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Title: The Microscope. Its History, Construction, and Application 15th
ed.

Author: Jabez Hogg

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*** START OF THE PROJECT GUTENBERG EBOOK THE
MICROSCOPE. ITS HISTORY, CONSTRUCTION, AND APPLICATION
15TH ED. ***

RADIOLARIA.

THE

MICROSCOPE
ITS HISTORY, CONSTRUCTION, AND APPLICATION

BEING A FAMILIAR INTRODUCTION TO THE USE OF
THE INSTRUMENT, AND THE STUDY OF
MICROSCOPICAL SCIENCE

By JABEZ HOGG, M.R.C.S., F.R.M.S.,
FORMERLY AND FOR TWENTY-FIVE YEARS SURGEON TO THE ROYAL WESTMINSTER
OPHTHALMIC
HOSPITAL; PAST PRESIDENT OF THE MEDICAL MICROSCOPICAL SOCIETY; HONORARY
FELLOW OF THE ACADEMY OF SCIENCES, PHILADELPHIA; OF THE MEDICO-LEGAL
SOCIETY, NEW YORK; OF THE BELGIAN MICROSCOPICAL SOCIETY, ETC.; AUTHOR
OF “ELEMENTS OF NATURAL PHILOSOPHY,” “A MANUAL OF
OPHTHALMOSCOPIC SURGERY,” ETC.

WITH UPWARDS OF
NINE HUNDRED
ENGRAVED FIFTEENTH EDITION
AND COLOURED RE-CONSTRUCTED,
ILLUSTRATIONS BY RE-WRITTEN,
TUFFEN WEST REVISED, AND
AND An 18th Century ENLARGED
OTHER ARTISTS Microscope. THROUGHOUT
LONDON AND NEW YORK
GEORGE ROUTLEDGE & SONS, LIMITED
1898

BRADBURY, AGNEW, & CO. I.D., PRINTERS,
LONDON AND TONBRIDGE.

PREFACE TO THE FIFTEENTH EDITION.

T HE First Edition of this work appeared in 1854, a time in the history of
the Microscope when the instrument, as an aid to original scientific
research, may be said to have been in its infancy. Then certainly it was
seldom employed in the laboratory or the medical schools. Now, however,
as I anticipated, it has asserted its proper position, and has at length become
one of the most important auxiliaries to science, and a direct incentive to
original work, while it has doubtless exercised considerable influence over
the student’s power of observation, and materially assisted in his studies, let
his ultimate object and pursuits be what they may.

The greater use made of the Microscope has likewise conferred benefits of
untold value upon the arts and industries of the country, thereby adding to
the national prosperity in ways as manifold as unique. The Microscope has
also proved of immense value in the promotion of the health of the
community, and the art and science of healing, since the theory of medicine
has become a science, resting on the minute microscopical examination of
animal tissues.

The work of research in the sister sciences and by other methods has,
during the last decade, received a corresponding impetus, while it has
undoubtedly tended towards elaboration and specialisation in all
departments. In consequence, the progress of microscopical science has
become more dependent upon the specialist for gaining accurate knowledge
and for certain important details seen to be branching out in many
directions. There never was a time when the instrument was so constantly
and generally resorted to and with so much confidence and advantage, as
the present. It has shown itself equal to the task imposed—that of teaching
the eye to see things that are new, and also, what is perhaps of more
importance, to perceive things which had been entirely overlooked. The
older defects, perhaps, arose from two causes; the want of more careful
training of the organ of vision, and the want of sufficient power and

precision in the optical part of the Microscope itself. Both of these obstacles
have been to a considerable extent removed, and all educational systems are
looked upon as incomplete without a knowledge of the Microscope.

A step has already been taken in another direction, that of furnishing special
forms of instruments, better adapted to the uses to which they will hereafter
be put, and purposely designed for chemical and analytical processes, for
petrological pursuits, the geometrical measurement of crystals, for special
work in connection with manufacturing industries, for the dairyman, and
the farmer. For the detection of adulterations—that of butter, for example—
a newer form of instrument has been devised, namely, a “Butro-
refractometer,” by the help of which any adulteration of this universal
article of diet will at once be revealed. The form of instrument upon which
the optician has expended a greater amount of skill than perhaps on any
other is the Bacteriological Microscope, as may be inferred from the larger
space I have devoted to this important adjunct, since by original research,
there can be no doubt a still greater future is in store for science in this
special department of microscopy. But perfect success in this direction
remains very much with the practical optician, and the further
improvements made in the optical part of the instrument, since it is admitted
that the highest theoretical perfection has not yet been reached.

It is a commonplace remark that every question solved is a step towards
new problems waiting solution. It is equally obvious that many difficulties
must be encountered by every author who uses his best endeavours to
supply a standard volume or even a fairly comprehensive text-book on the
Microscope, one that will remain a sure guide for any lengthened period.
Such a success I regard as scarcely possible. I may, however, notice that my
earlier work has met with a great amount of appreciation, and its utility
acknowledged in the past by a demand almost unprecedented, edition after
edition being called for.

It is hardly necessary to add that my task has been accomplished with an
earnest desire to assist in diffusing a love for an instrument which has been
my constant companion for upwards of sixty years.1 Moreover, I have a
firm conviction of the real utility of the Microscope in the work of
education, its practical value in many branches of science, art, and

manufacturing industries. These are my chief reasons for applying myself
once more to the task of revision, rewriting, and rearranging and bringing
this book as far as possible into line with the knowledge gained in chemical
pathology and bacteriology.

It will be noticed that in the first part, my subjects have as far as possible
been treated from a historical point of view. This method has enabled me to
affix dates of introduction of special inventions and improvements made in
the instrument and its appliances. The enlargement of my pages has enabled
me to devote more space to bacteriological processes, and by the further
addition of plates and several hundred illustrations to more fully elucidate
the subject matter of my text. In an Appendix I have introduced a selection
of “Formulæ and Methods” of staining, mounting, etc., also tables of the
“Metrical System,” now in general use in the laboratory; together with
comparative thermometric values, all of which I trust may prove of service
to the student.

Before bringing these few prefatory remarks to a close, a pleasing duty
devolves upon me—that of tendering my thanks for cordial aid received
from Professor Dr. Edgar Crookshank in dealing with his special subject,
Bacteriology. From his valuable “Text-Book on Bacteriology” I have
extracted much useful matter. I am equally indebted to Professor Marshall
Ward, F.R.S., Cambridge, for much information on “Economic Botany,”
and the great advances made in the knowledge of the uses of plants, and the
industrial value of bacteria in particular. My acknowledgments are also due
to the Messrs. Warne for many illustrations placed at my disposal, and for
useful facts derived from their “Royal Natural History.” It will, however, be
seen that the results of a large amount of independent observation have been
consigned to my pages. As the references show, recourse has been had to
original sources for trustworthy, reliable information on many subjects.
These are constantly, almost daily, being added to, as is made manifest by
the numerous periodical publications of the day devoted to this and kindred
sciences; the foremost and most important among which is that almost
exclusively given to microscopical science, “The Journal of the Royal
Microscopical Society of London,” the perusal of which I commend to my
readers.

London, July, 1898.

PREFACE TO THE FIRST EDITION.

T HE Author of this Publication entered upon his task with some
hesitation and diffidence; but the reasons which influenced him to
undertake it may be briefly told, and they at once explain his motives, and
plead his justification, for the work which he now ventures to submit to the
indulgent consideration of his readers.

It had been to him for some time a subject of regret that one of the most
useful and fascinating studies—that which belongs to the domain of
microscopic observation—should be, if not wholly neglected, at best but
coldly and indifferently appreciated by the great mass of the general public;
and he formed a strong opinion that this apathy and inattention were mainly
attributable to the want of some concise, yet sufficiently comprehensive,
popular account of the Microscope, both as regards the management and
manipulation of the instrument, and the varied wonders and hidden realms
of beauty that are disclosed and developed by its aid. He saw around him
valuable, erudite, and splendid volumes, which, however, being chiefly
designed for circulation amongst a special class of readers, were necessarily
published at a price that renders them practically unattainable by the great
bulk of the public. They are careful and beautiful contributions to the
objects of science, but they do not adequately bring the value and charm of
microscopic studies home, so to speak, to the firesides of the people. Day
after day, new and interesting discoveries, and amplifications of truth
already discerned, have been made, but they have been either sacrificed in
serials, or, more usually, devoted to the pages of class publications; and thus
this most important and attractive study has been, in a great measure, the
province of the few only, who have derived from it a rich store of
enlightenment and gratification: the many not having, however,
participated, to any great extent, in the instruction and entertainment which
always follow in the train of microscopical science.2

The manifold uses and advantages of the Microscope crowd upon us in
such profusion, that we can only attempt to enumerate them in the briefest
and most rapid manner in these prefatory pages.

It is not many years since this invaluable instrument was regarded in the
light of a costly toy; it is now the inseparable companion of the man of
science. In the medical world, its utility and necessity are fully appreciated,
even by those who formerly were slow to perceive its benefits; now,
knowledge which could not be obtained even by the minutest dissection is
acquired readily by its assistance, which has become as essential to the
anatomist and pathologist as are the scalpel and bedside observation. The
smallest portion of a diseased structure, placed under a Microscope, will tell
more in one minute to the experienced eye than could be ascertained by
long examination of the mass of disease in the ordinary method.
Microscopic agency, in thus assisting the medical man, contributes much to
the alleviation of those multiplied “ills which flesh is heir to.” So fully
impressed were the Council of the Royal College of Surgeons with the
importance of the facts brought to light in a short space of time, that, in
1841, they determined to establish a Professorship of Histology, and to form
a collection of preparations of the elementary tissues of both animals and
vegetables, healthy and morbid, which should illustrate the value of
microscopical investigations in physiology and medical science. From that
time, histological anatomy deservedly became an important branch of the
education of the medical student.

In the study of Vegetable Physiology, the Microscope is an indispensable
instrument; it enables the student to trace the earliest forms of vegetable
life, and the functions of the different tissues in the growth of plants.
Valuable assistance is derived from its agency in the detection of
adulterations. In the examination of flour, an article of so much importance
to all, the Microscope enables us to judge of the size and shape of the
starch-grains, their markings, their isolation and agglomeration, and thus to
distinguish the starch-grains of one meal from those of another. It detects
these and other ingredients, invisible to the naked eye, whether combined in
atoms or aggregated in crystals, which adulterate our food, our drink, and
our medicines. It discloses the lurking poison in the minute crystallisations
which its solutions precipitate. “It tells the murderer that the blood which

stains him is that of his brother, and not of the other life which he pretends
to have taken; and as a witness against the criminal, it on one occasion
appealed to the very sand on which he trod at midnight.”

The zoologist finds in the Microscope a necessary coadjutor. To the
geologist it reveals, among a multiplicity of other facts, “that our large coal-
beds are the ruins of a gigantic vegetation; and the vast limestone rocks,
which are so abundant on the earth’s surface, are the catacombs of myriads
of animal tribes, too minute to be perceived by the unaided vision.”

By “conducting the eye to the confines of the visible form,” the Microscope
proves an effective auxiliary in defining the geometric properties of bodies.
Its influence as an instrument of research upon the structure of bodies has
been compared to that of the galvanic battery, in the hands of Davy, upon
Chemistry. It detects the smallest structural difference, heretofore
inappreciable, and, as an ally of Chemistry, enables us to discover the very
small changes of form and colour effected by test-fluids upon solids; and
dissects for us, so to speak, the most multiplex compounds. It opens out to
the mind an extended and vast tract, opulent in wonders, rich in beauties,
and boundless in extent.

The Microscope not only assists studies, and develops objects of profound
interest, but also opens up innumerable sources of entertainment and
amusement, in the ordinary conventional acceptation of these terms;
disclosing to us peculiarities and attractions in abundance; impressing us
with the wonderful and beautifully skilful adaptation of all parts of creation,
and filling our minds with additional reverence and admiration for the
beneficent and Almighty Creator.

The Author will conclude these prefatory observations with a few words in
explanation of his arrangements, by way of dealing with the instrument and
development of his subject. He has sought, in the volume that he now lays
before the public, to point out and elucidate at once in a practical manner
and in a popular style, the vast fund of utility and amusement which the
Microscope affords, and has endeavoured to touch upon most of the
interesting subjects for microscopic observation as fully as the restrictions
of a limited space, and the nature of the succinct summary, would permit.
To have dwelt upon each in complete detail would have necessitated the

issue of many expensive volumes—and this would have entirely frustrated
the aim which the writer had in view; he has, therefore, contented himself
with the humble, but, he trusts, not useless, task of setting up a finger-post,
so to say, to direct the inquirer into the wider road. In the section of the
work devoted to the minuter portion of creation, he has ventured to dwell
somewhat longer, in the belief that that department is more especially the
province of the microscopist. He has arranged his topics under special
headings, and in separate chapters, for the sake of perspicuity and precision;
and has brought the ever-welcome aid of illustration to convey his
explanatory remarks more vividly to the minds of his readers.

Finally, it is the Author’s hope that, by the instrumentality of this volume,
he may possibly assist in bringing the Microscope, and its valuable and
delightful studies, before the general public in a more familiar,
compendious, and economical form than he found it at the period of its
publication, so that, in these days of a diffused taste for reading and the
spread of cheap publications, he may thus supply further exercise for the
intellectual faculties; contribute to the additional amusement and instruction
of the family circle, and aid the student of nature in investigating the
wonderful and exquisite works of the Almighty. If it shall be the good
fortune for this work, which is now confided with great diffidence to the
consideration of the public, to succeed, in however slight a degree, in
furthering this design, the Author will feel fully repaid for the amount of
time and labour expended.

London, May, 1854.

CONTENTS.

PART I.
PAGE
Early History of the Microscope 1
CHAPTER I.
Light—Its Propagation—Refraction—Spherical and Chromatic
Aberration—Human Eye—Formation of Images—Visual Angle
—Abbé’s Theory of Microscopic Vision—Definition of Aperture
—Numerical Aperture—Abbé’s Apertometer—Stereoscopic
Binocular Vision 12
CHAPTER II.
Simple and Compound Microscopes—Evolution of the Modern
Achromatic Microscope—Improvements in the Modern
Microscope: The Ross-Jackson, Powell and Lealand, Beck’s,
Baker’s, Pillischer, Zeiss, Leitz, Watson’s, Van Heurck’s
Edinburgh Students’, Swift’s, and Bacteriological 72
CHAPTER III.
Applied Optics—Eye-pieces—Achromatic Objectives and
Condensers—Magnifying Power of Eye-pieces and Objectives—
Method of Employing the Condenser—Its Illumination Mirror
—Accessories of the Microscope—Finders—Micrometers—
Camera-Lucida—Microscope in Position for Drawing—Abbé’s
Test-Plate—Micro-Photography—Polarisation of Light—The
Micro-Spectroscope 139
CHAPTER IV.
Practical Microscopy—Manipulation—Mode of Employing the 258
Microscope—Working Accessories—Methods of Preparing,
Hardening, Staining, and Section Cutting—Cutting Hard

Structures—Injecting Apparatus, Material, etc., employed in
Bacteriological Investigations—Warm Chambers and
Incubators—Preparing, Mounting, Cementing, and Collecting
Objects
PART II.
CHAPTER I.
Microscopic Forms of Life—Thallophytes, Pteridophyta,
Phanerogamæ—Structure and Properties of the Cell—
Pathogenic Fungi and Moulds—Parasitic Diseases of Plants,
Man, and Animals—Industrial Uses of Fungi and
Saccharomycetes—Desmidiaceæ and Diatomaceæ—Lichens,
Mosses, Ferns, Flowering Plants 353
CHAPTER II.
Sub-Kingdom Protozoa—Rhizopoda—Gromia and Foraminifera—
Infusoria—Ciliata—Rotifera—Porifera—Sponges 478
CHAPTER III.
Zoophytes—Cœlenterata—Medusæ—Corals—Hydrozoa—
Mollusca—Annulosa—Worms—Entozoa—Annelida—
Crustacea 515
CHAPTER IV.
Arthropoda—Insecta—Arachnida—Acarina—Ixodidæ—Mites
and Ticks 583
CHAPTER V.
Vertebrata—Internal and External Structures 633
CHAPTER VI.
The Mineral and Geological Kingdoms 670
APPENDIX.
Dr. Mercer on Illumination, and Abbé’s Theory—Micro-
Spectroscope—Formulæ, Methods, Cementing, Clearing,
Hardening, and Mounting—Tables, Metric and Thermometric 672
INDEX 691

DESCRIPTION OF PLATES,
COLOURED AND PLAIN.

