonstrated as existing between what used to be called the "corruptible"
matter of the earth and the "incorruptible" matter of the heavens.

This process of unification of the cosmos—this levelling of the celestial
with the sublunary—was carried no farther until the fact unexpectedly
emerged from a vast and complicated mass of observations, that the
magnetism of the earth is subject to subtle influences, emanating, certainly
from some, and presumably from all of the heavenly bodies; the inference
being thus rendered at least plausible, that a force not less universal than
gravity itself, but with whose modes of action we are as yet unacquainted,
pervades the universe, and forms, it might be said, an intangible bond of

sympathy between its parts. Now for the investigation of this influence two
roads are open. It may be pursued by observation either of the bodies from
which it proceeds, or of the effects which it produces—that is to say, either
by the astronomer or by the physicist, or, better still, by both concurrently.
Their acquisitions are mutually profitable; nor can either be considered as
independent of the other. Any important accession to knowledge respecting
the sun, for example, may be expected to cast a reflected light on the still
obscure subject of terrestrial magnetism; while discoveries in magnetism or
its alter ego electricity must profoundly affect solar inquiries.

The establishment of the new method of spectrum analysis drew far closer
this alliance between celestial and terrestrial science. Indeed, they have
come to merge so intimately one into the other, that it is no easier to trace
their respective boundaries than it is to draw a clear dividing-line between
the animal and vegetable kingdoms. Yet up to the middle of the last century,
astronomy, while maintaining her strict union with mathematics, looked
with indifference on the rest of the sciences; it was enough that she
possessed the telescope and the calculus. Now the materials for her
inductions are supplied by the chemist, the electrician, the inquirer into the
most recondite mysteries of light and the molecular constitution of matter.
She is concerned with what the geologist, the meteorologist, even the
biologist, has to say; she can afford to close her ears to no new truth of the
physical order. Her position of lofty isolation has been exchanged for one of
community and mutual aid. The astronomer has become, in the highest
sense of the term, a physicist; while the physicist is bound to be something
of an astronomer.

This, then, is what is designed to be conveyed by the "foundation of
astronomical or cosmical physics." It means the establishment of a science
of Nature whose conclusions are not only presumed by analogy, but are
ascertained by observation, to be valid wherever light can travel and gravity
is obeyed—a science by which the nature of the stars can be studied upon
the earth, and the nature of the earth can be made better known by study of
the stars—a science, in a word, which is, or aims at being, one and

universal, even as Nature—the visible reflection of the invisible highest
Unity—is one and universal.

It is not too much to say that a new birth of knowledge has ensued. The
astronomy so signally promoted by Bessel[403]—the astronomy placed by
Comte[404] at the head of the hierarchy of the physical sciences—was the
science of the movements of the heavenly bodies. And there were those who
began to regard it as a science which, from its very perfection, had ceased
to be interesting—whose tale of discoveries was told, and whose farther
advance must be in the line of minute technical improvements, not of novel
and stirring disclosures. But the science of the nature of the heavenly
bodies is one only in the beginning of its career. It is full of the audacities,
the inconsistencies, the imperfections, the possibilities of youth. It promises
everything; it has already performed much; it will doubtless perform much
more. The means at its disposal are vast and are being daily augmented.
What has so far been secured by them it must now be our task to extricate
from more doubtful surroundings and place in due order before our readers.

FOOTNOTES:
[347] Wolf, Gesch. der Astr., p. 655.

[348] Manuel Johnson, Mem. R.A.S., vol. xxvi., p. 197.

[349] Astronomie Théorique et Pratique, t. iii., p. 20.

[350] Wolf, Gesch. der Astr., p. 654.

[351] Month. Not., vol. xvii., p. 241.

[352] Mem. R.A.S., vol. xxvi., p. 200.

[353] Astr. Nach., No. 495.

[354] Gehler's Physikalisches Wörterbuch, art. Sonnenflecken, p. 851.

[355] Zweite Abth., p. 401.

[356] Annalen der Physik (Poggendorff's), Bd. lxxxiv., p. 580.

[357] Phil. Trans., vol. cxlii., p. 103.

[358] Mittheilungen der Naturforschenden Gesellschaft, 1852, p. 183.

[359] Archives des Sciences, t. xxi., p. 194.

[360] Neue Untersuchungen, Mitth. Naturf. Ges., 1852, p. 249.

[361] Phil. Trans., vol. xci., p. 316.

[362] Evidence of an eleven-yearly fluctuation in the price of food-grains in
India was collected some years ago by Mr. Frederick Chambers. Nature, vol.
xxxiv., p. 100.

[363] Bibl. Un. de Genève, t. li., p. 336.

[364] Neue Untersuchungen, p. 269.

[365] Die Sonne und ihre Flecken, p. 30. Arago was the first who attempted to
decide the question by keeping, through a series of years, a parallel register of
sun-spots and weather; but the data regarding the solar condition amassed at the
Paris Observatory from 1822 to 1830 were not sufficiently precise to support
any inference.

[366] Phil. Trans., vol. xxix., p. 421.

[367] Ibid., vols. cxliii., p. 558, cxlvi., p. 505.

[368] Observations on Light and Colours, p. 35.

[369] Phil. Trans., vol. lxxv., p. 190.

[370] Denkschriften (Munich. Ac. of Sc.), 1814, 1815, Bd. v., p. 197.

[371] Edinburgh Journal of Science, vol. v., p. 77. See also Phil. Mag., Feb.,
1834, vol. iv., p. 112.

[372] Ed. Phil. Trans., vol. xxi., p. 411.

[373] On the Absorption of Light by Coloured Media, Ed. Phil. Trans., vol. ix.,
p. 445 (1823).

[374] Phil. Mag., vol. xxvii, (ser. iii.), p. 81.

[375] Report Brit. Ass., 1835, p. 11 (pt. ii.). Electrodes are the terminals from
one to the other of which the electric spark passes, volatilising and rendering
incandescent in its transit some particles of their substance, the characteristic
light of which accordingly flashes out in the spectrum.

[376] Phil. Mag., vol. xx., p. 93.

[377] Annalen der Physik, Bd. cxiii., p. 357.

[378] Phil. Trans., vol. xcii., p. 378.

[379] Denkschriften, Bd. v., p. 202.

[380] Ibid., p. 220; Edin. Jour. of Science, vol. viii., p. 9.

[381] Denkschriften, Bd. v., p. 222.

[382] Arch. des Sciences, 1849, p. 43.

[383] Phil. Trans., vol. cl., p. 159, note.

[384] Ed. Phil. Trans., vol. xii., p. 528.

[385] Phil. Trans., vol. cxxvi., p. 453. "I conceive," he says, "that this result
proves decisively that the sun's atmosphere has nothing to do with the
production of this singular phenomenon" (p. 455). And Brewster's well-founded
opinion that it had much to do with it was thereby, in fact, overthrown.

[386] Monatsberichte, Berlin, 1859, p. 664.

[387] Abhandlungen, Berlin, 1861, pp. 80, 81.
[388] Ibid., 1861, p. 77; Annalen der Physik, Bd. cxix., p. 275. A similar
conclusion, reached by Balfour Stewart in 1858, for heat-rays (Ed. Phil. Trans.,
vol. xxii., p. 13), was, in 1860, without previous knowledge of Kirchhoff's work,
extended to light (Phil. Mag., vol. xx., p. 534); but his experiments wanted the
precision of those executed at Heidelburg.