FRONTISPIECE.

RADIOLARIA.

In this Plate Fig. 1 shows the elegant lattice-sphere of Rhizosphæra; Fig. 2
represents Sphærozoum, whose skeleton consists of loose spicules,
arranged tangentially; Actinomma, Fig. 3, possesses three concentric
lattice-spheres, joined by radiating spines; Figs. 4, 5, and 6, represent
Lithomespilus, Ommatocampe, and Carpocanium; Fig. 7 represents a
deep-sea form (Challengeria), whose oval case is formed of a regular,
very fine-meshed, network; Fig. 8 depicts the elegant lattice-sphere of
Heliosphæra; Figs. 9 and 10, Clathrocyclas and Dictyophimus.

PLATE I.—Page 400.

PROTOPHYTA. THALLOPHYTES.

Fig. 1. Peziza bicolor—2. Truffle: a. ascus of spores; b. mycelium—3.
Sphæria herbarum: a. piece of dead plant, with S. herbarum natural
size; b. section of same, slightly magnified; d. Ascus with spores, and
paraphyses more magnified—4. Peziza pygmæa—5. Apical form of
same—6. P. corpulasis: Ascus with spores and paraphyses, merely
given as a further illustration of structure in Peziza—7. Yeast healthy
—8. Yeast exhausted—9. Phyllactinia guttata—10. Yeast with favus

spores and mycelium of fungus—11. Favus ferment, with oïdium and
bacteria—12. Puccinia spores, growing in a saccharine solution—13.
Aerobic bacteria—14. Spores and mycelia from eczema produced by
yeast—15. Volvox globator—16. Amœboid condition of portion of
volvox—17. Puccinia buxi—18. Ditto, more enlarged—(17 to 20
illustrate Ascomycetes.)—19. Æcidium grossulariæ from transverse
section of leaf of currant: a. spermogones on upper surface; b.
perithecia with spores—20. Phragmidium bulbosum, development of
—21. Palmella parietina, trans. section through a spermogone,
showing green gonidia and spermatia escaping—22. Æcidium
berberida, from leaf of berberry—23. Vaucheria sessilis—24.
Stephanosphæra pluvialis: a. Full-grown example, germ cells spindle-
shaped with flagella; b. Resting-cell; c. division into four; d. Free-
swimming ciliated young specimen; e. Amœboid condition—25. a, b,
c, d, e, f and g, Development of lichen gonidia—26. Palmella stellaris
(lichen), vertical section through apothecium, showing asci, spores,
and paraphyses, with gonidia and filamentous medulla: a.
Spermatophore with spermatia—27. Moss gonidia assuming amœboid
form.

Typical forms of Protophyta; 7 to 14, modes of development or rudimentary
conditions; Confervoideæ, 23; Vaucheria, Stephanosphæra, 24; Volvox, 15,
&c.

PLATE II.—Page 412.

PROTOPHYTA. ALGÆ.

Fig. 27. Ceramium acanthonotum—28. Closterium, Triploceras gracilis—
29. Cosmarium radiatum—30. Micrasterias denticulata—31.
Docidium pristidæ—32. Callithamnion plumula—33. Diatoma, living:
a. Licmophora splendida; b. Achnanthes longipes; c. Grammatophora
marina. These figures are intended to show the general character of the
endochrome and growth of frustule—34. Callithamnion refractum—
35. Jungermannia albicans; b. representing elater and spores—36. Leaf

with antheridia, or male elements, represented more magnified at a to
the left of the figure—37. Ceramium echinotum—38. Pleurosigma
angulatum, side view—39. Delesseria hypoglossum—40. Pleurosigma
angulatum, front view, endochrome not represented—41. Ceramium
flabelligerum.

PLATE III.—Page 479.

PROTOZOA.

Figs. 43, 44, 45, 46, 47, 48, 49, 50, 51, 52. These figures are from drawings
made by Major Owen, to illustrate forms of living Polycystina,
sketched from life; these convey a faint idea of the richly coloured
appearance of the natural structure; Figs. 48 to 52—53. Gregarina
lumbricorum, round form—54. Gregarina lumbricorum, the usual
elongated form—55. Gregarina serpulæ—56. Gregarina Sieboldii;
illustration of septate form, with reflexed hook-like processes—57.
Gregarina lumbricorum, encysted—58. Gregarina lumbricorum, more
advanced and pseudo-navicellæ forming—59. Gregarina lumbricorum,
free pseudo-navicella of—60, 61. Gregarina lumbricorum, amœboid
forms of—62. Cruciate sponge-spicule—63. Astromma Humboldtii—
64. Eözoon Canadense, represents appearance of a portion of the
natural size—65. Eözoon Canadense, magnified, showing portions of
cell-walls left uncoloured, the animal sarcode inhabiting it coloured
dark green as in nature, and converted by fossilisation into a silicious
mineral; the narrow bands passing between these are processes
(stolons) of the same substance—66. Actinophrys sol, budding—67.
Euglena viridis: a. contracted; b. elongated form—68. Acineta
tuberosa—69. Œcistes longicornis (Davis)—70. Oxytricha gibba (side
view)—71. Oxytricha pellionella—72. Thuricola valvata, expanded—
73. Cyclidium (glaucoma)—74. Oxytricha scintillans—75 to 79, 80 to
85, illustrate types of Foraminifera discovered by Major Owen, living
—75. Globigerina acerosa, n. sp., broken open to show interior—76.
Globigerina, n. sp., broken open to show interior—77. Globigerina
hirsuta—78. Globigerina universa—79 and 81. G. Bulloides—80.

Conochilus vorticella—82. Globigerina inflata, sinistral shell—83.
Pulvinulina Micheliniana—84. P. Canariensis—85. P. Menardii.

PLATE IV.—Page 514.

METAZOA. BRYOZOA.

Fig. 86. Hartea elegans—87. Side view of Synapta spicula—88.
Ophioglypha rosula (very immature specimen): a. Claw hooks; b.
palmate spicula. The development of this species is described by G.
Hodge, in “Transactions of Tyneside Naturalists’ Field-Club”—89.
Spine of a star-fish, particularly interesting as showing the reticular
calcareous network obtaining in this as in all other hard parts of the
Echinodermata—90. Very minute Spatangus, obtained from stomach
of a bream: many of the spines are gone, but the structure of the shell
is intact and forms a beautiful object, interesting in connection with the
source whence obtained—91. Ophioglypha neglecta: wriggling or
brittle starfish. The plate does not admit of a figure on a scale
sufficient to show the full beauty of this object—92. Tubularia
Dumortierii—93. Pedicellaria mandibulata from Uraster glacialis—94.
Pedicellaria forcepiforma, from the same—95. Cristatella mucedo; 96.
Edge-view of statoblast; 97. early stage in development of same—98.
Lophopus crystallinus—99. Plumatella repens with ova, on submerged
stem—100. Tænia echinococcus—101. Hydatids from human liver—
102. Bilharzia hæmatobia—103. Amphistoma conicum—104. Trichina
spiralis from fleshy part of Hambrc’ pork—105. Trichina spiralis male,
separated from muscle.—106, 107. Fasciola gigantea.

PLATE V.—Page 556.

MOLLUSCA.

Fig. 108. Velutina lævigata, portion of lingual membrane—109. Velutina
lævigata, part of mandible—110. Hybocystis blennius, portion of

palate—111. Sepia officinalis, portion of palate—112. Aplysia hybrida,
part of mandible—113. Loligo vulgaris, part of palate—114. Haliotis
tuberculatus, part of palate—115. Cistula catenata, part of palate—116.
Patella radiata, part of palate—117. Acmæa virginea, part of palate—
118. Cymba olla, part of palate—119. Scapander ligniarius—120.
Oneidoris bilamellata, part of palate—121. Testacella Maugei, part of
palate—122. Pleurobranchus plumula, part of mandible—123. Turbo
marmoratus, part of palate.

Lingual membranes of Mollusca; drawings made from specimens in the
collection formed by F. E. Edwards, Esq., now in the British Museum.
Typical examples of the numerous forms of Odontophors met with in
Gasteropod and Cephalopod Mollusca.

PLATE VI.—Page 582.

INSECTA.

Fig. 124. Egg of Caradrina morpheus, mottled rustic moth—125. Egg of
tortoise-shell butterfly, Vanessa urticæ—126. Egg of common
footman, Lithosia complanula—127. Egg of shark moth, Cucullia
umbratica—128. Maple-aphis—129. Egg shell of acarus, empty—130.
Egg of house-fly—131. Mouth of Tsetse-fly, Glossina morsitans—132.
Vapourer moth, Orgyia antiqua: antenna of male—133. Vapourer
moth: antenna of female; a. branch more magnified to show
rudimentary condition of the parts—134. Tortoise-shell butterfly; head
in profile, showing large compound eye, one of the palpi, and spiral
tongue—135. Tortoise-beetle, Cassida viridis; under surface of left
fore-foot, to show the bifurcate tenent appendages, one of which is
given at a more magnified. This form of appendage is characteristic of
the family. “West on Feet of Insects,” Linn. Trans. vol. xxiii. tab. 43-
136. Egg of blue argus butterfly, Polyommatus argus—137. Egg of
mottled umber, Erannis defoliaria—138. Egg of Ennomos erosaria,
thorn-moth—139. Egg of Aspilates gilvaria, straw-belle—140. Blow-
fly, Musca vomitoria: left fore-loot, under-surface, to show tenent

hairs; a b more magnified; a from below, b from the side—141.
House-fly larva—142. Amara communis: left fore-foot, under-surface,
to show form of tenent appendages, of which one is given more
magnified at a. These, in ground beetles, are met with only in the
males, believed to be used for sexual purposes. These appendages are
carefully protected when not in use, as explained by West—143.
Ephydra riparia: left fore-foot, under-surface. This fly is met with
sometimes in immense numbers on the water in salt-marshes; it has no
power of climbing on glass, as seen by the structure of the tenent hairs;
the central tactile organ also is peculiar, the whole acting as a float, one
attached to each foot, enabling the fly to rest on the surface of the
water; a. an enlarged external hair—144. Egg of bot-fly, the larva just
escaping—145. Egg of parasite of pheasant—146. Egg of Scatophaga
—147. Egg of parasite of magpie—148. Egg of Jodis vernaria, small
emerald moth.

PLATE VII.—Page 633.

VERTEBRATA.

Fig. 149. Toe of mouse, integuments, bone of foot, and vessels—150.
Tongue of mouse, showing erectile papillæ and muscular layer—151.
Brain of rat, showing vascular supply—152. Vertical section of tongue
of cat, fungi-form papillæ and capillary loops passing into them,
vessels—153. Kidney of cat, showing Malpighian turfts and arteries—
154. Small intestine of rat, with villi and layer of mucous membrane
exposed—155. Nose of mouse, showing vascular supply to roots of
whiskers—156. Vascular supply to internal gill of tadpole, during one
phase of development—157. Section through sclerotic coat and retina
of cat’s eye, showing vascular supply of choroid vessels cut cross-
ways—158. Interior of fully-developed tadpole, exhibiting heart,
vascular arrangement and vascular system throughout body and tail.

This plate is designed to show the value, in certain cases, of injected
preparations in the delineation of animal structures. By thus artificially

restoring the blood and distending the tissues, a better idea is obtained of
the relative condition of parts during life.

PLATE VIII.—Page 220.

POLARISCOPE OBJECTS.

Fig. 158. New Red Sandstone—159. Quartz—163. Granite—161. Sulph.
Copper—162. Saliginine—163. Sulph. Iron and Cobalt, crystallized in
the way described by Thomas—164. Borax—165. Sulph. Nickel and
Potash—166. Kreatine—167. Starch granules—168. Aspartic Acid—
169. Fibro-cells, orchid.—170. Equisetum cuticle—171. Holothuria
spicula, Australia—172. Holothuria spicula, Port Essington—173.
Deutzia scabra; upper and under surface—174. Cat’s tongue, process
—175. Prawn shell, exuvia with crystals of lime—176. Grayling scale
—177. Scyllium caniculum scale—178. Rhinoceros horn, transverse
section—179. Horse hoof—180. Dytiscus, elytra with crystals of lime.

PLATE IX.—Page 362.

TYPICAL PLATE OF BACTERIA AND SCHIZOMYCETES.

Fig. 1. Cocci, singly, and varying in size—2. Cocci in chains or rosaries
(streptococcus)—3. Cocci in a mass (staphylococcus)—4 and 5. Cocci
in pairs (diplococcus)—6. Cocci in groups of four (merismopedia)—7.
Cocci in packets (sarcina)—8. Bacterium termo—9. Bacterium termo
× 4000 (Dallinger and Drysdale)—10. Bacterium septicæmiæ
hæmorrhagicæ—11. Bacterium pneumoniæ crouposæ—12. Bacillus
subtilis—13. Bacillus murisepticus—14. Bacillus diphtheriæ—15.
Bacillus typhosus (Eberth)—16. Spirillum undula (Cohn)—17.
Spirillum volutans (Cohn)—18. Spirillum choleræ Asiaticæ—19.
Spirillum Obermeieri (Koch)—20. Spirochæta plicatilis (Flügge)—21.
Vibrio rugula (Prazmowski)—22. Cladothrix Försteri (Cohn)—23.
Cladothrix dichotoma (Cohn)—24. Monas Okenii (Cohn)—25. Monas

Warmingii (Cohn)—26. Rhabdomonas rosea (Cohn)—27. Spore-
formation of Bacillus alvei—28. Spore-formation (Bacillus anthracis)
—29. Spore-formation in bacilli cultivated from rotten melon (Fränkel
and Pfeiffer)—30. Spore-formation in bacilli cultivated from earth
(Fränkel and Pfeiffer)—31. Involution-form of Crenothrix (Zopf)—32.
Involution-forms of Vibrio serpens (Warming)—33. Involution-forms
of Vibrio rugula (Warming)—34. Involution-forms of Clostridium
polymyxa (Prazmowski)—35. Involution-forms of Spirillum choleræ
Asiaticæ—36. Involution-forms of Bacterium aceti (Zopf and Hansen)
—37. Spirulina-form of Beggiatoa alba (Zopf)—38. Various thread-
forms of Bacterium merismopedioides (Zopf)—39. False-branching of
Cladothrix (Zopf).

PLATE X.—Page 420.

DESMIDIACEÆ.

Fig. 1. Euastrum oblongum—2. Micrasterias rotata—3. Desmidium
quadrangulatum—4. Didymoprium Grevillii—5. Micrasterias,
sporangium of—6. Didymoprium Borreri—7. Cosmarium Ralfsii—8,
9. Xanthidiæ—10. X. armatum—11. Cosmarium crenatum—12. C.
Sphærozosma vertebratum—13, 17. Sporangia of Cosmarium—14. X.
fasiculatum—18. Staurastrum hirsutum—19. Arthrodesmus
convergens—15. Staurastrum tumidum—16. Staurastrum dilitatum—
21. Penium—22. Euastrum Didelta—23. Docidium clavatum—24.
Pediastrum biradiatum—25. Closterium, showing conjugation or self-
division—26. Volvox, parent cell about to break up—27. Penium
Jennerii—28. Aptogonum desmidium—29. Pediastrum pertusum—30.
Ankistrodesmus falcatus—31. Parent cell of Closterium—32.
Staurastrum gracilis.—33. Conjugation of Penium margaritaceum—
34. Spirotænia—35. Closterium

PLATE XI.—Page 428.

DIATOMACEÆ.

Fig. 1. Arachnoidiscus—2. Actinocyclus (Bermuda)—3. Cocconeis (Algoa
Bay)—4. Coccinodiscus (Bermuda)—5. Isthmia enervis—6.
Zygoceros rhombus—7. Campilodiscus clypeus—8. Biddulphia—9.
Gallionella sulcata—10. Triceratium, found in Thames mud—11.
Gomphonema geminatum, with their stalk-like attachments—12.
Dictyocha fibula—13. Eunotia—14. Cocconema—15. Fragilaria
pectinalis—16. Meridion circulare—17. Diatoma flocculosum.

PLATE XII.—Page 438.

MICRO-PHOTOGRAPH OF TEST DIATOMS.
Taken with Zeiss’s 3 mm. N.A. 1·40 by Mr. A. A. Carvell for the Author.