[389] Miscellaneous Works, vol. i., p. 189.

[390] Ed. Phil. Trans., vol. ix., p. 458.

[391] Ibid., vol. xii., p. 519.

[392] Quart. Jour. Chem. Soc., vol. x. p. 79.

[393] A facsimile accompanied Sir H. Roscoe's translation of Kirchhoff's
"Researches on the Solar Spectrum" (London, 1862-63).

[394] Estimated by Kirchhoff's at a trillion to one. Abhandl., 1861, p. 79.

[395] Phil. Mag., vol. xxvii. (3rd series), p. 90.

[396] L'Institut, Feb. 7, 1849, p. 45; Phil. Mag., vol. xix. (4th series), p. 193.

[397] Ann. d. Phys., vol. cxviii., p. 110.
[398] Phil. Mag., vol. ix. (4th series), p. 327.

[399] Spectra may be produced by diffraction as well as by refraction; but we
are here only concerned with the subject in its simplest aspect.

[400] Astrologia Gallica (1661), p. 189.

[401] Pos. Phil., vol. i., pp. 114, 115 (Martineau's trans.).

[402] Proem Astronomiæ Pars Optica (1640), Op., t. ii.

[403] Pop. Vorl., pp. 14, 19, 408.

[404] Pos. Phil., p. 115.

CHAPTER II
SOLAR OBSERVATIONS AND THEORIES

The zeal with which solar studies have been pursued during the last half
century has already gone far to redeem the neglect of the two preceding
ones. Since Schwabe's discovery was published in 1851, observers have
multiplied, new facts have been rapidly accumulated, and the previous
comparative quiescence of thought on the great subject of the constitution
of the sun, has been replaced by a bewildering variety of speculations,
conjectures, and more or less justifiable inferences. It is satisfactory to find
this novel impulse not only shared, but to a large extent guided, by our
countrymen.

William Rutter Dawes, one of many clergymen eminent in astronomy,
observed, in 1852, with the help of a solar eye-piece of his own devising,
some curious details of spot-structure.[405] The umbra—heretofore taken for
the darkest part of the spot—was seen to be suffused with a mottled,
nebulous illumination, in marked contrast with the striated appearance of
the penumbra; while through this "cloudy stratum" a "black opening"
permitted the eye to divine farther unfathomable depths beyond. The hole
thus disclosed—evidently the true nucleus—was found to be present in all
considerable, as well as in many small maculæ.

Again, the whirling motions of some of these objects were noticed by him.
The remarkable form of one sketched at Wateringbury, in Kent, January 17,
1852, gave him the means of detecting and measuring a rotatory movement
of the whole spot round the black nucleus at the rate of 100 degrees in six
days. "It appeared," he said, "as if some prodigious ascending force of a
whirlwind character, in bursting through the cloudy stratum and the two
higher and luminous strata, had given to the whole a movement resembling

its own."[406] An interpretation founded, as is easily seen, on the
Herschelian theory, then still in full credit.

An instance of the same kind was observed by Mr. W. R. Birt in 1860,[407]
and cyclonic movements are now a recognised feature of sun-spots. They
are, however, as Father Secchi[408] concluded from his long experience, but
temporary and casual. Scarcely three per cent. of all spots visible exhibit the
spiral structure which should invariably result if a conflict of opposing, or
the friction of unequal, currents were essential, and not merely incidental to
their origin. A whirlpool phase not unfrequently accompanies their
formation, and may be renewed at periods of recrudescence or dissolution;
but it is both partial and inconstant, sometimes affecting only one side of a
spot, sometimes slackening gradually its movement in one direction, to
resume it, after a brief pause, in the opposite. Persistent and uniform
notions, such as the analogy of terrestrial storms would absolutely require,
are not to be found. So that the "cyclonic theory" of sun-spots, suggested by
Herschel in 1847,[409] and urged, from a different point of view, by Faye in
1872, may be said to have completely broken down.

The drift of spots over the sun's surface was first systematically investigated
by Carrington, a self-constituted astronomer, gifted with the courage and
the instinct of thoughtful labour.

Born at Chelsea in May, 1826, Richard Christopher Carrington entered
Trinity College, Cambridge, in 1844. He was intended for the Church, but
Professor Challis's lectures diverted him to astronomy, and he resolved, as
soon as he had taken his degree, to prepare, with all possible diligence, to
follow his new vocation. His father, who was a brewer on a large scale at
Brentford, offered no opposition; ample means were at his disposal;
nevertheless, he chose to serve an apprenticeship of three years as observer
in the University of Durham, as though his sole object had been to earn a
livelihood. He quitted the post only when he found that its restricted
opportunities offered no farther prospect of self-improvement.

He now built an observatory of his own at Redhill in Surrey, with the design
of completing Bessel's and Argelander's survey of the northern heavens by
adding to it the circumpolar stars omitted from their view. This project,
successfully carried out between 1854 and 1857, had another and still larger
one superposed upon it before it had even begun to be executed. In 1852,
while the Redhill Observatory was in course of erection, the discovery of
the coincidence between the sun-spot and magnetic periods was announced.
Carrington was profoundly interested, and devoted his enforced leisure to
the examination of records, both written and depicted, of past solar
observations. Struck with their fragmentary and inconsistent character, he
resolved to "appropriate," as he said, by "close and methodical research,"
the eleven-year period next ensuing.[410] He calculated rightly that he
should have the field pretty nearly to himself; for many reasons conspire to
make public observatories slow in taking up new subjects, and amateurs
with freedom to choose, and means to treat them effectually, were scarcer
then than they are now.

The execution of this laborious task was commenced November 9, 1853. It
was intended to be merely a parergon—a "second subject," upon which
daylight energies might be spent, while the hours of night were reserved for
cataloguing those stars that "are bereft of the baths of ocean." Its results,
however, proved of the highest interest, although the vicissitudes of life
barred the completion, in its full integrity, of the original design. By the
death, in 1858, of the elder Carrington, the charge of the brewery devolved
upon his son; and eventually absorbed so much of his care that it was found
advisable to bring the solar observations to a premature close, on March 24,
1861.

His scientific life may be said to have closed with them. Attacked four years
later with severe, and, in its results, permanent illness, he disposed of the
Brentford business, and withdrew to Churt, near Farnham, in Surrey. There,
in a lonely spot, on the top of a detached conical hill known as the "Devil's
Jump," he built a second observatory, and erected an instrument which he
was no longer able to use with pristine effectiveness; and there, November

27, 1875, he died of the rupture of a blood vessel on the brain, before he
had completed his fiftieth year.[411]

His observations of sun-spots were of a geometrical character. They
concerned positions and movements, leaving out of sight physical
peculiarities. Indeed, the prudence with which he limited his task to what
came strictly within the range of his powers to accomplish, was one of
Carrington's most valuable qualities. The method of his observations,
moreover, was chosen with the same practical sagacity as their objects. As
early as 1847, Sir John Herschel had recommended the daily self-
registration of sun-spots,[412] and he enforced the suggestion, with more
immediate prospect of success, in 1854.[413] The art of celestial
photography, however, was even then in a purely tentative stage, and
Carrington wisely resolved to waste no time on dubious experiments, but
employ the means of registration and measurement actually at his
command. These were very simple, yet very effective. To the "helioscope"
employed by Father Scheiner[414] two centuries and a quarter earlier, a
species of micrometer was added. The image of the sun was projected upon
a screen by means of a firmly-clamped telescope, in the focus of which
were placed two cross-wires forming angles of 45° with the meridian. The
six instants were then carefully noted at which these were met by the edges
of the disc as it traversed the screen, and by the nucleus of the spot to be
measured.[415] A short process of calculation then gave the exact position of
the spot as referred to the sun's centre.

From a series of 5,290 observations made in this way, together with a great
number of accurate drawings, Carrington derived conclusions of great
importance on each of the three points which he had proposed to himself to
investigate. These were: the law of the sun's rotation, the existence and
direction of systematic currents, and the distribution of spots on the solar
surface.

Grave discrepancies were early perceived to exist between determinations
of the sun's rotation by different observers. Galileo, with "comfortable

generality," estimated the period at "about a lunar month";[416] Scheiner, at
twenty-seven days.[417] Cassini, in 1678, made it 25·58; Delambre, in 1775,
no more than twenty-five days. Later inquiries brought these divergences
within no more tolerable limits. Laugier's result of 25·34 days—obtained in
1841—enjoyed the highest credit, yet it differed widely in one direction
from that of Böhm (1852), giving 25·52 days, and in the other from that of
Kysæus (1846), giving 25·09 days. Now the cause of these variations was
really obvious from the first, although for a long time strangely overlooked.
Scheiner pointed out in 1630 that different spots gave different periods,
adding the significant remark that one at a distance from the solar equator
revolved more slowly than those nearer to it.[418] But the hint was wasted.
For upwards of two centuries ideas on the subject were either retrograde or
stationary. What were called the "proper motions" of spots were, however,
recognised by Schröter,[419] and utterly baffled Laugier,[420] who despaired
of obtaining any concordant result as to the sun's rotation except by taking
the mean of a number of discordant ones. At last, in 1855, a valuable course
of observations made at Capo di Monte, Naples, in 1845-6, enabled C. H. F.
Peters[421] to set in the clearest light the insecurity of determinations based
on the assumption of fixity in objects plainly affected by movements
uncertain both in amount and direction.

Such was the state of affairs when Carrington entered upon his task.
Everything was in confusion; the most that could be said was that the
confusion had come to be distinctly admitted and referred to its true source.
What he discovered was this: that the sun, or at least the outer shell of the
sun visible to us, has no single period of rotation, but drifts round, carrying
the spots with it, at a rate continually accelerated from the poles to the
equator. In other words, the time of axial revolution is shortest at the
equator and lengthens with increase of latitude. Carrington devised a
mathematical formula by which the rate or "law" of this lengthening was
conveniently expressed; but it was a purely empirical one. It was a concise
statement, but implied no physical interpretation. It summarised, but did not
explain the facts. An assumed "mean period" for the solar rotation of 25·38

days (twenty-five days nine hours, very nearly), was thus found to be
actually conformed to only in two parallels of solar latitude (14° north and
south), while the equatorial period was slightly less than twenty-five, and
that of latitude 50° rose to twenty-seven days and a half.[422] These curious
results gave quite a new direction to ideas on solar physics.

The other two "elements" of the sun's rotation were also ascertained by
Carrington with hitherto unattained precision. He fixed the inclination of its
axis to the ecliptic at 82° 45′; the longitude of the ascending node at 73° 40′
(for the epoch 1850 A.D.). These data—which have scarcely yet been
improved upon—suffice to determine the position in space of the sun's
equator. Its north pole is directed towards a star in the coils of the Dragon,
midway between Vega and the Pole-star; its plane intersects that of the
earth's orbit in such a way that our planet finds itself in the same level on or
about the 3rd of June and the 5th of December, when any spots visible on
the disc cross it in apparently straight lines. At other times, the paths
pursued by them seem curved—downward (to an observer in the northern
hemisphere) between June and December, upward between December and
June.

A singular peculiarity in the distribution of sun-spots emerged from
Carrington's studies at the time of the minimum of 1856. Two broad belts of
the solar surface, as we have seen, are frequented by them, of which the
limits may be put at 6° and 35° of north and south latitude. Individual
equatorial spots are not uncommon, but nearer to the poles than 35° they are
a rare exception. Carrington observed—as an extreme instance—in July,
1858, one in south latitude 44°; and Peters, in June, 1846, watched, during
several days, a spot in 50° 24′ north latitude. But beyond this no true
macula has ever been seen; for Lahire's reported observation of one in
latitude 70° is now believed to have had its place on the solar globe
erroneously assigned; and the "veiled spots" described by Trouvelot in
1875[423] as occurring within 10° of the pole can only be regarded as, at the
most, the same kind of disturbance in an undeveloped form.

But the novelty of Carrington's observations consisted in the detection of
certain changes in distribution concurrent with the progress of the eleven-
year period. As the minimum approached, the spot-zones contracted
towards the equator, and there finally vanished; then, as if by a fresh
impulse, spots suddenly reappeared in high latitude, and spread downwards
with the development of the new phase of activity. Scarcely had this remark
been made public,[424] when Wolf[425] found a confirmation of its general
truth in Böhm's observations during the years 1833-36; and a perfectly
similar behaviour was noted both by Spörer and Secchi at the minimum
epoch of 1867. The ensuing period gave corresponding indications; and it
may now be looked upon as established that the spot-zones close in towards
the equator with the advance of each cycle, their activity culminating, as a
rule, in a mean latitude of about 16°, and expiring when it is reduced to 6°.
Before this happens, however, a completely new disturbance will have
manifested itself some 35° north and south of the equator, and will have
begun to travel over the same course as its predecessor. Each series of sun-
spots is thus, to some extent, overlapped by the succeeding one; so that
while the average interval from one maximum to the next is eleven years,
the period of each distinct wave of agitation is twelve or fourteen.[426]
Curious evidence of the retarded character of the maximum of 1883-4 was
to be found in the unusually low latitude of the spot-zones when it occurred.
Their movement downward having gone on regularly while the crisis was
postponed, its final symptoms were hence displaced locally as well as in
time. The "law of zones" was duly obeyed at the minima of 1890[427] and
1901, and Spörer found evidence of conformity to it so far back as 1619.
[428] His researches, however, also showed that it was in abeyance during
some seventy years previously to 1716, during which period sun-spots
remained persistently scarce, and auroral displays were feeble and
infrequent even in high northern latitudes. An unaccountable suspension of
solar activity is, in fact, indicated.[429]

Gustav Spörer, born at Berlin in 1822, began to observe sun-spots with the
view of assigning the law of solar rotation in December, 1860. His assiduity

and success with limited means attracted attention, and a Government
endowment was procured for his little solar observatory at Anclam, in
Pomerania, the Crown Prince (afterwards Emperor Frederick) adding a
five-inch refractor to its modest equipment. Unaware of Carrington's
discovery (not made known until January, 1859), he arrived at and
published, in June, 1861,[430] a similar conclusion as to the equatorial
quickening of the sun's movement on its axis. Appointed observer in the
new Astrophysical establishment at Potsdam in 1874, he continued his sun-
spot determinations there for twenty years, and died July 7, 1895.