Fig. 1. Portion of Surirella gemma, magnified × 1,000—2. Broken Frustule
of Pleurosigma angulatum, × 750—3 and 5. Triceratium favus ×—
1,000—4. Navicula rhomboides × 1,300—6. Pleurosigma formosum,
showing black dots—7. P. formosum, showing white dots, × 750.

PLATE XIII.—Page 454.

PHANEROGAMIÆ—ELEMENTARY TISSUE OF PLANTS.

Fig. 1. Elementary ovid cells—2. Branching tissue—2A and 3. Spiral
vessels from Opuntia vulgaris—4. Stellate tissue, section of rush—5.
Mushroom spawn—6. Starch from Tous-les-mois—7. Starch from
sago—8. Starch from rice—9. Wheat-starch—10. Rhubarb starch in
isolated cells—11. Maize-starch—12. Oat-starch—13. Barley-starch—
14. Section of Potato cells, filled with healthy starch—15. Potato
starch more highly magnified—16. Section of Potato with nearly all
starch absent—17. Potato with starch destroyed by fungoid disease—

18. Ciliated spermagones—19. Hairs of stinging-nettle—20. Section of
cellular parenchyma of ripe strawberry.

PLATE XIV.—Page 472.

STELLATE AND CRYSTALLINE TISSUE.

Fig. 1. Epidermis of husk of wheat, spiral vessels and silicious crystals—2.
Section of cane, silicious cell walls, internal portion filled with
granular bodies—3. Cuticular layer of the onion, showing crystals of
calcium carbonate and oxalate—4. Cells of garden rhubarb, with
crystalline bodies and raphides—4a. Another layer filled with starch
grains—5. Section of pear, testa, sclerogenous and granular tissue—6.
Stellate hairs, sinuous cells and silicious parenchyma of leaf of Deutzia
scabra, under surface—7. Silicious cuticle layer of grass, Pharus
cristatus.

PLATE XV.—Page 482.

RHIZOPODA.—GROMIA.—FORAMINIFERA.

Fig. 1. Astrorhiza limicola—2. Lieberkühnia paludosa—3. Micro-gromia
socialis undergoing fission—4. A colony of Hertwig’s Micro-gromia
socialis—5. G. Lieberkühnia—6. Egg-shaped Gromia, G. oviformis,
with pseudopodia extended, magnified 500 diameters. “Hertwig Ueber
Micro-gromia, archiv. für Mickr. Anat. bdx.”

PLATE XVI.—Page 510.

SPONGE SPICULES.

Fig. 1. A portion of sponge, Halichondria simulans, showing silicious
spicula imbedded in the sarcode matrix—2. Spicula divested of its
matrix by acid—3. Gemmule Spongilla fluviatallis enclosed in spicula
—4. Birotulate spicula from same—5. Gemmule after being steeped in
acid showing reticulated coating of birotulate spicula—6. Gemmules
of Geodia—7. Gemmule in more advanced stage of growth—8.
Skeleton of the acerate form covered by rows of spines—9. Showing
rings of growth and horny covering, and bundles of spicula of the
genus Verongia—10. Sphero-stellate spicula of Tethya—11.
Tricuspidanchorate and sphero-stellate spicula—12. Acuate-bi-clavate
and other forms of spicula from Geodia—13. Clavate spicula covered
with short spines.

PLATE XVII.—Page 518.

ZOOPHYTES, ASTEROIDS, NUDIBRANCHS, AND ECHINOIDS.

Fig. 1. a. Astrophyton scutatum—b. Doris pinnatifida, back and side view
—c. Æquorea Forbesina—d. Medusæ bud—e. Thaumantias corynetes
—f. Echinus in an early free stage—g. Echinus sphæra—h. Cydippe
pyleus—i. Ascidiæ—k. Botryllus violaceus, on a Fucus—l. Corystes
cassivelaunus—m. Eurynome aspera—n. Ophiocoma rosula—o.
Pagurus Prideauxii—p. Ebalia Permantii.

PLATE XVIII.—Page 558.

SHELLS OF MOLLUSCA.

Fig. 1. Transverse section of spine of Echinus—2. Another section of
Echinus, showing reticulated structure, the calcareous portion
dissolved out by acid—3. Horizontal section of shell of Haliotis
splendens, showing stellate pigment—4. Shell of crab with granules in
articular layer—5. Another section of same shell, showing hexagonal
structure—6. Horizontal section of coach-spring shell, Terebratulata

rubicunda, showing radiating perforations—7. Transverse section of
shell of the Pinna ingens—8. Crystals of carbonate of lime, from
oyster shell.

PLATE XIX.—Page 636.

VERTEBRATA.

Fig. 1. a. Spheroidal epithelium cells, filled with central nuclei and granular
matter; b. mucous membrane of stomach, showing cells, with open
mouths of tubes at the bottom of each, magnified 50 diameters—2. a.
Diagram of a portion of the involuted mucous membrane, showing
continuation of its elements in the follicles and villi, with a nerve
entering the submucous tissue. The upper surface of one villus is
covered with cylindrical epithelium; the other denuded, and with dark
line of basement membrane running around it; b. epithelium cells,
separated and magnified 200 diameters, a central nucleus, with a
nucleolus, seen in centre; c. pavement epithelium cells, from the
mucous membrane of bronchial or air tubes with nuclei, and nucleoli
in some; d. vibratile or ciliated epithelium, nuclei visible, and cilia at
the upper free surface, magnified 200 diameters—3. a. is one of the
tubular follicles from a pig’s stomach, cut obliquely to display upper
part of cavity, and the cylindrical epithelium forming its walls, a few
cells detached; b. shows a section of a lymphatic, with capillary blood-
vessels, distributed beneath the mucous surfaces—4. Cells of adipose
tissue, or fat, magnified 100 diameters—5. a single fat-cell separated,
and magnified 250 diameters—6. A capillary of blood-vessels
distributed through tissue—7. Section of the Tendo-Achillis as it joins
the cartilage, showing stellate cells of tendon, seen to be gradually
coalescing to form round or oval cells of cartilage—8. A vertical
section of cartilage, with clusters of cells arranged in columns previous
to their conversion into bone—9. A small transverse section of the
same, showing the gradual change of the cartilage cells at a. into the
true bone cells, lacunæ, at b. with characteristic canaliculi—10. A
stellate nerve corpuscle, with tubular processes issuing forth, at a.

filled with corpuscles containing black pigment, above which is a
corpuscle the nucleus of which is seen to have nucleoli; at b. a
corpuscle enclosed within sheath, and filled with granular matter taken
from the root of a spinal nerve—11. The continuity of muscle, the
upper portion, with connective tissue of the lower portion, from the
tongue of a lamb—12. Branched muscle, ending in stellate connective
cells, from the upper lip of the rat—13. Choroidal black pigment-cells
from the human eye.

PLATE XX.—Page 658.

BONE STRUCTURE.

Figs. 1. and 2. Transverse section of the human clavicle (collar bone),
showing Haversian canals, concentric laminæ, and concentric
arrangement of bone cells—3. Transverse section of the femur of an
ostrich—4. Transverse section of humerus (fore-arm) bone of a turtle,
Chelonia mydas—5. Horizontal section of the lower jaw-bone of a
conger eel, in which no Haversian canals are present—6. A portion of
the cranium of a siren, Siren lacertina—7. Portion of bone taken from
the shaft of humerus of a Pterodactyle, showing elongated bone-cells
characteristic of the order Reptilia—8. Horizontal section of a scale, or
flattened spine, from the skin of a Trygon (sting-ray), showing large
Haversian canals, numerous wavy parallel tubes, also bone-cells with
canaliculi communicating as in dentine.

ERRATA.
Prefaces, page vi., line 22 Insert “a” into “admitted.”
Prefaces, page x., line 13 Insert “the” before “assistance.”
Prefaces, page xii. Insert “for” at commencement of line 24.
Page 33, line 13 For “Rabbit” read “Kitten.”
Page 486, 4th line from bottom Strike out “The late.”
Page 511, two lines from
bottom For “Plate XIII.” read “Plate XVI.”
Page 584, 5th line from bottom Insert “Stalk-Eyed” before
“crustaceans.”
Page 624, line 12 For “or” read “and.”
Page 633, Plate VII. Numbering of figures—
For “152” read “158.”
For “152a” read “152.”

(Professor Abbe, erroneously referred to more than once as “the late” is, the
author is happy to say, in excellent health).

THE MICROSCOPE.

PART I.

Early History of the Microscope.

The instrument known as the Microscope derives its designation from two
Greek words, μικρὸς (mikros), small, and σκοπέω (skopeo), to see or
observe; and is an optical instrument by means of which objects are so
magnified that details invisible or indistinct to the naked eye are clearly
seen. Its origin, so far as yet can be traced back, seems to be of a doubtful
nature. It is tolerably certain the ancients had little or no conception of the
magnifying power of lenses; this may be surmised from their writings. The
elder Pliny incidentally states that the physicians of his day cauterised by
means of “a globe of crystal.” The learned Greek physician, Galen,
however, demonstrates conclusively that in the first and second centuries of
our era the use of magnifying lenses was quite unknown either to Greek or
Roman. Moreover, the writings of Archimedes, Ptolemy, and other learned
men, show that, although they had some idea of the action of refraction at
plane surfaces, as of water, yet of the refraction at curved surfaces they had
formed no conception. Indeed, they refer quite indiscriminately to the
spherical form, or the disc, or the plane surface of the water, but not one of
them speaks of the lenticular form, or the curvature of their surfaces.

As to the more powerful optical instruments, the telescope and microscope,
although it would appear that Alhazen in the 10th or 11th century, Roger
Bacon in the 13th, and Fracastoro and Baptist Porta in the 16th, had formed
some idea that lenses might be made and combined so that distant objects
might be seen clearer, or near ones magnified beyond the power of normal
vision; yet we hold with Kepler, that no instrument analogous to our
telescope was known before the early part of the 17th century.

The combination of lenses associated with the name of Galileo, was, he
tells us, of Dutch origin, and of a date anterior to that of his telescope,

constructed by him in 1609; and this would appear to be the probable origin
of the microscope consisting of a combination of a convex object lens with
a concave eye lens.3

It now appears almost impossible to assign the exact date of the first
production of the microscope (as distinguished from the simple magnifying
lens), but those who have made a special investigation, agree that it must
have been invented between 1590 and 1609, and that either of the three
spectacle-makers of Middelburg, Holland, Hans Janssen, his son Zacharias
Janssen, and Hans Lippershey, may have been the inventor, the probabilities
being in favour of the Janssens, and there the question must remain.

The history of the modern microscope, like that of nations and arts, has had
its brilliant periods, in which it shone with uncommon splendour, and was
cultivated with extraordinary ardour; these periods have been succeeded by
intervals marked with no discovery, and in which the science seemed to
fade away, or at least to lie dormant, till some favourable circumstance—the
discovery of a new object, or some new improvement in the instruments of
observation—awakened the attention of the curious, and reanimated the
spirit of research. Thus, soon after the invention of the microscope, the field
it presented to observation was cultivated by men of the first rank in
science, and who enriched almost every branch of natural history by the
discoveries made by means of this instrument.

The Modern Microscope.

To the celebrated Dr. Hooke belongs the honour of publishing an account of
the compound instrument in 1665 in his “Micrographia.” His first claim,
however, is founded on the application of a lamp adjustable on a pillar,
together with a glass globe of water and a deep plano-convex condensing
lens. By means of this arrangement, he says, “The light can be directed
more directly on the object under examination.” In the further description
given of his microscope, he explains: “It has four draw-tubes for
lengthening the body, and a third lens to the optical combination.” This, it
would appear, was only brought into use when he wished to see the whole
object at once: “The middle-glass lens, conveying a very great company of

radiating pencils (of light) which would stray away; but when I had
occasion to examine the small parts of a body, I took out the middle glass
and made use of one eye-glass with the object-glass.”

From Hooke’s description I gather that he also introduced the ball-and-
socket movement into the construction of the body of his instrument. This
has found many imitators since his day; some of them have gone so far as to
claim the invention as one quite new. For small accessories, where the
leverage need not be considered, the ball-and-socket has proved convenient
enough; but not, however, if applied to the stand of the microscope. Hooke,
in his early work, expressed dissatisfaction with the English-made lenses he
had in use. He complains of the “apertures of the object-glasses, which are
so small that very few rays are admitted; none will admit a sufficient
number of rays to magnifie the object beyond a determinate bigness.” So
we may take it that he thus early discovered the great importance of an
increase in the aperture of his microscope. Other improvements of
importance were made, and he was the first to describe a useful method of
estimating the magnifying power of his lenses, and the difficulty of
distinguishing between a prominence and a depression in the object under
investigation, which he was made more fully aware of when preparing
drawings for the illustration of his “Micrographia Illustrata”; this would be
in 1664, if not earlier. His book created no little sensation on its first
appearance, and it soon became scarce. Hooke (says Mr. Mayall) “must
undoubtedly be credited with the first suggestion of immersion lenses.”
Nevertheless, in his “Lectures and Collections,” published in 1676, he
appears to be no longer enthusiastic over his double microscope, and once
more he reverts to the simpler instrument of his earlier days. Whether this
change of opinion was due to the publication of Leeuwenhoek’s
observations with his simple microscopes it is impossible to say.

As early as 1673 Leeuwenhoek communicated some important discoveries
made by a simple microscope of his own construction to the Royal Society;
he, however, gave no particulars of the construction of the instrument. Dr.
Adams, writing to his friend (Sir) Hans Sloane, says: “They appear to be
spherules lodged between two plates of gold or brass, in a hole whose
diameter appears to be no bigger than that of a small pin’s head.” At his
death he bequeathed to the Royal Society a cabinet containing twenty-six of

these microscopes; the cabinet and the microscopes long ago disappeared,
but not before they were carefully examined and described by Mr. Henry
Baker, F.R.S. In his report to the Royal Society, he says: “They consisted of
a series of convex-lenses, ranging in power from 1·20 to 1·5, and
magnifying from 160 to 40 diameters.” This must now be regarded as an
eventful period in the history of the microscope, since Leeuwenhoek’s
discoveries created a great sensation throughout Europe. And all further
improvements in compound instruments appear to have been laid aside for
some considerable period in consequence: and the pocket instrument of
Wilson, together with that of his scroll standard (seen on the cover of this
book), and which was one of the first simple microscopes with a mirror
mounted on the base in a line with the optic axis.

The discoveries once more made, and at a much later period (1738), by Dr.
Nathaniel Lieberkuhn with his simple microscopes, and by means of which
he discovered the minute structure of the mucous membrane of the
alimentary canal, and which alone would have immortalised his name had
we not preserved in use to this day an important adjunct of every modern
instrument, the Lieberkuhn reflector.

In the Museum of the Royal College of Surgeons of England, there is a
small cabinet of two drawers, containing a set of twelve of his simple
microscopes, each being provided with an original injection. The form of
the instrument is shown in Figs. 1 and 2. a b represents a piece of brass
tubing about an inch long and an inch in diameter and provided with a cap
at each extremity. The one at a carries a small double-convex lens of half an
inch focal length; while at b there is fixed a condensing lens three-quarters
of an inch in diameter. In Fig. 2 the instrument is seen in section, and
explains itself. It is held by the handle in such a position that the rays of
light, from a lamp or a white cloud, may fall on the condenser b, and
concentrate on the speculum l. This again further condenses the rays on the
disc c, where the object is held, and its adjustment made by the milled-head
screw d, so as to bring it within the focus of the lens a.

From this digression I pass on to the evolution of the compound
microscope. The earliest workable form known was that designed by
Eustachio Divini, who brought it to the notice of the Royal Society in 1668.

It consisted of two plano-convex lenses, combined with
their convex surfaces retained in apposition. His idea was
subsequently improved upon by a London optician. Not
long afterwards, Philip Bonnani published an account of
his improved compound microscope; and we are
certainly indebted to him for two or more forms of the
movable horizontal microscopes, and for the compound
condenser fitted with focussing gear for illuminating
transparent objects by transmitted light. I must, however,
pass by the many changes made in the structure and form
of the instrument by the celebrated Dr. Culpeper, Scarlet,
Cuff, and many other inventors.