The time had now evidently come for a fundamental revision of current
notions respecting the nature of the sun. Herschel's theory of a cool, dark,
habitable globe, surrounded by, and protected against, the radiations of a
luminous and heat-giving envelope, was shattered by the first dicta of
spectrum analysis. Traces of it may be found for a few years subsequent to
1859,[431] but they are obviously survivals from an earlier order of ideas,
doomed to speedy extinction. It needs only a moment's consideration of the
meaning at last found for the Fraunhofer lines to see the incompatibility of
the new facts with the old conceptions. They implied not only the presence
near the sun, as glowing vapours, of bodies highly refractory to heat, but
that these glowing vapours formed the relatively cool envelope of a still
hotter internal mass. Kirchhoff, accordingly, included in his great memoir
"On the Solar Spectrum," read before the Berlin Academy of Sciences, July
11, 1861, an exposition of the views on the subject to which his memorable
investigations had led him. They may be briefly summarised as follows:

Since the body of the sun gives a continuous spectrum, it must be either
solid or liquid,[432] while the interruptions in its light prove it to be
surrounded by a complex atmosphere of metallic vapours, somewhat cooler
than itself. Spots are simply clouds due to local depressions of temperature,
differing in no respect from terrestrial clouds except as regards the kinds of
matter composing them. These sun-clouds take their origin in the zones of
encounter between polar and equatorial currents in the solar atmosphere.

This explanation was liable to all the objections urged against the "cumulus
theory" on the one hand, and the "trade-wind theory" on the other. Setting
aside its propounder, it was consistently upheld perhaps by no man eminent
in science except Spörer; and his advocacy of it proved ineffective to secure
its general adoption.

M. Faye, of the Paris Academy of Sciences, was the first to propose a
coherent scheme of the solar constitution covering the whole range of new
discovery. The fundamental ideas on the subject now in vogue here made
their first connected appearance. Much, indeed, remained to be modified
and corrected; but the transition was finally made from the old to the new
order of thought. The essence of the change may be conveyed in a single
sentence. The sun was thenceforth regarded, not as a mere heated body, or
—still more remotely from the truth—as a cool body unaccountably spun
round with a cocoon of fire, but as a vast heat-radiating machine. The
terrestrial analogy was abandoned in one more particular besides that of
temperature. The solar system of circulation, instead of being adapted, like
that of the earth, to the distribution of heat received from without, was seen
to be directed towards the transportation towards the surface of the heat
contained within. Polar and equatorial currents, tending to a purely
superficial equalisation of temperature, were replaced by vertical currents
bringing up successive portions of the intensely heated interior mass, to
contribute their share in turn to the radiation into space which might be
called the proper function of a sun.

Faye's views, which were communicated to the Academy of Sciences,
January 16, 1865,[433] were avowedly based on the anomalous mode of
solar rotation discovered by Carrington. This may be regarded either as an
acceleration increasing from the poles to the equator, or as a retardation
increasing from the equator to the poles, according to the rate of revolution
we choose to assume for the unseen nucleus. Faye preferred to consider it a
retardation produced by ascending currents continually left behind as the
sphere widened in which the matter composing them was forced to travel.
He further supposed that the depth from which these vertical currents rose,

and consequently the amount of retardation effected by their ascent to the
surface, became progressively greater as the poles were approached, owing
to the considerable flattening of the spheroidal surface from which they
started;[434] but the adoption of this expedient has been shown to involve
inadmissible consequences.

The extreme internal mobility betrayed by Carrington's and Spörer's
observations led to the inference that the matter composing the sun was
mainly or wholly gaseous. This had already been suggested by Father
Secchi[435] a year earlier, and by Sir John Herschel in April, 1864;[436] but it
first obtained general currency through Faye's more elaborate presentation.
A physical basis was afforded for the view by Cagniard de la Tour's
experiments in 1822,[437] proving that, under conditions of great heat and
pressure, the vaporous state was compatible with a very considerable
density. The position was strengthened when Andrews showed, in 1869,[438]
that above a fixed limit of temperature, varying for different bodies, true
liquefaction is impossible, even though the pressure be so tremendous as to
retain the gas within the same space that enclosed the liquid. The opinion
that the mass of the sun is gaseous now commands a very general assent;
although the gaseity admitted is of such a nature as to afford the consistence
rather of honey or pitch than of the aeriform fluids with which we are
familiar.

On another important point the course of subsequent thought was
powerfully influenced by Faye's conclusions in 1865. Arago somewhat
hastily inferred from experiments with the polariscope the wholly gaseous
nature of the visible disc of the sun. Kirchhoff, on the contrary, believed
(erroneously, as we now know) that the brilliant continuous spectrum
derived from it proved it to be a white-hot solid or liquid. Herschel and
Secchi[439] indicated a cloud-like structure as that which would best
harmonise the whole of the evidence at command. The novelty introduced
by Faye consisted in regarding the photosphere no longer "as a defined
surface, in the mathematical sense, but as a limit to which, in the general

fluid mass, ascending currents carry the physical or chemical phenomena of
incandescence."[440] Uprushing floods of mixed vapours with strong
affinities—say of calcium or sodium and oxygen—at last attain a region
cool enough to permit their combination; a fine dust of solid or liquid
compound particles (of lime or soda, for example) there collects into the
photospheric clouds, and descending by its own weight in torrents of
incandescent rain, is dissociated by the fierce heat below, and replaced by
ascending and combining currents of similar constitution.

This first attempt to assign the part played in cosmical physics by chemical
affinities was marked by the importation into the theory of the sun of the
now familiar phrase dissociation. It is indeed tolerably certain that no such
combinations as those contemplated by Faye occur at the photospheric
level, since the temperature there must be enormously higher than would be
needed to reduce all metallic earths and oxides; but molecular changes of
some kind, dependent perhaps in part upon electrical conditions, in part
upon the effects of radiation into space, most likely replace them. The
conjecture was emitted by Dr. Johnstone Stoney in 1867[441] that the
photospheric clouds are composed of carbon-particles precipitated from
their mounting vapour just where the temperature is lowered by expansion
and radiation to the boiling-point of that substance. But this view, though
countenanced by Ångström,[442] and advocated by Hastings of Baltimore,
[443] and other authorities,[444] is open to grave objections.[445]

In Faye's theory, sun-spots were regarded as simply breaks in the
photospheric clouds, where the rising currents had strength to tear them
asunder. It followed that they were regions of increased heat—regions, in
fact, where the temperature was too high to permit the occurrence of the
precipitations to which the photosphere is due. Their obscurity was
attributed, as in Dr. Brester's more recent Théorie du Soleil, to deficiency of
emissive power. Yet here the verdict of the spectroscope is adverse and
irreversible.

After every deduction, however, has been made, we still find that several
ideas of permanent value were embodied in this comprehensive sketch of
the solar constitution. The principal of these were; first, that the sun is a
mainly gaseous body; secondly, that its stores of heat are rendered available
at the surface by means of vertical convection-currents—by the bodily
transport, that is to say, of intensely hot matter upward, and of
comparatively cool matter downward; thirdly, that the photosphere is a
surface of condensation, forming the limit set by the cold of space to this
circulating process, and that a similar formation must attend, at a certain
stage, the cooling of every cosmical body.

To Warren de la Rue belongs the honour of having obtained the earliest
results of substantial value in celestial photography. What had been done
previously was interesting in the way of promise, but much could not be
claimed for it as actual performance. Some "pioneering experiments" were
made by Dr. J. W. Draper of New York in 1840, resulting in the production
of a few "moon-pictures" one inch in diameter;[446] but slight
encouragement was derived from them, either to himself or others. Bond of
Cambridge (U.S.), however, secured in 1850 with the Harvard 15-inch
refractor that daguerreotype of the moon with which the career of extra-
terrestrial photography may be said to have formally opened. It was shown
in London at the Great Exhibition of 1851, and determined the direction of
De la Rue's efforts. Yet it did little more than prove the art to be a possible
one.

Warren de la Rue was born in Guernsey in 1815, and died in London April
19, 1889. Educated at the École Sainte-Barbe in Paris, he made a large
fortune as a paper manufacturer in England, and thus amply and early
provided the material supplies for his scientific campaign. Towards the end
of 1853 he took some successful lunar photographs. They were remarkable
as the first examples of the application to astronomical light-painting of the
collodion process, invented by Archer in 1851; and also of the use of
reflectors (De la Rue's was one of thirteen inches, constructed by himself)
for that kind of work. The absence of a driving apparatus was, however,

very sensibly felt; the difficulty of moving the instrument by hand so as
accurately to follow the moon's apparent motion being such as to cause the
discontinuance of the experiments until 1857, when the want was supplied.
De la Rue's new observatory, built in that year at Cranford, was expressly
dedicated to celestial photography; and there he applied to the heavenly
bodies the stereoscopic method of obtaining relief, and turned his attention
to the delicate business of photographing the sun.

A solar daguerreotype was taken at Paris, April 2, 1845,[447] by Foucault
and Fizeau, acting on a suggestion from Arago. But the attempt, though far
from being unsuccessful, does not, at that time, seem to have been repeated.
Its great difficulty consisted in the enormous light-power of the object to be
represented, rendering an inconceivably short period of exposure
indispensable, under pain of getting completely "burnt-up" plates. In 1857
De la Rue was commissioned by the Royal Society to construct an
instrument specially adapted to the purpose for the Kew Observatory. The
resulting "photoheliograph" may be described as a small telescope (of 3-1/2
inches aperture and 50 focus), with a plate-holder at the eye-end, guarded in
front by a spring-slide, the rapid movement of which across the field of
view secured for the sensitive plate a virtually instantaneous exposure. By
its means the first solar light-pictures of real value were taken, and the
autographic record of the solar condition recommended by Sir John
Herschel was commenced and continued at Kew during fourteen years—
1858-72. The work of photographing the sun is now carried on in every
quarter of the globe, from Mauritius to Massachusetts, and the days are few
indeed on which the self-betrayal of the camera can be evaded by our chief
luminary. In the year 1883 the incorporation of Indian with Greenwich
pictures afforded a record of the state of the solar surface on 340 days; and
364 were similarly provided for in 1897 and 1899.

The conclusions arrived at by photographic means at Kew were
communicated to the Royal Society in a series of papers drawn up jointly
by De la Rue, Balfour Stewart, and Benjamin Loewy, in 1865 and

subsequent years. They influenced materially the progress of thought on the
subject they were concerned with.

By its rotation the sun itself offers opportunities for bringing the
stereoscope to bear upon it. Two pictures, taken at an interval of twenty-six
minutes, show just the amount of difference needed to give, by their
combination, the maximum effect of solidity.[448] De la Rue thus obtained,
in 1861, a stereoscopic view of a sun-spot and surrounding faculæ,
representing the various parts in their true mutual relations. "I have
ascertained in this way," he wrote,[449] "that the faculæ occupy the highest
portions of the sun's photosphere, the spots appearing like holes in the
penumbræ, which appeared lower than the regions surrounding them; in one
case, parts of the faculæ were discovered to be sailing over a spot
apparently at some considerable height above it." Thus Wilson's inference
as to the depressed nature of spots received, after the lapse of not far from a
century, proof of the most simple, direct, and convincing kind. A careful
application of Wilson's own geometrical test gave results only a trifle less
decisive. Of 694 spots observed, 78 per cent. showed, as they traversed the
disc, the expected effects of perspective;[450] and their absence in the
remaining 22 per cent. might be explained by internal commotions
producing irregularities of structure. The absolute depth of spot-cavities—at
least of their sloping sides—was determined by Father Secchi through
measurement of the "parallax of profundity"[451]—that is, of apparent
displacements attendant on the sun's rotation, due to depression below the
sun's surface. He found that in every case it fell short of 4,000 miles, and
averaged not more than 1,321, corresponding, on the terrestrial scale, to an
excavation in the earth's crust of 1-1/5 miles. Of late, however, the reality of
even this moderate amount of depression has been denied. Mr. Howlett's
persevering observations, extending over a third of a century, the results of
which were presented to the Royal Astronomical Society in December,
1894,[452] availed to shatter the consensus of opinion which had so long
been maintained on the subject of spot-structure.[453] It has become
impossible any longer to hold that it is uniformly cavernous; and what seem

like actually protruding umbræ are occasionally vouched for on
unimpeachable authority.[454] We can only infer that the forms of sun-spots
are really more various than had been supposed; that they are peculiarly
subject to disturbance; and that the level of the nuclei may rise and fall
during the phases of commotion, like lavas within volcanic craters.

The opinion of the Kew observers as to the nature of such disturbances was
strongly swayed by another curious result of the "statistical method" of
inquiry. They found that of 1,137 instances of spots accompanied by faculæ,
584 had those faculæ chiefly or entirely on the left, 508 showed a nearly
equal distribution, while 45 only had faculous appendages mainly on the
right side.[455] Now the rotation of the sun, as we see it, is performed from
left to right; so that the marked tendency of the faculæ was a lagging one.
This was easily accounted for by supposing the matter composing them to
have been flung upwards from a considerable depth, whence it would reach
the surface with the lesser absolute velocity belonging to a smaller circle of
revolution, and would consequently fall behind the cavities or "spots"
formed by its abstraction. An attempt, it is true, made by M. Wilsing at
Potsdam in 1888[456] to determine the solar rotation from photographs of
faculæ had an outcome inconsistent with this view of their origin. They
unexpectedly gave a uniform period. No trace of the retardation poleward
from the equator, shown by the spots, could be detected in their movements.
But the experiment was obviously inconclusive;[457] and M. Stratonoff's[458]
repetition of it with ampler materials gave a full assurance that faculæ rotate
like spots in periods lengthening as latitude augments.

The ideas of M. Faye were, on two fundamental points, contradicated by the
Kew investigators. He held spots to be regions of uprush and of heightened
temperature; they believed their obscurity to be due to a downrush of
comparatively cool vapours. Now M. Chacornac, observing, at Ville-
Urbanne, March 6, 1865, saw floods of photospheric matter visibly
precipitating themselves into the abyss opened by a great spot, and carrying
with them small neighbouring maculæ.[459] Similar instances were

repeatedly noted by Father Secchi, who considered the existence of a kind
of suction in spots to be quite beyond question.[460] The tendency in their
vicinity, to put it otherwise, is centripetal, not centrifugal; and this alone
seems to negative the supposition of a central uprush.