Benjamin Martin’s Microscope.—Benjamin Martin,
about 1742, was busily engaged in making improvements
in the microscope, and I may say he was certainly the
first to provide accurate results for determining the exact
magnifying power of any object-lens, so that the observer
might state the exact amplification in a certain number of
diameters. He devised numerous improvements in the
mechanism and optical arrangements of the instrument;
the rack and pinion focussing adjustments; the inclining
movements to the pillar carrying the stage; and the Fig. 1.
rectangular mechanical motions to the stage
itself. He was familiar with the principles of
achromatism, since it appears he produced an
achromatic objective about 1759, and he is said
to have sent an achromatic objective to the
Royal Society about that date. But an
ingeniously constructed microscope by Martin
found its way to George the Third, the
grandfather of our Queen, and afterwards came Fig. 2.—Lieberkuhn’s Microscope.
into the possession of the late Professor John
Quekett, of the Royal College of Surgeons, who presented it to the Royal
Microscopical Society of London. This microscope will ever associate
Martin’s name with the earliest and best form of the instrument, even
should he not receive full recognition as the inventor of the achromatic

microscope. On this account I introduce a carefully made drawing of so
singularly perfect a form of the early English microscope to the notice of
my readers. (Fig. 3.) The description given of it by the late Professor
Quekett is as follows:—“It stands about two feet in height, and is supported
on a tripod base, A; the central part of the stem, B, is of triangular figure,
having a rack at the back, upon which the stage, O, and frame, D,
supporting the mirror, E, are capable of being moved up or down. The
compound body, F, is three inches in diameter; it is composed of two tubes,
the inner of which contains the eye-piece, and can be raised or depressed by
rack and pinion, so as to increase or diminish the magnifying power. At the
base of the triangular bar is a cradle joint, G, by which the instrument can
be inclined by turning the screw-head, H (connected with an endless screw
acting upon a worm-wheel). The arm, I, supporting the compound body, is
supplied with a rack and pinion, K, by which it can be moved backwards
and forwards, and a joint is placed below it, upon which the body can be
turned into the horizontal position; another bar, carrying a stage and mirror,
can be attached by a screw, L N, so as to convert it into a horizontal
microscope. The stage, O, is provided with all the usual apparatus for
clamping objects, and a condenser can be applied to its under surface; the
stage itself may be removed, the arm, P, supporting it, turned round on the
pivot, C, and another stage of exquisite workmanship placed in its stead, the
under surface of which is shown at Q.”

This stage is strictly a micrometer one, having rectangular movements and a
fine adjustment, the movements being accomplished by the fine-threaded
screws, the milled heads of which are graduated. The mirror, E, is a double
one, and can be raised or depressed by rack and pinion; it is also capable of
removal, and an apparatus for holding large opaque objects, such as
minerals, can be substituted for it. The accessory instruments are very
numerous, and amongst the more remarkable may be mentioned a tube, M,
containing a speculum, which can take the place of the tube, R, and so form
a reflecting microscope. The apparatus for holding animalcules or other live
objects, which is represented at S, as well as a plate of glass six inches in
diameter, with four concave wells ground in it, can be applied to the stage,
so that each well may be brought in succession under the magnifying
power. The lenses belonging to this microscope are twenty-four in number;
they vary in focal length from four inches to one-tenth of an inch; ten of

them are supplied with Lieberkuhns. A
small arm, capable of carrying single
lenses, can be supplied at T, and when
turned over, the stage of the instrument
becomes a single microscope; there are
four lenses suitable for this purpose,
their focal length varying from one-
tenth to one-fortieth of an inch. The
performance of all the lenses is
excellent, and no pains appear to have
been spared in their construction. There
are numerous other pieces of accessory
apparatus, all remarkable for the beauty
of their workmanship.4

In addition to the movements described
by Quekett, the body-tube with its
support can be moved in an arc
concentrically with the axis of the
triangular pillar, on the top of which it
is fitted with a worm-wheel and
endless-screw mechanism, actuated by
Fig. 3.—Martin’s Universal Microscope. 1782. the screw-head, T, below. It must
therefore be admitted that Martin led
the way far beyond his contemporaries, both in the design and the evolution
of the microscope. Furthermore, in his “New Elements of Optics,” 1759, he
dealt with the principle of achromatism, by the construction of an
achromatic telescope.

At a somewhat later period there lived in London a philosophical
instrument maker of some repute, George Adams, who published in 1746 a
quarto book, entitled “Micrographia Illustrata, or the Knowledge of the
Microscope Explained.” This work fairly well describes “the nature, uses,
and magnifying powers of microscopes in general, together with full
directions how to prepare, apply, examine, and preserve minute objects.”
Adams’ book was the first of the kind published in this country, and it
contributed in no small degree to the advancement of microscopical

science. Adams writes: “We owe the construction of the variable
microscope to the ingenuity and generosity of a noble person. The apparatus
belonging to it is more convenient, more certain, and more extensive than
that of any other at present extant; consequently, the advantage and pleasure
attending the observations in viewing objects through it must be as
extensive in proportion.” This is believed to apply to Martin’s several
microscopes, and that especially constructed for the king, afterwards
improved upon by Adams. Another early form of microscope, Wilson
Simple Scroll (1746), stamped on the cover of this book, and has thus
become familiar to microscopists, was also made by Adams.

We now closely approach a period fertile in the improvement of the
microscope, and in the discoveries made by its agency. The chief of those
among the honoured names of the time we find Trembley, Ellis, Baker,
Adams, Hill, Swammerdam, Lyonet, Needham, and a few others. Adams
somewhat sarcastically observes “that every optician exercises his talents in
improving (as he calls it) the microscope, in other words, in varying its
construction and rendering it different in form from that sold by his
neighbour; or at the best rendering it more complex and troublesome to
manage.” There were no doubt good reasons for these and other strictures
upon inventors as well as makers of microscopes, even in the Adams’ day.
In the year 1787 the “Microscopical Essays” of his son were published, in
which he described all the instruments in use up to that period.

Looking back, and taking a general survey of the work of nearly two
centuries in the history of the microscope, it cannot be said that either in its
optical or mechanical construction any great amount of progress was made.
This in part may have arisen from the fact that no pressing need was felt for
either delicate focussing or higher magnification. At all events, it was not
until the application of achromatism to the instrument that new life was
infused into its use, and a great impetus was given to its development, both
optically and mechanically.

In the year 1823 a strong desire became manifest for improved forms of the
instrument, in France by M. Selligue, by Frauenhofer in Munich, by Amici
in Modena, by M. Chevalier in Paris, and by Dr. Goring, Mr. Pritchard, and
Mr. Tully in London. The result was that in 1824 a new form of achromatic

object-glass was constructed of nine-tenths of an inch focal length,
composed of three lenses, and transmitting a pencil of eighteen degrees; and
which, as regards accurate correction throughout the field, was for some
years regarded as perfect.

Sir David Brewster was the first to suggest the great importance of
introducing materials of a more highly refracting nature into the
construction of lenses. He wrote: “There can be no essential improvement
expected in the microscope unless from the discovery of some transparent
substance which, like the diamond, combines a high refractive with a low
dispersive power.” Having experienced the greatest difficulty in getting a
small diamond cut into a prism in London, he did not conceive it
practicable to grind, polish, and form it into a lens.

Mr. Pritchard, however, was led to make the experiment, and on the 1st of
December, 1824, “he had the pleasure of first looking through a diamond
microscope.” Dr. Goring also tried its performance on various objects, both
as a single microscope and as an objective of a compound instrument, and
satisfied himself of its superiority over other kinds of lenses. But here Mr.
Pritchard’s labours did not end. He subsequently found that the diamond
used had many flaws in it, which led him to abandon the idea of finishing it.
Having been prevented from resuming his operations on this refractory
material for a time he made a third attempt, and met with another
unexpected defect; he found that some lenses, unlike the first, gave a double
or triple image instead of a single one, in consequence of some of their parts
being either harder or softer than others. These defects were found to be due
to polarisation. Mr. Pritchard having learned how to decide whether a
diamond is fit for a magnifier or not, subsequently succeeded in making two
planoconvex lenses of adamant; these proved to be perfect for microscopic
purposes. “One of these, of one-twentieth of an inch in focal length, is now
in the possession of his Grace the Duke of Buckingham; the other, of one-
thirtieth of an inch focus, is in his own hands.”

“In consequence of the high refracting power of a diamond lens over a glass
lens, the former material may be at least one-third as thin as that of the
latter, and if the focal length of both be equal, say, one-eightieth of an inch,
the magnifying power of the diamond lens will be 2,133 diameters, whereas

that of glass will be only 800.” At a date (1812) before Brewster proposed
diamond lenses he demonstrated a simple method of rendering both single
and compound microscopes achromatic. “Starting,” he says, “with the
principle that all objects, however delicate, are best seen when immersed in
fluid, he placed an object on a slip of glass, and put above a drop of oil,
having a greater dispersive power than the single concave lens, which
formed the object-glass of the microscope. The lens was then made to touch
the fluid, so that the surface of the fluid was formed into a concave lens,
and if the radius of the outward surface was such as to correct the
dispersion, we should have a perfect achromatic microscope.” Here we
have the immersion system foreshadowed. Shortly after these experiments
of Brewster’s were in progress, Dr. Goring is said to have discovered that
the structure of certain bodies could be readily seen in some microscopes
and not in others. These bodies he named test objects. He then examined
these tests with the achromatic combinations of the Tullys, and was led to
the discovery that “the penetrating power of the microscope depends upon
its angle of aperture.”

“While these practical investigations were in progress,” writes Andrew
Ross, “the subject of achromatism engaged the attention of some of the
most profound mathematicians in England, Sir John Herschel, and
Professors Airy and Barlow. Mr. Coddington and others contributed largely
to the theoretical examination of the subject; and although the results of
their labours were not applicable to the microscope, they essentially
promoted its improvement.”

About this period (1812) Professor Amici, of Modena, was experimentally
engaged in the improvement of the achromatic object-glass, and he invented
a reflecting microscope superior to those of Newton, Baker, or Smith, made
as early as 1738, and long ago abandoned. In 1815 Amici made further
experiments, and introduced the immersion system; while Frauenhofer, of
Munich, about the same time constructed object-glasses for the microscope
of a single achromatic lens, in which the two glasses, although placed in
juxtaposition, were not cemented together.

Dolland, it has been said, introduced achromatic lenses; but although he
constructed many achromatic telescopes, he did not apply the same

principle to microscopes, and those which he sold were only modifications
of the compound microscope of Cuff.

Dr. Wollaston employed a new form of combination in a microscope
constructed for his own use, and by which “he was able to see distinctly the
finest markings upon the scales of the Lepisma and Podura, and upon those
of the gnat’s wing.” His doublet is still employed, and to which I shall refer
under “Simple Microscopes.”

Fig. 3a.—Sir David Brewster’s Microscope, of the early part of the century,
recently presented to the British Museum.

CHAPTER I.

Elementary Optics.

Value of Inductive Science—Light: Its Propagation, Refraction, Reflection
—Spherical and Chromatic Aberrations—Human Eye, formation of
Images of External Objects in—Visual Angle increased—Abbe’s
Theory of Microscopic Vision.

The advances made in physics and mechanics during the 17th and 18th
centuries fairly opened the way to the attainment of greater perfection in all
optical instruments. This has been particularly exemplified with reference to
the invention of the microscope, as briefly sketched out in the previous
chapter. Indeed, in the first half of the present century the microscope can
scarcely be said to have held a position of importance among the scientific
instruments in frequent use. Since then, however, the zoologist and botanist
by its aid have laid bare the intimate structure of plants and animals, and
thereby have opened up a vast kingdom of minute forms of life previously
undreamt of; and in connection with chemistry a new science has been
founded, that of bacteriology.

For these reasons it will be of importance to the student of microscopy to
begin at the beginning, and it will be my endeavour to introduce to his
notice such facts in physical optics as are closely associated with the
formation of images, and, so to speak, systematise such stepping stones for
work hereafter to be accomplished. Elementary principles only will be
adduced, and without attempting to involve my readers in intricate
mathematical problems, and which for the most part are unnecessary for the
attainment of the object in view. I therefore pass at once to the consideration
of the propagation of light through certain bodies.

The microscope, whether simple or compound, depends for its magnifying
power on the influence exerted by lenses in altering the course of the rays
of light passing through them being REFRACTED. Refraction takes place in
accordance with two well-known laws of optics. When a ray of light passes
from one transparent medium to another it undergoes a change of direction
at the surface of separation, so that its course in the second medium makes
an angle with its course in the first. This change of direction is a resultant of
refraction. The broken appearance presented by a stick partly immersed in
water, and viewed in an oblique position, is an illustration of the law of
refraction. Liquids have a greater refractive power than air or gases. As a
rule, with some few exceptions, the denser of the two substances has the
greater refractive power; hence it is customary in enumerating some of the
laws of optics to speak of the denser medium and the rarer medium. The
more correct designation would be the more refractive and the less
refractive.5

Let R I (Fig. 4) be a ray
incident at I on the surface of
separation of two media, and
let I S′ be the course of the ray
after refraction. Then the
angles which R I and I S make
with the normal are the angle
of incidence and the angle of
refraction respectively, and the
first law of refraction is that
these angles lie in the same
plane, or the plane of
refraction is the same as the
plane of incidence. The law
which connects the magnitudes
of these angles, and which was
discovered by Snell, a Dutch Fig. 4.—Law of Refraction.
philosopher, can only be stated
either by reference to a geometrical construction, or by using the language
of trigonometry. Describe a circle about the point of incidence, I as a centre,
and drop perpendiculars from the points where it cuts the rays on the

normal. The law is that these perpendiculars, R′ P′, S′ P, will have a constant
ratio, or the sines of the angles of incidence and refraction are in a constant
ratio; that is, so long as the media through which the ray first passes, and by
which it is afterwards refracted, remain the same, and the light also of the
same kind, then it is referred to as the law of sines.

Indices of Refraction.

The ratio of the sine of the angle of incidence to the sine of the angle of
refraction, when a ray passes from one medium to another is termed the
relative index of refraction. When a ray passes from vacuum into any
medium, this ratio is always greater than unity, and is called the absolute
index of refraction, or simply the index of refraction for the medium in
question.

The absolute index of air is so small that it may be neglected in comparison
with those of solids and liquids; but strictly speaking, the relative index for
a ray passing from air into a given substance must be multiplied by the
absolute index of the air, in order to obtain the true index of refraction.

Critical Angle.—It will be seen
from the law of sines that, when the
incident ray is in the less refractive
of the two media, to every possible
angle of incidence there is a
corresponding angle of refraction.
The angle referred to is termed the
critical angle, and is readily
computed if the relative index of
Fig. 5.—Vision through a Glass Plate. refraction be given. When the media
are air and water, this angle is about
48° 30′. For air and ordinary kinds of glass its value varies from 38° to 41°.

The phenomenon of total reflection may be observed in several familiar
instances. For example, if a glass of water, with a spoon in it, is held above
the level of the eye, the under side of the surface is seen to shine like a

mirror, and the lower part of the spoon is seen reflected in it. Effects of the
same kind are observed when a ray of sunlight passes into an aquarium—on
the other hand rays falling normally on a uniform transparent plate of glass
with parallel faces keep their course; but objects viewed obliquely through
the same are displaced from their true position. Let S (Fig. 5) be a luminous
point which sends light to an eye not directly opposite to it, on the other
side of a parallel plate. The emergent rays which enter the eye are parallel to
the incident rays; but as they have undergone lateral displacement, their
point of concourse is changed from S to S′, and this is accordingly the
image of S. The rays in such a case which compose the pencil that enters
the eye will not exactly meet in any one point; there will be two focal lines,
just as in the case of spherical mirrors. The displacement produced, as seen
in the figure referred to above, increases with the thickness of the plate, its
index of refraction, and the obliquity of incidence. This furnishes one of the
simplest means of measuring the index of refraction of a glass substance,
and is thus employed in Pichot’s refractometer (“Deschanel”).

Refraction through a Prism.—A
prism is a portion of a refracting
medium bounded by two plane
surfaces, inclined at a definite angle
to one another. The two plane
surfaces are termed the faces of the
prism, and their inclination to one
another is the refracting angle of the
prism. A prism preserves the
property of bending rays of light
from their original course by Fig. 6.—Refraction through a Prism.
refraction. A cylinder may be
regarded as the limit of a prism whose sides increase in number and
diminish in size indefinitely: it may also be regarded as a pyramid whose
apex is removed to an indefinite distance.