A fresh witness was by this time at hand. The application of the
spectroscope to the direct examination of the sun's surface dates from
March 4, 1866, when Sir Norman Lockyer (to give him his present title)
undertook an inquiry into the cause of the darkening in spots.[461] It was
made possible by the simple device of throwing upon the slit of the
spectroscope an image of the sun, any part of which could be subjected to
special scrutiny, instead of, as had hitherto been done, admitting rays from
every portion of his surface indiscriminately. The answer to the inquiry was
prompt and unmistakable, and was again, in this case, adverse to the French
theorist's view. The obscurations in question were found to be produced by
no deficiency of emissive power, but by an increase of absorptive action.
The background of variegated light remains unchanged, but more of it is
stopped by the interposition of a dense mass of relatively cool vapours. The
spectrum of a sun-spot is crossed by the same set of multitudinous dark
lines, with some minor differences, visible in the ordinary solar spectrum.
We must then conclude that the same vapours (speaking generally) which
are dispersed over the unbroken solar surface are accumulated in the umbral
cavity, the compression incident to such accumulation being betrayed by the
thickening of certain lines of absorption. But there is also a general
absorption, extending almost continuously from one end of the spot-
spectrum to the other. Using, however, a spectroscope of exceptionally high
dispersive power, Professor Young of Princeton, New Jersey, succeeded in
1883 in "resolving" the supposed continuous obscurity of spot-spectra into a
countless multitude of fine dark lines set very close together.[462] Their
structure was seen still more perfectly, about five years later, by M. Dunér,
[463] Director of the Upsala Observatory, who traced besides some shadowy
vestiges of the crowded doublets and triplets forming the array, from the
spots on to the general solar surface. They cease to be separable in the blue

part of the spectrum; and the ultra-violet radiations of spots show nothing
distinctive.[464]

As to the movements of the constipated vapours forming spots, the
spectroscope is also competent to supply information. The principle of the
method by which it is procured will be explained farther on. Suffice it here
to say that the transport, at any considerable velocity, to or from the eye of
the gaseous material giving bright or dark lines, can be measured by the
displacement of such lines from their previously known normal positions.
In this way movements have been detected in or above spots of enormous
rapidity, ranging up to 320 miles per second. But the result, so far, has been
to negative the ascription to them of any systematic direction. Uprushes and
downrushes are doubtless, as Father Cortie remarks,[465] "correlated
phenomena in the production of a sun-spot"; but neither seem to
predominate as part of its regular internal economy.

The same kind of spectroscopic evidence tells heavily against a theory of
sun-spots started by Faye in 1872. He had been foremost in pointing out
that the observations of Carrington and Spörer absolutely forbade the
supposition that any phenomenon at all resembling our trade-winds exists in
the sun. They showed, indeed, that beyond the parallels of 20° there is a
general tendency in spots to a slow poleward displacement, while within
that zone they incline to approach the equator; but their "proper
movements" gave no evidence of uniformly flowing currents in latitude.
The systematic drift of the photosphere is strictly a drift in longitude; its
direction is everywhere parallel to the equator. This fact being once clearly
recognised, the "solar tornado" hypothesis at once fell to pieces; but M.
Faye[466] perceived another source of vorticose motion in the unequal
rotating velocities of contiguous portions of the photosphere. The "pores"
with which the whole surface of the sun is studded he took to be the smaller
eddies resulting from these inequalities; the spots to be such eddies
developed into whirlpools. It only needs to thrust a stick into a stream to
produce the kind of effect designated. And it happens that the differences of

angular movement adverted to attain a maximum just about the latitudes
where spots are most frequent and conspicuous.

There are, however, grave difficulties in identifying the two kinds of
phenomena. One (already mentioned) is the total absence of the regular
swirling motion—in a direction contrary to that of the hands of a watch
north of the solar equator, in the opposite sense south of it—which should
impress itself upon every lineament of a sun-spot if the cause assigned were
a primary producing, and not merely (as it possibly may be) a secondary
determining one. The other, pointed out by Young,[467] is that the cause is
inadequate to the effect. The difference of movement, or relative drift,
supposed to occasion such prodigious disturbances, amounts, at the utmost,
for two portions of the photosphere 123 miles apart, to about five yards a
minute. Thus the friction of contiguous sections must be quite insignificant.

A view better justified by observation was urged by Secchi in and after the
year 1872, and was presented in an improved form by Professor Young in
his excellent little book on The Sun, published in 1882.[468] Spots are
manifestly associated with violent eruptive action, giving rise to the faculæ
and prominences which usually garnish their borders. It is accordingly
contended that upon the withdrawal of matter from below by the flinging up
of a prominence must ensue a sinking-in of the surface, into which the
partially cooled erupted vapours rush and settle, producing just the kind of
darkening by increased absorption told of by the spectroscope. Round the
edges of the cavity the rupture of the photospheric shell will form lines of
weakness provocative of further eruptions, which will, in their turn, deepen
and enlarge the cavity. The phenomenon thus tends to perpetuate itself, until
equilibrium is at last restored by internal processes. A sun-spot might then
be described as an inverted terrestrial volcano, in which the outbursts of
heated matter take place on the borders instead of at the centre of the crater,
while the cooled products gather in the centre instead of at the borders.

But on the earth, the solid crust forcibly represses the steam gathering
beneath until it has accumulated strength for an explosion, while there is no

such restraining power that we know of in the sun. Zöllner, indeed, adapted
his theory of the solar constitution to the special purpose of procuring it; yet
with very partial success, since almost every new fact has proved adverse to
his assumptions. Volcanic action is essentially spasmodic. It implies
habitual constraint varied by temporary outbreaks, inconceivable in a
gaseous globe, such as we believe the sun to be.

If the "volcanic hypothesis" represented the truth, no spot could possibly
appear without a precedent eruption. The real order of the phenomenon,
however, is exceedingly difficult to ascertain; nor is it perhaps invariable.
Although, in most cases, the "opening" shows first, that may be simply
because it is more easily seen. According to Father Sidgreaves,[469] the
disturbance has then already passed the incipient stage. He considers it
indeed "highly probable that the preparatory sign of a new spot is always a
small, bright patch of facula."

This sequence, if established, would be fatal to Lockyer's theory of sun-
spots, communicated to the Royal Society, May 6, 1886,[470] and further
developed some months later in his work on The Chemistry of the Sun.
Spots are represented in it as incidental to a vast system of solar
atmospheric circulation, starting with the polar out- and up-flows indicated
by observations during some total eclipses, and eventuating in the plunge
downward from great heights upon the photosphere of prodigious masses of
condensed materials. From these falls result, primarily, spots; secondarily,
through the answering uprushes in which chemical and mechanical forces
co-operate, their girdles of flame-prominences. The evidence is, however,
slight that such a circulatory flow as would be needed to maintain this
supposed cycle of occurrences really prevails in the sun's atmosphere; and a
similar objection applies to an "anticyclonic theory" (so to designate it)
elaborated by Egon von Oppolzer in 1893.[471] August Schmidt's optical
rationale of solar phenomena[472] was, on the other hand, a complete
novelty, both in principle and development. Attractive to speculators from
its recondite nature and far-reaching scope, it by no means commended

itself to practical observers, intolerant of finding the all but palpable
realities of their daily experience dealt with as illusory products of "circular
refraction."

A singular circumstance has now to be recounted. On the 1st of September,
1859, while Carrington was engaged in his daily work of measuring the
positions of sun-spots, he was startled by the sudden appearance of two
patches of peculiarly intense light within the area of the largest group
visible. His first idea was that a ray of unmitigated sunshine had penetrated
the screen employed to reduce the brilliancy of the image; but, having
quickly convinced himself to the contrary, he ran to summon an additional
witness of an unmistakably remarkable occurrence. On his return he was
disappointed to find the strange luminous outburst already on the wane;
shortly afterwards the last trace vanished. Its entire duration was five
minutes—from 11.18 to 11.23 A.M., Greenwich time; and during those five
minutes it had traversed a space estimated at 35,000 miles! No perceptible
change took place in the details of the group of spots visited by this
transitory conflagration, which, it was accordingly inferred, took place at a
considerable height above it.[473]

Carrington's account was precisely confirmed by an observation made at
Highgate. Mr. R. Hodgson described the appearance seen by him as that "of
a very brilliant star of light, much brighter than the sun's surface, most
dazzling to the protected eye, illuminating the upper edges of the adjacent
spots and streaks, not unlike in effect the edging of the clouds at sunset."
[474]

This unique phenomenon seemed as if specially designed to accentuate the
inference of a sympathetic relation between the earth and the sun. From the
28th of August to the 4th of September, 1859, a magnetic storm of
unparalleled intensity, extent, and duration, was in progress over the entire
globe. Telegraphic communication was everywhere interrupted—except,
indeed, that it was, in some cases, found practicable to work the lines
without batteries, by the agency of the earth-currents alone:[475] sparks

issued from the wires; gorgeous auroræ draped the skies in solemn crimson
over both hemispheres, and even within the tropics; the magnetic needle
lost all trace of continuity in its movements, and darted to and fro as if
stricken with inexplicable panic. The coincidence was drawn even closer. At
the very instant[476] of the solar outburst witnessed by Carrington and
Hodgson, the photographic apparatus at Kew registered a marked
disturbance of all the three magnetic elements; while, shortly after the
ensuing midnight, the electric agitation culminated, thrilling the earth with
subtle vibrations, and lighting up the atmosphere from pole to pole with the
coruscating splendours which, perhaps, dimly recall the times when our
ancient planet itself shone as a star.

Here then, at least, the sun was—in Professor Balfour Stewart's phrase
—"taken in the act"[477] of stirring up terrestrial commotions. Nor have
instances since been wanting of an indubitable connection between
outbreaks of individual spots and magnetic disturbances. Four such were
registered in 1882; and symptoms of the same kind, including the beautiful
"Rose Aurora," marked the progress across the sun of the enormous spot-
group of February, 1892—the largest ever recorded at Greenwich. This
extraordinary formation, which covered about 1/300 of the sun's disc,
survived through five complete rotations.[478] It was remarkable for a
persistent drift in latitude, its place altering progressively from 17° to 30°
south of the solar equator.

Again, the central passage of an enormous spot on September 9, 1898,
synchronised with a sharp magnetic disturbance and brilliant aurora;[479]
and the coincidence was substantially repeated in March, 1899,[480] when it
was emphasised by the prevalent cosmic calm. The theory of the connection
is indeed far from clear. Lord Kelvin, in 1892,[481] pronounced against the
possibility of any direct magnetic action by the sun upon the earth, on the
ground of its involving an extravagant output of energy; but the fact is
unquestionable that—in Professor Bigelow's words—"abnormal agitations
affect the sun and the earth as a whole and at the same time."[482]

The nearer approach to the event of September 1, 1859, was
photographically observed by Professor George E. Hale at Chicago, July
15, 1892.[483] An active spot in the southern hemisphere was the scene of
this curiously sudden manifestation. During an interval of 12m. between
two successive exposures, a bridge of dazzling light was found to have
spanned the boundary-line dividing the twin-nuclei of the spot; and these,
after another 27m., were themselves almost obliterated by an overflow of
far-spreading brilliancy. Yet two hours later, no trace of the outburst
remained, the spot and its attendant faculæ remaining just as they had been
previously to its occurrence. Unlike that seen by Carrington, it was
accompanied by no exceptional magnetic phenomena, although a "storm"
set in next day.[484] Possibly a terrestrial analogue to the former might be
discovered in the "auroral beam" which traversed the heavens during a
vivid display of polar lights, November 17, 1882, and shared, there is every
reason to believe, their electrical origin and character.[485]

Meantime M. Rudolf Wolf, transferred to the direction of the Zürich
Observatory, where he died, December 6, 1893, had relaxed none of his zeal
in the investigation of sun-spot periodicity. A laborious revision of the
entire subject with the aid of fresh materials led him, in 1859,[486] to the
conclusion that while the mean period differed little from that arrived at in
1852 of 11.11 years, very considerable fluctuations on either side of that
mean were rather the rule than the exception. Indeed, the phrase "sun-spot
period" must be understood as fitting very loosely the great fact it is taken
to represent; so loosely, that the interval between two maxima may rise to
sixteen and a half or sink below seven and a half years.[487] In 1861[488]
Wolf showed, and the remark was fully confirmed at Kew, that the shortest
periods brought the most acute crises, and vice versâ; as if for each wave of
disturbance a strictly equal amount of energy were available, which might
spend itself lavishly and rapidly, or slowly and parsimoniously, but could in
no case be exceeded. The further inclusion of recurring solar commotions
within a cycle of fifty-five and a half years was simultaneously pointed out;
and Hermann Fritz showed soon afterwards that the aurora borealis is

subject to an identical double periodicity.[489] The same inquirer has more
recently detected both for auroræ and sun-spots a "secular period" of 222
years,[490] and the Kew observations indicate for the latter, oscillations
accomplished within twenty-six and twenty-four days,[491] depending, most
likely, upon the rotation of the sun. This is certainly reflected in magnetic,
and perhaps in auroral periodicity. The more closely, in fact, spot-
fluctuations are looked into, the more complex they prove. Maxima of one
order are superposed upon, or in part neutralised by, maxima of another
order;[492] originating causes are masked by modifying causes; the larger
waves of the commotion are indented with minor undulations, and these
again crisped with tiny ripples, while the whole rises and falls with the
swell of the great secular wave, scarcely perceptible in its progress because
so vast in scale.