Let S I (Fig. 6) be an incident ray in the plane of the principal section of the
prism. If the external medium be air, or other substance of less refractive
power than the prism, the ray on entering the same will be bent nearer to the
normal, taking such a course as I E, and on leaving the prism will be bent

away from the normal, taking the course E B. The effect of these two
refractions is, therefore, to turn the ray away from the edge (or refracting
angle) of the prism. In practice, the prism is usually so placed that I E, the
path of the ray through the prism, makes equal angles with the two faces at
which refraction occurs. If the prism is turned very far from this position,
the course of the ray may be altogether different from that represented in
the figure; it may enter at one face, be internally reflected at another, and
come out at the third.

It is evident, therefore, that the minimum number of sides, i.e., the
bounding faces, exclusive of the ends, which a prism can have is three. In
this form, it constitutes a most valuable instrument of research in physical
optics. A convex lens is practically merely a curved form of two prisms
combined, their bases being brought into contact; on the other hand the
concave lens is simply a reversal of the position of the apices brought into
contact, as shown in Fig. 11. Both convex and concave lenses are therefore
closely related to the prism.

Reflection.—The laws that govern the change of direction which a ray of
light experiences when it strikes upon the surfaces of separation of two
media and is thrown back into the same medium from which it approached
is as follows:—When the reflecting surface is plain the direction of the
reflected ray makes with the normal to the surface the same angle which the
incident ray makes with the same normal; or, as it is usually expressed, the
angles of reflection and incidence are equal. When the surfaces are curved
the same law holds good. In all cases of reflection the energy of the ray is
diminished, so that reflection must always be accompanied by absorption.
The latter probably precedes the former. Most bodies are visible by light
reflected from their surfaces, but before this takes place the light has
undergone a modification, namely, that which imparts colour peculiar to the
bodies viewed. When light impinges upon the surface of a denser medium
part is reflected, part absorbed, and part refracted. But for a certain angle
depending upon the refractive index of the refracting medium no refraction
takes place. This angle is termed the angle of total reflection, since all the
light which is not absorbed is wholly reflected.

Multiple images are produced by a transparent parallel plate of glass. If the
glass be silvered at the back, as it usually is in the microscope-mirror, the
second image is brighter than the first, but as the angle of incidence
increases the first image gains upon the second; and if the luminous object
be a lamp or candle, a number of images, one behind the other, will be
visible to an eye properly placed in front. This is due to the fact that the
reflecting power of a surface of glass increases with the angle of incidence.

Fig. 7.—Conjugate Foci of Curved Surfaces.

Concave Surfaces.—Rays of light proceeding from any given point in
front of a concave spherical mirror, are reflected so as to meet in another
point, and the line joining the two points passes through the centre of the
sphere. The relation between them is or should be mutual, hence they are
termed conjugate foci. By a focus in general is meant a point in which a
number of rays of light meet, and the rays which thus meet, taken
collectively, are termed a pencil. Fig. 7 represents two pencils of rays
whose foci, S s, are conjugate, so that, if either of them be regarded as an
incident pencil, the other will be the corresponding reflected pencil. Each
point, in fact, sends a pencil of rays which converge, after reflection, to the
conjugate focus. The principal focal distance is half the radius of curvature.
But it will not escape attention that concave mirrors have two reflecting
surfaces, a front and a back. This, however, does not practically disturb its
virtual focus, since the achromatic condenser when brought into use collects
and concentrates the light received from the mirror upon an object for the
purpose of rendering it more distinctly visible to the eye when viewing an

object placed on the stage of the microscope. The images seen in a plane
mirror are always virtual, and any spherical mirror, whether concave or
convex, is nearly equivalent to a plane mirror when the distance of the
object from its surface is small in comparison with the radius of curvature.

Lenses.

Forms of Lenses.—A lens is a portion of a refracting medium bounded by
two surfaces which are portions of spheres, having a common axis, termed
the axis of the lens. Lenses are distinguished by different names, according
to the nature of their surfaces.

Lenses with sharp edges (thicker at
the centre) are convergent or positive
lenses. Lenses with blunt edges
(thinner at the centre) are divergent
or negative lenses. The first group
comprises:—(1) The bi-convex lens;
(2) the plano-convex lens; (3) the
Fig. 8.—Converging and Diverging Lenses. convergent meniscus. The second
group:—(4) The concave lens; (5)
the plano-concave lens; (6) the divergent meniscus (Fig. 8).

Principal Focus.—A lens is usually a solid of revolution, and the axis of
revolution is termed the principal axis of the lens. When the surfaces are
spherical it is the line joining the centre of curvature.

From the great importance of lenses, especially convex lenses, in practical
optics, it will be necessary to explain their properties somewhat at length.

Fig. 9.—Principal Focus of a Convex Lens.

Principal Focus of Convex Lens.—When rays which were originally
parallel to the principal axis pass through a convex lens (Fig. 9), the effect
of the two refractions which they undergo, one on entering and the other on
leaving the lens, is to make them all converge approximately to one point F,
which is called the principal focus. The distance A F of the principal focus
from the lens is called the principal focal distance, or more briefly and
usually, the focal length of the lens. The radiant point and its image after
refraction are known as the conjugate foci. In every lens the right line
perpendicular to the two surfaces is the axis of the lens. This is indicated by
the line drawn through the several lenses, as seen in the diagram (Fig. 8).
The point where the axis cuts the surface of the lens is termed the verte.

Parallel rays falling on a double-convex lens are brought to a focus in the
centre of its diameter; conversely, rays diverging from that point are
rendered parallel. Hence the focus of a double-convex lens will be at just
half the distance, or half the length, of the focus of a plano-convex lens
having the same curvature on one side. The distance of the focus from the
lens will depend as much on the degree of curvature as upon the refracting
power (termed the index of refraction) of the glass of which it may be
formed. A lens of crown-glass will have a longer focus than a similar one of
flint-glass; since the latter has a greater refracting power than the former.
For all ordinary practical purposes we may consider the principal focus—as
the focus for parallel rays is termed—of a double-convex lens to be at the
distance of its radius, that is, in its centre of curvature; and that of a plano-

convex lens to be at the distance of twice its radius, that is, at the other end
of the diameter of its sphere of curvature. The converse of all this occurs
when divergent rays are made to fall on a convex lens. Rays already
converging are brought together at a point nearer than the principal focus;
whereas rays diverging from a point within the principal focus are rendered
still more diverging, though in a diminished degree. Rays diverging from
points more distant than the principal focus on either side, are brought to a
focus beyond it: if the point of divergence be within the circle of curvature,
the focus of convergence will be beyond it; and vice-versâ. The same
principles apply equally to a plano-convex lens; allowance being made for
the double distance of its principal focus; and also to a lens whose surfaces
have different curvatures; the principal focus of such a lens is found by
multiplying the radius of one surface by the radius of the other, and dividing
this product by half the sum of the radii.

Fig. 10.—Principal Focus of Concave Lens.

In the case of a concave lens (Fig. 10), rays incident parallel to the principal
axis diverge after passing through; and their directions, if produced
backwards, would approximately meet in a point F; this is its principal
focus. It is, however, only a virtual focus, inasmuch as the emergent rays do
not actually pass through it, whereas the principal focus of a converging
lens is real.

Optical Centre of a Lens.—Secondary Axes.—Let O and O′ (Fig. 11) be
the centres of the two spherical surfaces of a lens. Draw any two parallel

radii, O I, O′ E, to meet these
surfaces, and let the joining line I E
represent a ray passing through the
lens. This ray makes equal angles
with the normals at I and E, since
these latter are parallel by
construction; hence the incident and
emergent rays S I, E R also make
equal angles with the normals, and
are therefore parallel. In fact, if
tangent planes (indicated by the
dotted lines in the figure) are drawn
at I and E, the whole course of the
Fig. 11.—Principal Centre of Lens. ray S I E R will be the same as if it
had passed through a plate bounded
by these planes.

Let C be the point in which the line I E cuts the principal axis, and let R, R′
denote the radii of the two spherical surfaces. Then from the similarity of
the triangles O C I, O′ C E, we have (O C)/(C O′) = R′/R; which shows that
the point C divides the line of centres O O′ in a definite ratio depending
only on the radii. Every ray whose direction on emergence is parallel to its
direction before entering the lens, must pass through the point C in
traversing the lens; and conversely, every ray which in its course through
the lens traverses the point C, has parallel directions at incidence and
emergence. The point C which possesses this remarkable property is called
the centre, or optical centre, of the lens.

This diagram may also be taken to prove my former proposition, that the
convex lens is practically a form of two prisms combined.

Fig. 12.—Conjugate Foci, one Real, the other Virtual.

Conjugate Foci, one Real, one Virtual.—When two foci are on the same
side of the lens, one (the most distant of the two) must be virtual. For
example, in Fig. 12, if S, S′ are a pair of conjugate foci, one of them S being
between the principal focus F and the lens, rays sent to the lens at a
luminous point at S, will, after emergence, diverge as if from S′; and rays
coming from the other side of the lens, if they converge to S′ before
incidence, will in reality be made to meet in S. As S moves towards the
lens, S′ moves in the same direction more rapidly; and they become
coincident at the surface of the lens.

Formation of Real Images.—Let A B (Fig. 13) be an object in front of a
lens, at a distance less than the principal focal length. It will have a real
image on the other side of the lens. To determine the position of the image
by construction, draw through any point A of the object a line parallel to the
principal axis, meeting the lens in A′. The ray represented by this line will,
after refraction, pass through the principal focus, F, and its intersection with
the secondary axis, A O, determines the position of a, the focus conjugate to
A. We can in like manner determine the position of b, the focus conjugate to
B, another point of the object; and the joining line a b will then be the
magnified image of the line A B. It is evident that if a b were the object, A
B would be the image.

Fig. 13.—Real and Magnified Image.

The figures 12 and 13 represent the cases in which the distance of the object
is respectively greater and less than twice the focal length of the lens.

The focal length of a lens is determined by the convexity of its surfaces and
the refractive power of the material of which it is composed, being
shortened either by an increase of refractive power, or diminution of the
radii of curvature of the faces of the lens. The increase or decrease of
spherical aberration is determined by the shape or curvature of the lens; it is
less in the bi-convex than in other forms. When a lamp or other source of
light is placed at the focus of the rays constituting that portion of its light
which falls upon the lens, the light is so refracted as to become parallel.
Should the source of light be brought nearer to the lens than the focus the
refracted rays are still divergent, though not to the same extent; on the other
hand, if the source be beyond the focus, the refracted rays are rendered
convergent so as to meet at a point which is mathematically related to the
distance of the luminous source from the focus. The former arrangement is
that with which we are most familiar, since it is the ordinary magnifying
glass.

Concave Lenses.

The refracting influence of a concave lens (Fig. 14) will be precisely the
opposite of that of a convex. Rays which fall upon it in a parallel direction

will be made to diverge as if from the principal focus, which is here called
the negative focus. This will be, for a plano-concave lens, at the distance of
the diameter of the sphere of curvature; and for a double-concave, in the
centre of that sphere.

Fig. 14.—A Virtual Image formed by Concave Lens.

In Fig. 14 A B is the object and a b the image. Rays incident from A and B
parallel to the principal axis will emerge as if they came from the principal
focus F; hence, the points a b are determined by the intersections of the
dotted lines in the figure with the secondary axis, O A, O B. An eye on the
other side of the lens sees the image a b, which is always virtual, erect and
diminished.

In the construction of the microscope, either simple or compound, the
curvature of the lenses employed is usually spherical. Convergent lenses,
with spherical curvatures, have the defect of not bringing all the rays of
light which pass through them to one and the same focus. Each circle of
rays from the axis of the lens to its circumference has a different focus, as
shown in Fig. 15. The rays a a, which pass through the lens near its
circumference, are seen to be more refracted, or come to a focus at a shorter
distance behind it than the rays b b, which pass through near its centre or
axis, and are less refracted. The consequence of this defect of lenses with
spherical curvatures, which is called spherical aberration, is that a well-
defined image or picture is not formed by them, for when the object is
focussed, for the circumferential rays, the picture projected to the eye is

rendered indistinct by a halo or confusion produced by the central rays
falling in a circle of dissipation, before they have come to a focus. On the
other hand, when placed in the focus of the central rays, the picture formed
by them is rendered indistinct by the halo produced by the circumferential
rays, which have already come to a focus and crossed, and now fall in a
state of divergence, forming a circle of dissipation. The grosser defects of
spherical aberration are corrected by cutting off the passage of the rays a a,
through the circumferences of the lens, by means of a stop diaphragm, so
that the central rays, b b, only are concerned in the formation of the image.
This defect is reduced to a minimum, by using the meniscus form of lens,
which is the segment of an ellipsoid instead of a sphere.

Fig. 15.—Spherical Aberration of Lens.

The ellipse and the hyperbola are forms of lenses in which the curvature
diminishes from the central ray, or axis, to the circumference b; and
mathematicians have shown that spherical aberration may be practically got
rid of by employing lenses whose sections are ellipses or hyperbolas. The
remarkable discovery of these forms of lenses is attributed to Descartes,
who mathematically demonstrated the fact.

If a l, a l′, for example (Fig. 16) be part of an ellipse whose greater axis is to
the distance between its foci f f as the index of refraction is to unity, then
parallel rays r l′, r′′ l incident upon the elliptical surface l′ a l, will be

refracted by the single action of that surface into lines which would meet
exactly in the farther focus f, if there were no second surface intervening
between l a l′ and f. But as every useful lens must have two surfaces, we
have only to describe a circle l a′ l′ round f as a centre, for the second
surface of the lens l′ l.

Fig. 16.—Converging Meniscus.

As all the rays refracted at the surface l a l′ converge accurately to f, and as
the circular surface l a′ l′ is perpendicular to every one of the refracted rays,
all these rays will go on to f without suffering any refraction at the circular
surface. Hence it should follow, that a meniscus whose convex surface is
part of an ellipsoid, and whose concave surface is part of any spherical
surface whose centre is in the farther focus, will have no appreciable
spherical aberration, and will refract parallel rays incident on its convex
surface to the farther focus.

The spherical form of lens is that most generally used in the construction of
the microscope. If a true elliptical or hyperbolic curve could be ground,
lenses would very nearly approach perfection, and spherical aberration
would be considerably reduced. Even this defect can be further reduced in

practice by observing a certain ratio between the
radii of the anterior and posterior surfaces of lenses;
thus the spherical aberration of a lens, the radius of
one surface of which is six or seven times greater
than that of the other, will be much reduced when its
more convex surface is turned forward to receive
parallel rays, than when its less convex surface is
turned forwards. It should be borne in mind that in
lenses having curvatures of the kind the object
would only be correctly seen in focus at one point—
the mathematical or geometrical axis of the lens. Fig. 17.—Aplanatic Doublet.

Chromatic Aberration.—We have yet to deal with one of the most
important of the phenomena of light, CHROMATIC ABERRATION, upon the
correction of which, in convex lenses in particular, the perfection of the
objective of the microscope so much depends. Chromatism arises from the
unequal refrangibility and length of the different coloured rays of light that
together go to make up white light; but which, when treated of in optics, is
always associated with achromatism, so that a combination of prisms, or
lenses, is said to be achromatic when the coloured rays arising from the
dispersion of the pencil of light refracted through them are combined in due
proportions as they are in perfectly white light.

A lens, however, of uniform material will not form a single white image,
but a series of images of all colours of the spectrum, arranged at different
distances, the violet being nearest, and the red the most remote, every other
colour giving a blurred image; the superposition of these and the blending
of the different elementary rays furnishing a complete explanation of the
beautiful phenomenon of the rainbow. Sharpness of outline is rendered
quite impossible in such a case, and this source of confusion is known as
chromatic aberration.

In order to ascertain whether it is possible to remedy this evil by combining
lenses of two different materials, Newton made some trials with a
compound prism composed of glass and water (the latter containing a little
sugar of lead), and he found it impossible by any arrangement of these two,
or by other substances, to produce deviation of the transmitted light without

separation into its component colours. If this ratio were the same for all
substances, as Newton supposed, achromatism would be impossible; but, in
fact, its value varies greatly, and is far greater for flint than for crown glass.
If two prisms of these substances, of small refracting angles, be combined
into one, with their edges turned in opposite directions, they will
achromatise each other.

The chromatism of lenses may, however, be somewhat further reduced by
stopping out the marginal rays, but as the most perfect correction possible is
required when lenses are combined for microscopic uses, other means of
correction are resorted to, as will be seen hereafter. I shall first proceed to
show the deviations which rays of white light undergo in traversing a lens.