The idea that solar maculation depends in some way upon the position of
the planets occurred to Galileo in 1612.[493] It has been industriously sifted
by a whole bevy of modern solar physicists. Wolf in 1859[494] found reason
to believe that the eleven-year curve is determined by the action of Jupiter,
modified by that of Saturn, and diversified by influences proceeding from
the earth and Venus. Its tempting approach to agreement with Jupiter's
period of revolution round the sun, indeed, irresistibly suggested a causal
connection; yet it does not seem that the most skilful "coaxing" of figures
can bring about a fundamental harmony. Carrington pointed out in 1863,
that while, during eight successive periods, from 1770 downwards, there
were approximate coincidences between Jupiter's aphelion passages and
sun-spot maxima, the relation had been almost exactly reversed in the two
periods preceding that date;[495] and Wolf himself finally concluded that the
Jovian origin must be abandoned.[496] M. Duponchel's[497] prediction,
nevertheless, of an abnormal retardation of the maximum due in 1881
through certain peculiarities in the positions of Uranus and Neptune about
the time it fell due, was partially verified by the event, since, after an
abortive phase of agitation in April, 1882, the final outburst was postponed
to January, 1894. The interval was thus 13.5 instead of 11.1 years; and it is

noticeable that the delay affected chiefly the southern hemisphere.
Alternations of activity in the solar hemispheres were indeed a marked
feature of the maximum of 1884, which, in M. Faye's view,[498] derived
thence its indecisive character, while sharp, strong crises arise with the
simultaneous advance of agitation north and south of the solar equator. The
curve of magnetic disturbance followed with its usual strict fidelity the
anomalous fluctuations of the sun-spot curve. The ensuing minimum
occurred early in 1889, and was succeeded in 1894 by a maximum slightly
less feeble than its predecessor.[499]

It cannot be said that much progress has been made towards the disclosure
of the cause, or causes, of the sun-spot cycle. No external influence
adequate to the effect has, at any rate, yet been pointed out. Most thinkers
on this difficult subject provide a quasi-explanation of the periodicity in
question through certain assumed vicissitudes affecting internal processes;
[500] Sir Norman Lockyer and E. von Oppolzer reach the same end by
establishing self-compensatory fluctuations in the solar atmospheric
circulation; Dr. Schuster resorts to changes in the electrical conductivity of
space near the sun.[501] In all these theories, however, the course of
transition is arbitrarily arranged to suit a period, which imposes itself as a
fact peremptorily claiming admittance, while obstinately defying
explanation.

The question so much discussed, as to the influence of sun-spots on
weather, does not admit of a satisfactory answer. The facts of meteorology
are too complex for easy or certain classification. Effects owning
dependence on one cause often wear the livery of another; the meaning of
observed particulars may be inverted by situation; and yet it is only by the
collection and collocation of particulars that we can hope to reach any
general law. There is, however, a good deal of evidence to support the
opinion—the grounds for which were primarily derived from the labours of
Dr. Meldrum at Mauritius—that increased rainfall and atmospheric
agitation attend spot-maxima; while Herschel's conjecture of a more

copious emission of light and heat about the same epochs has recently
obtained some countenance from Savélieff's measures showing a gain in the
strength of the sun's radiation pari passu with increase in the number of
spots visible on his surface.[502]

The examination of what we may call the texture of the sun's surface
derived new interest from a remarkable announcement made by Mr. James
Nasmyth in 1862.[503] He had made (as he supposed) the discovery that the
entire luminous stratum of the sun is composed of a multitude of elongated
shining objects on a darker background, shaped much like willow-leaves, of
vast size, crossing each other in all possible directions, and possessed of
unceasing relative motions. A lively controversy ensued. In England and
abroad the most powerful telescopes were directed to a scrutiny
encompassed with varied difficulties. Mr. Dawes was especially emphatic
in declaring that Nasmyth's "willow-leaves" were nothing more than the
"nodules" of Sir William Herschel seen under a misleading aspect of
uniformity; and there is little doubt that he was right. It is, nevertheless,
admitted that something of the kind may be seen in the penumbræ and
"bridges" of spots, presenting an appearance compared by Dawes himself in
1852 to that of a piece of coarse straw-thatching left untrimmed at the
edges.[504]

The term "granulated," suggested by Dawes in 1864,[505] best describes the
mottled aspect of the solar disc as shown by modern telescopes and
cameras. The grains, or rather the "floccules," with which it is thickly
strewn, have been resolved by Langley, under exceptionally favourable
conditions, into "granules" not above 100 miles in diameter; and from these
relatively minute elements, composing, jointly, about one-fifth of the visible
photosphere,[506] he estimates that three-quarters of the entire light of the
sun are derived.[507] Janssen agrees, so far as to say that if the whole surface
were as bright as its brightest parts, its luminous emission would be ten to
twenty times greater than it actually is.[508]

The rapid changes in the forms of these solar cloud-summits are beautifully
shown in the marvellous photographs taken by Janssen at Meudon, with
exposures reduced at times to 1/100000 of a second! By their means, also,
the curious phenomenon known as the réseau photosphérique has been
made evident.[509] This consists in the diffusion over the entire disc of
fleeting blurred patches, separated by a reticulation of sharply-outlined and
regularly-arranged granules. The imperfect definition in the smudged areas
may be due to agitations in the solar or terrestrial atmosphere, unless it be—
as Dr. Schemer thinks possible[510]—merely a photographic effect. M.
Janssen considers that the photospheric cloudlets change their shape and
character with the progress of the sun-spot period;[511] but this is as yet
uncertain.

The "grains," or more brilliant parts of the photosphere, are now generally
held to represent the upper termination of ascending and condensing
currents, while the darker interstices (Herschel's "pores") mark the positions
of descending cooler ones. In the penumbræ of spots, the glowing streams
rushing up from the tremendous sub-solar furnace are bent sideways by the
powerful indraught, so as to change their vertical for a nearly horizontal
motion, and are thus taken, as it were, in flank by the eye, instead of being
seen end-on in mamelon-form. This gives a plausible explanation of the
channelled structure of penumbræ which suggested the comparison to a
rude thatch. Accepting this theory as in the main correct, we perceive that
the very same circulatory process which, in its spasms of activity, gives rise
to spots, produces in its regular course the singular "marbled" appearance,
for the recording of which we are no longer at the mercy of the fugitive or
delusive impressions of the human retina. And precisely this circulatory
process it is which gives to our great luminary its permance as a sun, or
warming and illuminating body.

FOOTNOTES:
[405] Mem. R. A. S., vol. xxi., p. 157.

[406] Ibid., p. 160.

[407] Month. Not., vol. xxi., p. 144.
[408] Le Soleil, t. i., pp. 87-90 (2nd ed., 1871).

[409] See ante, p. 58.

[410] Observations at Redhill (1863), Introduction.

[411] Month. Not., vol. xxxvi., p. 142.
[412] Cape Observations, p. 435, note.

[413] Month. Not., vol. x., p. 158.

[414] Rosa Ursina, lib. iii., p. 348.

[415] Observations at Redhill, p. 8.

[416] Op., t. iii., p. 402.

[417] Rosa Ursina, lib. iv., p. 601. Both Galileo and Scheiner spoke of the
apparent or "synodical" period, which is about one and a third days longer than
the true or "sidereal" one. The difference is caused by the revolution of the earth
in its orbit in the same direction with the sun's rotation on its axis.
[418] Rosa Ursina, lib. iii., p. 260.

[419] Faye, Comptes Rendus, t. lx., p. 818.

[420] Ibid., t. xii., p. 648.

[421] Proc. Am. Ass. Adv. of Science, 1885, p. 85.

[422] Observations at Redhill, p. 221.

[423] Am. Jour. of Science, vol. xi., p. 169.

[424] Month. Not., vol. xix., p. 1.

[425] Vierteljahrsschrift der Naturfors. Gesellschaft (Zürich), 1859, p. 252.

[426] Lockyer, Chemistry of the Sun, p. 428.

[427] Maunder, Knowledge, vol. xv., p. 130.

[428] Month. Mon., vol. l., p. 251.