If parallel rays of light pass through a double-convex lens the violet rays,
the most refrangible of them, will come to a focus at a point much nearer to
the lens than the focus of the red rays, which are the least refrangible; and
the intermediate rays of the spectrum will be focussed at points between the
red and the violet. A screen held at either of these foci will show an image
with prismatic fringes. The white light, A A′′ (Fig. 18), falling on the
marginal portion of the lens is so far decomposed that the violet rays are
brought to a focus at C, and crossing there, diverge again and pass on to F
F′, while the red rays, B B′′, do not come to a focus until they reach the
point D, and cross the divergent violet rays, E E′. The foci of the
intermediary rays of the spectrum (red, green, and blue) are intermediate
between these extremes. The distance, C D, limiting the blue or violet, and
the red is termed the longitudinal chromatic aberration of the lens. If the
image be received upon a screen placed at C, violet will predominate and
appear surrounded by a prismatic fringe, in which violet will predominate.
If the screen be now shifted to D, the image will have a predominant red
tint, surrounded by a series of coloured fringes in an inverted order to those
seen in the former experiment. The line E E′ joins the points of intersection
between the violet and red rays, and this marks the mean focus, the point
where the coloured rays will be least apparent.

Fig. 18.—Chromatic Aberration of Lens.

In the early part of this century the optical correction of chromatic
aberration was partially brought about by combining a convex lens of
crown-glass with a concave lens of flint-glass, in the proportion of which
these two kinds of glass respectively refract and disperse rays of light; so
that the one medium may by equal and contrary dispersion neutralise the
dispersion caused by the other, without at the same time wholly neutralising
its refraction. It is a curious fact that the media found most available for the
purpose should be a combination of crown and flint-glass, of crown-glass
whose index of refraction is 1·519, and dispersive power 0·036, and of
flint-glass whose index of refraction is 1·589, and dispersive power 0·0393.
The focal length of the convex crown-glass lens must be 41⁄3 inches, and
that of the concave flint-glass lens 72⁄3 inches, and the combined focal
length 10 inches. The diagram (Fig. 19) shows how rays of light are brought
to a focus, nearly free from colour. The small amount of residual colour in
such a combination is termed the secondary spectrum; the violet ray F Y,
crossing the axis of the lens at V, and going to the upper end P of the
spectrum, the red ray F B going to the lower end T. But as the flint-glass
lens l l, on the prism A a C, which receives the rays F V, F R, at the same
points, is interposed, these rays will unite at f, and form a small circle of
white light, the ray S F being now refracted without colour from its
primitive direction S F Y into the direction F f. In like manner, the
corresponding ray S F′ will be refracted to f, and a white colourless image
be the result.

Fig. 19.—Correction of Chromatic Aberration.

The achromatic aplanatic objective constructed on the optical formula
enunciated, did not meet all the difficulties experienced by the skilled
microscopist, in obtaining resolution of the finest test objects, and whereby
the intrinsic value of the objective (in his estimation) must stand or fall.
There were other disturbing residuary elements besides those of the
secondary spectrum, and which at a later period were met by the practical
skill of the optician, who applied the screw-collar, and by means of which
the back lens of the objective is made to approach the front lens, thus more
accurately shortening the distance between the eye-piece, where the image
is eventually formed, and the back lens of the objective.

In this diagram L L is a convex lens of crown-glass, and l l a concave one of
flint-glass. A convex lens will refract a ray of light (S) falling at F on it
exactly in the same manner as the prism A B C, whose faces touch the two
surfaces of the lens at the points where the ray enters, and quits. The ray S
F, thus refracted by the lens L L, or prism A B C, would have formed a
spectrum (P T) on a screen or wall, had there been no other lens.

Fig. 20.—Virtual Image formed by Convex Lens.

Formation of Virtual Images.—The normal eye possesses a considerable
power of adjusting itself to form a distinct image of objects placed at
varying distances; the nearer, within a certain limit, the larger it appears,
and the more distinctly the details are brought out. When brought within a
distance of two or three inches, the images become blurred or quite
indistinct, and when brought closer to the eye, cannot be seen at all, and it
simply obstructs the light. Now the utility of a convex lens, when
interposed between the object and the eye, consists in reducing the
divergence of the rays forming the several pencils which issue from it, and
send images to the retina in a state of moderate divergence, that is, as if they
had issued from an object beyond the nearest point of distinct vision, and so
that a more clearly defined image may reach the sensitive membrane of the
eye. But, not only is the course of the several rays in each pencil altered as
regards the rest, but the course of the pencils themselves is changed, so that
they enter the eye under an angle corresponding with that under which they
would have arrived from a larger object situated at a greater distance, and
thus the picture formed by any object corresponds in all respects with one
which would have been made by the same object increased in its
dimensions and viewed at the smallest ordinary distance of distinct vision.
For instance, let an object A B (Fig. 20) be placed between a convex lens
and its principal focus. Then the foci conjugate to the points A B are virtual,
and their positions can be found by construction from the consideration that
rays through A, B, parallel to the principal axis, will be refracted to F, the

principal focus on the other side. The refracted rays, if produced backwards,
must meet the secondary axis O A, O B in the required points. An eye
placed on the other side of the lens will accordingly see a virtual image
erect, magnified, and at a greater distance from the lens than the object.
This is the principle of the simple microscope.

The Human Eye.

To gain a clear insight into the mode in which a single lens serves to
magnify objects, it will be necessary to revert to the phenomena of ordinary
vision. An eye free from any defect has a considerable power of adjusting
itself to very considerable distances. One of the special functions of the eye
is bringing the rays of light, by a series of dioptric mechanisms, to a perfect
focus on its nervous sensitive layer, the retina. The eye in this respect has
been compared to a photographic camera. But this is not quite correct. The
retina is destined simply to receive the images furnished by the dioptric
apparatus, and has no influence upon the formation of these images. The
luminous rays are refracted by the dioptric apparatus; the images would be
formed quite as well—indeed, even better in certain cases—if the retina
were not there. The dioptric apparatus and its action are absolutely
independent of the retina.

The same laws with regard to the passage of the rays of light into the human
eye hold good, as those already enunciated in the previous pages. As to
change of direction when rays are passing obliquely from a medium of low
density to that of a higher density, i.e., it changes its course, and is bent
towards the perpendicular. On leaving the denser for the rarer medium it is
bent once more from the perpendicular. Again, by means of a convex lens,
the rays of light from one source will be refracted so as to meet at a point
termed the principal focus of vision.

In the eye there are several surfaces separating the different media where
refraction takes place. The refractive index of the aqueous humour and the
tears poured out by the lachrymal gland is almost equal to that of the
cornea. We may, therefore, speak of the refracting surfaces as three, viz.:
Anterior surface of cornea, anterior surface of lens, and posterior surface of

lens; and also of the refracting media as three—the aqueous humour, the
lens, and vitreous humour. These several bodies are so adapted in the
normal eye that parallel rays falling on the cornea are converged to a focus
at the most sensitive spot (the yellow spot, or fovea centralis) in the retina, a
point representing to the principal focus of the eye. A line drawn from this
point through the centre of the cornea is called the optic axis of the eye-ball.

Fig. 21.—Nerve and Stellate Cell Layer of Cornea,6 stained by chloride of gold;
magnified 300 diameters. a, Nerve cells. b, Stellate cells.

Fig. 22.—Anterior section of Eye, showing changed form of lens during the act of
accommodation, a voluntary action in the eye. M, Ciliary muscle; I, Iris; L, Lens;
V, Vitreous Humour; A, Aqueous Humour; C, Cornea and optic axis.

But as we are able to form a distinct image of near objects, and as we notice
when we turn our gaze from far to near objects there is a distinct feeling of
muscular effort in the eyes, there must be some means whereby the eye can
readily adapt itself for focussing near and distant objects. In a photographic
camera the focus can be readily altered, either by changing the lenses,
employing a lens of greater or less curvature, or by altering the distance of
the screen from the lens. The last method is obviously impossible in the
rigid eye-ball, and therefore the act of focussing for near and distant objects
is associated with a change in the curvature of the lens, a faculty of the eye
termed accommodation (Fig. 22), a change chiefly accomplished by the
ciliary (muscle) processes, which pull the lens forwards and inwards by
virtual contracting power of the ciliary muscle, and by which its suspensory
ligament is relaxed, and the front of the lens allowed to bulge forward. In
every case, however, accommodation is associated with contraction of the
iris, the special function of which is that of a limiting diaphragm (an iris-
diaphragm), Fig. 23.

In an ordinary spherical bi-convex lens, as already pointed out, the rays of
light passing through the periphery of the lens come to a focus at a nearer
point than the rays passing through the central portion. In this way a certain
amount of blurring of the image takes place, and which, in optical language,
is termed spherical aberration. This defect of the eye is capable of

correction in three possible ways, and which it may be well to repeat: 1. By
making the refractive index of the lens higher at its centre than at its
circumference; (2) By making the curvature of the lens less near the
circumference than at the centre; (3) By stopping out the peripheral rays of
light by a diaphragm. The two latter methods are those resorted to in most
optical instruments.

Fig. 23.—1. Equatorial section of Eyeball, showing Iris and Ciliary Processes,
after washing away the pigment, × three diameters.

2. Nerves of the Cornea of Kitten’s Eye, stained with iodine.

3. Fibres or Tubules of Lens, × 250, seen to be made up of superimposed crenated
layers, and is therefore not homogeneous in structure, but made up of a number of
extremely fine tubules, whose curvatures are nearly spherical.

In the human eye an attempt is made to apply all these methods, but the
most important is the third, that of applying the diaphragm formed by the
iris, a circular semi-muscular curtain lying just in front of the anterior
surface of the lens. The iris is also furnished with a layer of pigmental cells
which effectually stop out all peripheral rays of light that otherwise would
pass into the eye, creating circles of diffusion of a disturbing nature to
perfect vision. This delicate membrane, then, is kept in constant action by a
two-fold nerve supply, derived from five or six sources, which it is
unnecessary to describe at length. But the eye, with all its marvellous
adaptations, has an obvious defect, that of secondary or uncorrected
chromatic aberration.

Chromatic Aberration of the Eye.—White light, as previously explained,
is composed of different wave lengths; and accordingly as these undulations
are either longer or shorter, so do they produce on the eye the impression of
different colours. We have seen how a pencil of white light may, by means
of a prism, be decomposed into a multi-coloured band. In an ordinary
magnifying reading-glass these coloured fringes are always seen around the
margins. In practical optics chromatic aberration is partially corrected by
employing two different kinds of glass in the construction of certain
combined lenses. In the human eye chromatism cannot be corrected in this
way; hence a blue light and a red light placed at the same distance from the
eye appears to be unequally distant: the red light requiring greater
accommodation in the eye than the blue, and this accordingly appears to be
the nearer of the two.

This visual error may be experimentally shown and explained. There is a
kind of glass which at first sight appears dark blue or violet, but which
really contains a great deal of red. Take an ordinary microscope lamp,
having a metal or opaque chimney, and drill a circular hole in it, about 3
mm. in diameter. This opening should be just at the height of the flame;
cover it over with a piece of ground glass and a piece of the red-blue glass.
Thus will be formed a luminous point whose light is composed of red and
blue, i.e., of colours far apart from each other in the spectrum.

Fig. 24.—Chromatic Aberration of Eye, showing the wave differences of the blue
and red rays of light (Landolt).

If rays coming from this point enter the eye, the blue rays (Fig. 24), being
more strongly reflected than the red, will come to a focus sooner than the
latter. The red rays, on the contrary, will be brought to a focus later than the
blue, while the latter, past their focus, are diverging. Let A B C D (Fig. 24)
be the section of a pencil of rays given off from a red-blue point sufficiently
distant so that these rays may be regarded as parallel. The focus of the blue
is at b, that of the red at r.

An eye is adapted to the distance of the luminous point when the circle of
diffusion, received upon the retina, is at its minimum. This is the case when
the sentient layer of the retina lies between the two foci E. In this case the
point will appear as a small circle, composed of the two colours, that is to
say—violet. If the retina be in front of this point, at the focus of the blue
rays for instance, the eye will perceive a blue point surrounded by a red
circle, the latter being formed by the periphery of the luminous cone of red
rays, which are focussed only after having passed the retina. The blue point
will become a circle of diffusion larger in proportion as the retina is nearer
the dioptric system, or as the focus for blue is farther behind it. But the blue
circle will always be surrounded by a red ring. If, on the contrary, the retina
is behind the focus for red, the blue cone will be greater in diameter than the
red, and we shall have a red circle of diffusion, larger in proportion as the
retina is farther from the focus, but always surrounded by a blue ring M. If
the blue-red point is five metres, or more, distant, the emmetropic7 eye will
evidently see it more distinctly, i.e., as a small violet point; the hyperopic
eye, whose retina is situated in front of the focus of its dioptric system, will
see a blue circle, surrounded by red; the myopic eye, whose retina is behind
its focus, will see a red circle, surrounded by blue. The size of these circles
will be either larger or smaller when the principal focus of the eye is either
in front of or behind the retina.8

The refractive surfaces of a perfectly formed eye are very like an ellipsoid
of revolution with two axes, one of which, the major axis of the ellipse, is at
the same time the optic axis and that of rotation; the other is perpendicular
to it, and is equal in all meridians. Eyes, however, perfectly constructed are

rarely met with. The curvature of the cornea is nearly always greater in one
meridian than in another. Its surfaces then cannot be regarded as entirely
belonging to an ellipsoid of revolution, since the solid figure, of which the
former would constitute a part, has not only two axes, but three, and these
unequal. This irregularity is not, however, always great enough to produce
discomfort and it is therefore disregarded. But in other cases the difference
of curvature in the different meridians of the eye attain to a higher degree,
and vision falls far below the average.

The refractive anomaly alluded to is
termed astigmatism (from the Greek,
α privative, στιγμα, a point—
inability to see a point). The way in
which objects appear to such a
person will mainly result from the
way in which he sees a point. Take,
for example, the vertical to be the
most, and the horizontal to be the
least, refractive meridian: place a
vertical line (Fig. 25, I) at a stated
distance before the eye, and the line
will appear elongated, owing to the
diffusion image of each of the points
Fig. 25.—Lines as seen by the Astigmatic. composing it. It will also seem to be
somewhat broadened, as at II. If the
vertical meridian is adapted to the distance of the vertical, the line will
appear very diffuse and broadened, as at III. All these little diffusion lines
overlap each other, and give the line an elongated appearance. Hence a
straight line is seen distinctly by an astigmatic eye only when the meridian
to which it is perpendicular is perfectly adapted to its distance. A vertical
line is seen distinctly when the horizontal meridian is adapted to its
distance. It appears indistinct when its image is formed by the vertical
meridian. The way in which an astigmatic person sees points and lines led
to the discovery of this remarkable irregularity in the refraction of the eye.
The late Astronomer Royal, Sir George Airy, suffered for some years until,
indeed, he discovered how it could be corrected. This anomaly of curvature
of the refractive surfaces of the eye is now known to prevail largely among

the more civilised races of mankind. It is, then, of very great importance
when using high powers of the microscope. In most persons the visual
power of both eyes is rarely quite equal; on the other hand, the mind exerts
an important influence, dominates, as it were, the eye in the interpretation
of visual sensations and images. An example of this is presented in
Wheatstone’s pseudoscope, known to produce precisely the opposite effect
of his stereoscope—conveys, in fact, the converse of relief produced by the
latter and better known instrument.

Visual Judgment.—The apparent size of an object is determined by the
magnitude of the image formed on the retina, and this is inversely
proportional to the distance. Thus the size of an image on the retina of an
object two inches long at a distance of a foot, is equal to the image of an
object four inches long at a distance of two feet. An object can be seen if the
visual angle subtended by it is not less than sixty seconds. This is equivalent
to an image on the fovea centralis of the retina of about 4 µ9 across, and
which corresponds to the diameter of a cone: so that while we have had
under consideration the optical and physical conditions of human vision, we
have likewise taken a lesson on the action of lenses used in the construction
of the microscope.

The Theory of Microscopical Vision.

It has been said that no comparison can be instituted between microscopic
vision and macroscopic; that the images formed by minute objects are not
delineated microscopically under ordinary laws of diffraction, and that the
results are dioptrical. This assertion, however, cannot be accepted
unconditionally, as will be seen on more careful examination of the late
Professor Abbe’s masterly exposition of “The Microscopical Theory of
Vision,” and also his subsequent investigations on the estimation of aperture
and the value of wide-angled immersion objectives, published in the
“Journal of the Royal Microscopical Society.”

The essential point in Abbe’s theory of microscopical vision is that the
images of minute objects in the microscope are not formed exclusively on
the ordinary dioptric method (that is, in the same way in which they are

formed in the camera or telescope), but that they are largely affected by the
peculiar manner in which the minute construction of the object breaks up
the incident rays, giving rise to diffraction.

The phenomena of diffraction in general may be observed experimentally
by plates of glass ruled with fine lines. Fig. 26 shows the appearance
presented by a single candle-flame seen through such a plate, an uncoloured
image of the flame occupying the centre, flanked on either side by a row of
coloured spectra of the flame, which become dimmer as they recede from
the centre. A similar phenomenon may be produced by dust scattered over a
glass plate, and by other objects whose structure contains very minute
particles, or the meshes of very fine gauze wire, the rays suffering a
characteristic change in passing through such objects; that change
consisting in the breaking up of a parallel beam of light into a group of rays,
diverging with wide angle and forming a regular series of maxima and
minima of intensity of light, due to difference of phase of vibration.10

In the same way, in the microscope,
the diffraction pencil originating
from a beam incident upon, for
Fig. 26.
instance, a diatom, appears as a fan
of isolated rays, decreasing in
intensity as they are further removed from the direction of the incident
beam transmitted through the structure, the interference of the primary
waves giving a number of successive maxima of light with dark interspaces.

When a diaphragm opening is interposed between the mirror, and a plate of
ruled lines placed upon the stage such as Fig. 27, the appearance shown in
Fig. 27a, will be observed at the back of the objective on removing the eye-
piece and looking down the tube of the microscope. The centre circles are
the images of the diaphragm opening produced by the direct rays, while
those on the other side (always at right angles to the direction of the lines)
are the diffraction images produced by the rays which are bent off from the
incident pencil. In homogeneous light the central and lateral images agree
in size and form, but in white light the diffraction images are radially drawn
out, with the outer edges red and the inner blue (the reverse of the ordinary
spectrum), forming, in fact, regular spectra the distance separating each of

which varies inversely as the closeness of the lines, being for instance with
the same objective twice as far apart when the lines are twice as close.

Fig. 27. Fig. 27a.

The influence of these diffraction spectra may be demonstrated by some
very striking experiments, which show that they are not by any means
accidental phenomena, but are directly connected with the image which is
seen by the eye.

The first experiment shows that with the central beam, or any one of the
spectral beams alone, only the contour of the object is seen, the addition of
at least one diffraction spectrum being essential to the visibility of the
structure.

Fig. 28. Fig. 28a.

When by a diaphragm placed at the back of the objective, as in Fig. 28, we
cover up all the diffraction spectra of Fig. 27a, and allow only the central
rays to reach the image, the object will appear to be wholly deprived of fine
details, the outline alone will remain, and every delineation of minute
structure will disappear, just as if the microscope had suddenly lost its
optical power, as in Fig. 28a.

This experiment illustrates a case of the obliteration of structure by
obstructing the passage of the diffraction spectra to the eye-piece. The next
experiment shows how the appearance of fine structure may be created by
manipulating the spectra.

Fig. 29. Fig. 29a.

When a diaphragm such as that shown in Fig. 29 is placed at the back of the
objective, so as to cut off each alternate one of the upper row of spectra in
Fig. 27a, that row will obviously become identical with the lower one, and
if the theory holds good, we should find the image of the upper lines
identical with that of the lower. On replacing the eye-piece, we see that it is

so, the upper set of lines are doubled in number, a new line appearing in the
centre of the space between each of the old (upper) ones, and upper and
lower set having become to all appearance identical, as seen in Fig. 29a.

Fig. 30. Fig. 30a.

In the same way, if we stop off all but the outer spectra, as in Fig. 30, the
lines are apparently again doubled, as seen in Fig. 30a.

A case of apparent creation of structure, similar in principle to the
foregoing, though more striking, is afforded by a network of squares, as in
Fig. 31, having sides parallel to this page, which gives the spectra shown in
Fig. 31a, consisting of vertical rows for the horizontal lines and horizontal
rows for the vertical ones. But it is readily seen that two diagonal rows of
spectra exist at right angles to the diagonals of the squares, just as would
arise from sets of lines in the direction of the diagonals, so that if the theory
holds good we ought to find, on obstructing all the other spectra and
allowing only the diagonal ones to pass to the eye-piece, that the vertical
and horizontal lines have disappeared and are replaced by two new sets of
lines at right angles to the diagonals.

Fig. 31. Fig. 31a.

Fig. 32. Fig. 32a.

On inserting the diaphragm, Fig. 32, and replacing the eye-piece, we find in
the place of the old network the one shown in Fig. 32a, the squares being,
however, smaller in the proportion of 1 : √2, as they should be in
accordance with the theory propounded.

An object such as Pleurosigma angulatum, which gives six diffraction
spectra arranged as in Fig. 33, should, according to this theory, show
markings in a hexagonal arrangement. For there will be one set of lines at
right angles to b, a, e, another set at right angles to c, a, f, and a third at
right angles to g, a, d. These three sets of lines will obviously produce the
appearance shown in Fig. 33a.

Fig. 33. Fig. 33a.

A great variety of appearances may be produced with the same arrangement
of spectra. Any two adjacent spectra with the central beam (as b, c, a) will
form equilateral triangles and give hexagonal markings. Or by stopping off
all but g, c, e (or b, d, f), we again have the spectra in the form of equilateral

triangles; but as they are now further apart,
the sides of the triangles in the two cases
being as √3 : 1, the hexagons will be
smaller and three times as numerous. Their
sides will also be arranged at a different
angle to those of the first set. The hexagons
may be entirely obliterated by admitting
only the spectra g, c, or g, f, or b, f, etc.,
when new lines will appear at right angles,
or obliquely inclined, to the median line.
By varying the combinations of the spectra,
therefore, different figures of varying size
Fig. 34.
and positions are produced, all of which
cannot, of course, represent the true
structure. Not only, however, may the appearance of particular structure be
obliterated or created, but it may even be predicted before being seen under
the microscope. If the position and relative intensity of the spectra in any
particular case are given, the character of the resultant image, in some
instances, may be worked out by mathematical calculations. A remarkable
instance of such a prediction is to be found in the case recorded by Mr.
Stephenson, where a mathematical student who had never seen a diatom,
worked out the purely mathematical result of the interference of the six
spectra b-g of Fig. 33 (identical with P. angulatum), giving the drawing
copied in Fig. 34. The special feature was the small markings between the
hexagons, which had not, before this time, been noticed on P. angulatum.
On more closely scrutinizing a valve, stopping out the central beam and
allowing the six spectra only to pass, the small markings were found
actually to exist, though they were so faint they had previously escaped
observation until the result of the mathematical deduction had shown that
they ought to be seen.

These experiments seem to show that diffraction plays a very essential part
in the formation of microscopical images, since dissimilar structures give
identical images when the differences of their diffractive effect is removed,
and conversely similar structures may give dissimilar images when their
diffractive images are made dissimilar. Whilst a purely dioptric image
answers point for point to the object on the stage, and enables a safe

inference to be drawn as to the actual nature of that object, the visible
indications of minute structure in a microscopical image are not always or
necessarily conformable to the real nature of the object examined, so that
nothing more can safely be inferred from the image as presented to the eye,
than the presence in the object of such structural peculiarities as will
produce the particular diffraction phenomena on which these images
depend.

Further investigations and experiments led Abbe to discard so much of his
theoretical conclusions relating to superimposed images having a distinct
character as well as a different origin, and as to their capability of being
separated and examined apart from each other. In a later paper he writes: “I
no longer maintain in principle the distinction between the absorption
image or direct dioptrical image and the diffraction image, nor do I hold
that the microscopical image of an object consists of two superimposed
images of different origin or a different mode of production. Thus it appears
that both the absorption image and the diffraction image he held to be
equally of diffraction origin; but while a lens of small aperture would give
the former with facility, it would be powerless to reveal the latter, because
of its limited capacity to gather in the strongly-deflected rays due to the
excessively minute bodies the microscopical objective has to deal with.”11

Abbe’s theory of vision has been questioned by mathematicians, and since
his death Lord Rayleigh went more deeply into the question of “the theory
of the formation of optical images,” with special reference to the
microscope and telescope. He has shown that two lines cannot be fairly
resolved unless their components subtend an angle exceeding that
subtended by the wave-length of light at a distance equal to the aperture;
also, that the measure of resolution is only possible with a square aperture,
or one bounded by straight lines, parallel to the lines resolved.

Lord Rayleigh’s Theory of the Formation of Optical Images, with
Special Reference to the Microscope.12

Of the two methods adopted, that of Helmholtz’s consists in tracing the
image representative of a mathematical point in the object, the point being

regarded as self-luminous; that of Abbe’s the typical object was not, as we
have seen, a luminous point, but a grating illuminated by plane waves of
light. In the latter method, Lord Rayleigh argues that the complete
representation of the object requires the co-operation of all the spectra
which are focussed in the principal focal plane of the objective; when only a
few are present the representation is imperfect, and wholly fails when there
is only one. He then proceeds to show, by the aid of diagrams and
mathematical formula, how the resolving power can be adduced.

On further criticism of the Abbe spectrum theory, he observes “that
although the image ultimately formed may be considered to be due to the
spectra focussed to a given point, the degree of conformity of the image to
the object is another question. The consideration of the case of a very fine
grating, which might afford no lateral spectra at all, shows the incorrectness
of the usually accepted idea that if all the spectra are utilised the image will
still be incomplete, so that the theory (originally promulgated by Abbe)
requires a good deal of supplementing; while it is inapplicable when the
incident light is not parallel, and when the object is, for example, a double
point and not a grating. Even in the case of a grating, the spectrum theory is
inapplicable, if the grating is self-luminous; for in this case no spectra can
be formed since the radiations from the different elements of the grating
have no permanent phase-relations.” For these reasons Lord Rayleigh
advises that the question should be reconsidered from the older point of
view, according to which the typical object is a point and not a grating.
Such treatment will show that the theory of resolving power is essentially
the same for all instruments. The peculiarities of the microscope, arising
from the divergence-angles not being limited to be small, and from the
different character of the illumination, are theoretically only differences of
detail. These investigations can be extended to gratings, and the results so
obtained confirm for the most part the conclusions of the spectrum theory.

Furthermore, that the function of the condenser in microscopic practice in
throwing upon the object the image of the lamp-flame is to cause the object
to behave, at any rate in some degree, as if it were self-luminous, and thus
to obviate the sharply-marked interference bands which arise when
permanent and definite phase-relations are permitted to exist between the
radiations which issue from various points of the object. This is capable of

mathematical proof; and in the case where the illumination is such that each
point of the row or of the grating radiates independently, the limit to
resolution is seen to depend only on the width of the aperture, and thus to
be the same for all forms of aperture as for those of the rectangular. That
Abbe’s theory of microscopic vision is fairly open to the criticisms passed
on it by Lord Rayleigh must be taken for granted.

Definition of Aperture; Principles of Microscopic Vision.

It must be well within the last half-century that the achromatic objective-
glass for the microscope was brought to perfection and its value became
generally recognised. Prior to the discovery of the achromatic principle in
the construction of lenses it was assumed that the formation of the
microscopic image took place (as we have already seen) on ordinary
dioptric principles. As the image is formed in the camera or telescope, so it
was said to be in the microscope. This belief existed, it will be remembered,
at a time when dry objectives only were in favour and the use of the term
angle of aperture was misunderstood, when it was supposed that the
different media with diffraction-indices were used; and the angle of the
radiant pencil was believed not only to admit of a comparison of two
apertures in the same medium, but likewise to admit of a standard of
comparison when the media were entirely different in their refractive
qualities.

It was during my tenure of office as secretary of the Royal Microscopical
Society (1867 to 1873), that the aperture question, and also that of
numerical aperture, came under discussion, both being met by the majority
of the Fellows of the Society and practical opticians by a non-possumus.

Opticians alleged, that is, before the value of aperture became fully
recognised (1860), that the achromatic objective had reached a stage of
perfection, beyond which it was not possible to go; indeed, not only
opticians, but physicists of high standing, as Professor Helmholtz, who
made many important contributions to the theory of the microscope, and
who, after duly weighing all the known physical laws on which the
formation of images can be explained, emphatically stated that in his

opinion “the limit of possible improvement of the microscope as an
instrument of discovery had been very nearly reached.” A quarter of a
century ago I ventured to throw a doubt upon so questionable a statement. I
determined, if possible, to submit the aperture question to an exhaustive
examination. My views were accordingly submitted to two of the highest
authorities in this country—Sir George Airy, the then Astronomer Royal,
and Sir George Stokes, Professor of Physics at Cambridge University—both
of whom agreed with me that the possible increase of aperture would be
attended with great advantage to the objective, and open the way to an
extension of power resolution in the microscope.13 The discussion
afterwards took a warm turn, as will be seen on reference to “The Monthly
Microscopical Journals” of 1874, 1875 and 1876.

The confusion into which the aperture question at this period had lapsed
was no doubt due to the fact that its opponents had not yet grasped the true
meaning of the term aperture. It was believed to be synonymous with
“angular aperture,” much in use at the time. It will, however, appear quite
unaccountable that even the older opticians should have confounded the
latter with the former; and so entirely disregarded the fact that the angles of
the pencil of light admitted by the objective cannot serve as a measure of its
aperture, and that high refractive media can greatly reduce the value length
of waves of light.

When the medium in which the objective works is the same as air, it is not
that a comparison can be made by the angles of the radiant pencils only, but
by their sines. For example, if two dry objectives admit pencils of 60° and
180°, their real apertures are not as 1 : 3, but as 1 : 2 only. Aperture in fact
is computed by mathematicians by tracing the rays from the back focus
through the system of lenses to the front focus, the front focus being the
point at which the whole cone of rays converge as free as may be from
aberration. If the front focus be in air, no pencil greater than 82°, “double
the angle of total reflection,” can emerge from the plane front of the lens;
and, obviously, if no greater cone can emerge to a focus one way, neither
can any greater cone enter the body of the lens from the radiant. This angle,
then, of 82°, must be regarded as the limit for dry lenses or objectives.

This limit, it will be seen on more careful examination, is very nearly the
maximum angle that can be computed for a lens to have a front focus in air.
This can be proved by the consideration of the angle of the image of rays,
as they are radiated from the object itself in balsam: for although this angle
of image rays viewed as nascent from a self-luminous object capable of
scattering rays in all directions, may be 180° in the substance of the balsam
and cover-glass, of the 180° only 82° of the central portion will emerge into
air—all rays beyond this limit are internally reflected at the cover-glass.
This cone, then, of 82° becomes 180° in air, and a large part must
necessarily be lost by reflection at the first incidence on the plane front of
the lens. But with a formula permitting the use of a water medium between
the front lens and the cover-glass, the aperture of the image rays may reach
126°—double the critical angle from glass to water; and with an oil
medium, the aperture will be found to be limited only by the form of the
front lens that can be constructed by the optician.

To sum up, then, the effect of the immersion system, greatly assists in the
correction of aberration, gives increased magnification and angular
aperture, increase of working distance between the objective and object,
and renders admissible the use of the thicker glass-cover.

The aperture question would in all probability have remained unsolved
many years longer (ten or twelve years elapsed after I brought the question
under discussion before opticians gave way), but for the fortunate
circumstance that the eminent mathematical and practical optician,
Professor Abbe, of Jena, was about to visit London. This came off in the
early part of the seventies, when the late Mr. John Mayall and myself had
the good fortune to interview him. The subject discussed was naturally the
increase of aperture and the theory of microscopical vision. He readily at
our request undertook to re-investigate the question in all its bearings on the
microscope. It is almost unnecessary to add that the conclusions he came to,
and the results obtained, have proved of inestimable value to the
microscopist and practical optician, and it may well seem necessary to
explain somewhat at greater length the conclusions the learned Professor
came to, and by the adoption of which the microscope has been placed on a
more scientific basis than it had before attained to. Several papers were
published in extenso in the “Journal of the Royal Microscopical Society,”

and I am greatly indebted to Mr. Frank Crisp, LL.D., for an excellent
resumé of Abbe’s Monograph.14

The essential step in the consideration of aperture is, as I have said, to
understand clearly what is meant by the term. It will at once be recognised
that its definition must necessarily refer to its primary meaning of opening,
and must, in the case of an optical instrument, define its capacity for
receiving rays from the object, and transmitting them to the image received
at the eye-piece.

In the case of the telescope-objective, its capacity for receiving and
transmitting rays is necessarily measured by the expression of its absolute
diameter or “opening.” No such absolute measure can be applied in the case
of the microscope objective, the largest constructed lenses of which having
by no means the largest apertures, being, in fact, the lower powers of the
instrument, whose apertures are for the most part but small. The capacity of
a microscope objective for receiving and transmitting rays is, however, as
will be seen, estimated by its relative opening, that is, its opening in relation
to its focal length. When this relative opening has been ascertained, it may
be regarded as synonymous with that denoted in the telescope by absolute
opening. That this is so will be better appreciated by the following
consideration:—

In a single lens, the rays admitted within one meridional plane evidently
increase as the diameter of the lens (all other circumstances remaining the
same), and in the microscope we have, at the back of the lens, the same
conditions to deal with as are in front in the case of the telescope; the larger
or smaller number of emergent rays will therefore be measured by the clear
diameter, and as no rays can emerge that have not first been admitted, this
will give the measure of the admitted rays under similar circumstances.

If the lenses compared have different focal lengths but the same clear
“openings,” they will transmit the same number of rays to equal areas of an
image at a definite distance, because they would admit the same number if
an object were substituted for the image; that is, if the lens were used as a
telescope-objective. But as the focal lengths are different, the amplification
of the images is different also, and equal areas of these images correspond
to different areas of the object from which the rays are collected. Therefore,

the higher power lens with the same opening as the lower power, will admit
a greater number of rays in all from the same object, because it admits the
same number as the latter from a smaller portion of the object. Thus, if the
focal lengths of two lenses are as 2 : 1, and the first amplifies N diameters,
the second will amplify 2 N with the same distance of the image, so that the
rays which are collected to a given field of 1 mm. diameter of the image are
admitted from a field of 1/N mm. in the first case, and of 1/2N mm. in the
second. As the “opening” of the objective is estimated by the diameter (and
not by the area) the higher power lens admits twice as many rays as the
lower power, because it admits the same number from a field of half the
diameter, and, in general, the admission of rays by the same opening, but
different powers, must be in the inverse ratio of the focal lengths.

In the case of the single lens, therefore, its aperture is determined by the
ratio between the clear opening and the focal length. The same
considerations apply to the case of a compound objective, substituting,
however, for the clear opening of the single lens the diameter of the pencil
at its emergence from the objective, that is, the clear utilised diameter of the
back lens. All equally holds good whether the medium in which the
objective is placed is the same in the case of the two objectives or different,
as an alteration of the medium makes no difference in the power.

180° Oil Angle. (Numerical
Aperture 1·52.)

180° Water Angle. (Numerical
Aperture 1·33.)

180° Air Angle. 96° Water Angle.
82° Oil Angle. (Numerical Aperture
1·00.)

97° Air Angle. (Numerical Aperture
·75.)

60° Air Angle. (Numerical Aperture
·50.)

Fig. 35.—Relative diameters of the (utilized) back lenses of various dry and
immersion objectives of the same power (¼-in.) from an air angle of 60° to an
oil angle of 180°.

Thus we arrive at a general proposition for all kinds of objectives: 1st, when
the power is the same, the admission of rays (or aperture) varies with the
diameter of the pencil at its emergence; 2nd, when the powers are different,
the same aperture requires different openings in the ratio of the focal
lengths, or conversely with the same opening the aperture is in inverse ratio
to the focal lengths. We see, therefore, that just as in the telescope the
absolute diameter of the object-glass defines its aperture, so in the
microscope the ratio between the utilised diameter of the back lens and the
focal length of the objective defines its aperture also, and this is clearly a
definition of aperture in its primary and only legitimate meaning as
“opening;” that is, the capacity of the objective for admitting rays from the
object and transmitting them to the image.

If, by way of illustration, we compare a series of dry and oil-immersion
objectives, and commencing with small air angles, progress up to 180° air
angle, and then take an oil-immersion of 82° and progress again to 180° oil
angle, the ratio of opening to power progresses also, and attains its
maximum, not in the case of the air angle of 180° (when it is exactly
equivalent to the oil angle of only 82°), but is greatest at the oil angle of
180°. If we assume the objectives to have the same power throughout we
get rid of one of the factors of the ratio, and we have only to compare the
diameters of the emergent beams, and can represent their relations by
diagrams.

Fig. 35 illustrates five cases of different apertures of ¼-in. objectives, viz.:
those of dry objectives of 60°, 97°, and 180° air angle, a water-immersion
of 180° water angle, and an oil-immersion of 180° oil angle. The inner
dotted circles in the two latter cases are of the same size as that
corresponding to the 180° air angle.

A dry objective of the maximum air angle of 180° is only able to utilise a
diameter of back lens equal to twice the focal length, while an immersion
lens of even only 100° utilises a larger diameter, i.e., it is able to transmit
more rays from the object to the image than any dry objective is capable of
transmitting. Whenever the angle of an immersion lens exceeds twice the
critical angle for the immersion fluid, i.e., 96° for water or 82° for oil, its
aperture is in excess of that of a dry objective of 180°.

Fig. 36.

This excess will be seen if we take an oil-immersion objective of, say 122°
balsam angle, illuminating it so that the whole field is filled with the
incident rays, and use it first on an object not mounted in balsam, but dry.
We then have a dry objective of nearly 180° angular aperture, for, as will be
seen by reference to Fig. 36, the cover-glass is virtually the first surface of
the objective, as the front lens, the immersion fluid, and the cover-glass are
all approximately of the same index, and form, therefore, a front lens of
extra thickness. When the object is close to the cover-glass the pencil
radiating from it will be very nearly 180°, and the emergent pencil
(observed by removing the eye-piece) will be seen to utilise as much of the

back lens of the objective as is equal to twice the focal length, that is, the
inner of the two circles at the head of Fig. 35.

If now balsam be run in beneath the cover-glass so that the angle of the
pencil taken up by the objective is no longer 180°, but 122° only (that is,
smaller), the diameter of the emergent pencil is larger than it was before,
when the angle of the pencil was 180° in air, and will be approximately
represented by the outer circle of Fig. 35. As the power remains the same in
both cases, the larger diameter denotes the greater aperture of the
immersion objective over a dry objective of even 180° angle, and the excess
of aperture is made plainly visible.

Having settled the principle, it is still necessary, however, to find a proper
notation for comparing apertures. The astronomer can compare the
apertures of his various objectives by simply expressing them in inches, but
this is obviously not available to the microscopist, who has to deal with the
ratio of two varying quantities.

In consequence of a discovery made by Professor Abbe in 1873, that a
general relation existed between the pencil admitted into the front of the
objective and that emerging from the back of the objective, he was able to
show that the ratio of the semi-diameter of the emergent pencil to the focal
length of the objective could be expressed by the formula n Sin u, i.e., by
the sine of half the angle of aperture (u) multiplied by the refractive index
of the medium (n) in front of the objective (n being 1·0 for air, 1·33 for
water, and 1·52 for oil or balsam).

When, then, the values in any given cases of the expression n Sin u (which
is known as the “numerical aperture”) has been ascertained, the objectives
are instantly compared as regards their aperture, and, moreover, as 180° in
air is equal to 1·0 (since n = 1·0 and the sine of half 180° = 1·0) we see,
with equal readiness, whether the aperture is smaller or larger than that
corresponding to 180° in air. Thus, suppose we desire to compare the
apertures of three objectives, one a dry objective, the second a water
immersion, and the third an oil immersion; these would be compared on the
angular aperture view as, say 74° air angle, 85° water angle, and 118° oil
angle, so that a calculation must be worked out to arrive at the actual
relation between them. Applying, however, the numerical15 notation, which

gives ·60 for the dry objective, ·90 for the water immersion, and 1·30 for
the oil immersion, their relative apertures are immediately recognised, and
it is seen, for instance, that the aperture of the water immersion is somewhat
less than that of a dry objective of 180°, and that the aperture of the oil
immersion exceeds that of the latter by 30%.

The advantage of immersion, in comparison with dry objectives, becomes
at once apparent. Instead of consisting merely in a diminution of the loss of
light by reflection or increased working distance, it is seen that a wide-
angled immersion objective has a larger aperture than a dry objective of
maximum angle, so that for any of the purposes for which aperture is
essential an immersion must necessarily be preferred to a dry objective.

That pencils of identical angular extension but in different media are
different physically, will cease to appear in any way paradoxical if we recall
the simple optical fact that rays, which in air are spread out over the whole
hemisphere, are in a medium of higher refractive index such as oil
compressed into a cone of 82° round the perpendicular, i.e., twice the
critical angle. A cone exceeding twice the critical angle of the medium will
therefore embrace a surplus of rays which do not exist even in the
hemisphere when the object is in air.

The whole aperture question, notwithstanding the innumerable perplexities
which heretofore surrounded it, is in reality completely solved by these two
simple considerations: First, that “aperture” is to be applied in its ordinary
meaning as representing the greater or less capacity of the objective for
receiving and transmitting rays; and second, that when so applied the
aperture of an objective is determined by the ratio between its opening and
its focal length; the objective that utilises the larger back lens (or opening)
relatively to its focal length having necessarily the larger aperture. It would
hardly, therefore, serve any useful purpose if we were here to discuss the
various erroneous ideas that gave rise to the contention that 180° in air must
be the maximum aperture. Amongst these was the suggestion that the larger
emergent beams of immersion objectives were due to the fact that the
immersion fluid abolished the refractive action of the first plane surface
which, in the case of air, prevented there being any pencil exceeding 82°
within the glass. Also the very curious mistake which arose from the

assumption that a hemisphere did not magnify an object at its centre
because the rays passed through without refraction. A further erroneous
view has, however, been so widespread that it seems to be desirable to
devote a few lines to it, especially as it always appears at first sight to be
both simple and conclusive.

If a dry objective is used upon an
object in air, as in Fig. 37, the angle
Fig. 37. may approach 180°, but when the
object is mounted in balsam, as in
Fig. 37a, the angle at the object
cannot exceed 82°, all rays outside
that limit (shown by dotted lines)
Fig. 37a.
being reflected back at the cover-
glass and not emerging into air. On
using an immersion objective, however, the immersion fluid which replaces
the air above the cover-glass allows the rays formerly reflected back to pass
through to the objective, so that the angle at the object may again be nearly
180° as with the dry lens. The action of the immersion objective was,
therefore, supposed to be simply that it repaired the loss in angle which was
occasioned when the object was transferred from air to balsam, and merely
restored the conditions existing in Fig. 37a with the dry objective on a dry
object.

As the result of this erroneous supposition, it followed that an immersion
objective could have no advantage over a dry objective, except in the case
of the latter being used upon a balsam-mounted object, its aperture then
being (as was supposed) “cut down.” The error lies simply in overlooking
the fact that the rays which are reflected back when the object is mounted in
balsam Fig. 37a) are not rays which are found when the object is in air (Fig.
37), but are additional and different rays which do not exist in air, as they
cannot be emitted in a substance of so low a refractive index.

Lastly, it should also be noted that it is numerical and not angular aperture
which measures the quantity of light admitted to the objective by different
pencils.

Fig. 38. Fig. 38a.

First take the case of the medium being the same. The popular notion of a
pencil of light may be illustrated by Fig. 38, which assumes that there is
equal intensity of emission in all directions, so that the quantity of light
contained in any given pencils may be compared by simply comparing the
contents of the solid cones. The Bouguer-Lambert law, however, shows that
the quantity of light emitted by any bright point varies with the obliquity of
the direction of emission, being greater in a perpendicular than in an
oblique direction. The rays are less intense in proportion as they are more
inclined to the surface which emits them, so that a pencil is not correctly
represented by Fig. 38, but by Fig. 38a, the density of the rays decreasing
continuously from the vertical to the horizontal, and the squares of the sines
of the semi-angles (i.e., of the numerical aperture) constituting the true
measure of the quantity of light contained in any solid pencil.

If, again, the media are of different refractive indices, as air (1·0), water
(1·33), and oil (1·52), the total amount of light emitted over the whole 180°
from radiant points in these media under a given illumination is not the
same, but is greater in the case of the media of greater refractive indices in
the ratio of the squares of those indices (i.e., as 1·0, 1·77 and 2·25). The
quantity of light in pencils of different angle and in different media must
therefore be compared by squaring the product of the sines and the
refractive indices, i.e. (n Sin u2), for the square of the numerical aperture.

The fact is therefore made clear that the aperture of a dry objective of 180°
does not represent, as was supposed, a maximum, but that aperture
increases with the increase in the refractive index of the immersion fluid;
and it should be borne in mind that this result has been arrived at in strict

accordance with the ordinary propositions of geometrical optics, and
without any reference to or deductions from the diffraction theory of
Professor Abbe.

There still remains one other point for determination, namely, the proper
function of aperture in respect to immersion objectives of large aperture.
The explanation of the increased power of vision obtained by increase of
aperture was, that by the greater obliquity of the rays to the object “shadow
effects” were produced, a view which overlooked the fact, first, that the
utilisation of increased aperture depends not only on the obliquity of the
rays sent to the object, but also to the axis of the microscope; and exactly as
there is no acoustic shadow produced by an obstacle, which is only a few
multiples of the length of the sound waves, so there can be no shadow
produced by minute objects, only a few multiples from the light waves, the
latter then passing completely round the object. The Abbe diffraction
theory, however, supplies the true explanation of this, and shows that the
increased performance of immersion objectives of large aperture is directly
connected (as might have been anticipated) with the larger “openings” in
the proper sense of the term, which, as we have already explained, such
objectives really possess. Furthermore, in order that the image exactly
corresponds with the object, all diffracted rays must be gathered up by the
objective. Should any be lost we shall have not an actual image of the
object, but a spurious one. Now, if we have a coarse object, the diffracted
rays are all comprised within a narrow cone round the direct beam, and an
objective of small aperture will transmit them all. With a minute object,
however, the diffracted rays are widely spread out, so that a small aperture
can admit only a fractional part—to admit the whole or a very large part,
and consequently to see the minute structure of the object, or to see it truly,
a large aperture is necessary, and in this lies the value of aperture and of a
wide-angled immersion objective for the observation of minute structures.

Numerical Aperture.

Measure of Apertures of Objectives. N.A.—Numerical aperture, as it is
termed, is measured by the scale of measurement calculated by the late
Professor Abbe, and which has since been generally recognised and

adopted. He showed that even in lenses made for the same medium (as air)
their comparative aperture as compared with their focus was not correctly
measured by the angle of the rays grasped, but by the actual diameters of
the pencil of rays transmitted, which depend, as already seen, more upon
the back of the lens than the front. To get a geometric measure for
comparison, he took the radii, or half diameters (whose relative proportions
would be the same), and which geometrically are the sines of the semi-
angle of the outermost rays grasped. Abbe further showed that if this sine of
half the outside angle were multiplied by the refractive index of the medium
used we should have a number which would give the comparative aperture
of any lens, whatever the medium. This number, then, determines both the
numerical aperture and the resolving power of the objective.

The following table of numerical apertures shows the respective angular
pencils which they express in air, water and cedar oil, or glass.16 The first
column gives the numerical apertures from 0·20 to 1·33; the second, third,
and fourth, the air, water and oil (or balsam) angles of aperture from 23° 4′
air angle to 180° balsam angle. The theoretical resolving power in lines to
the inch is shown in the sixth column; the line E of the spectrum being
taken from about the middle of the green, the column giving “illuminating
power” being of less importance; while in using that of penetrating power, it
must be remembered that several data beside that of 1/a go to make up the
total depth of vision with the microscope.

ABRIDGED NUMERICAL APERTURE TABLE.
Limit of Resolving
Corresponding Angle Power,
(2 u) for in Lines to an Inch.
(1) (2) (3) (4) (5) (6) (7) (8) (9)
180° 122°
1·33 ... 128,225 138,989 168,907 1·769 ·752
0′ 6′
165° 120°
1·32 ... 127,261 137,944 167,637 1·742 ·758
56′ 33′
155° 117°
1·30 ... 125,333 135,854 165,097 1·690 ·769
38′ 35′

148° 114°
1·28 ... 123,405 133,764 162,557 1·638 ·781
42′ 44′
142° 111°
1·26 ... 121,477 131,674 160,017 1·588 ·794
39′ 59′
137° 109°
1·24 ... 119,548 129,584 157,477 1·538 ·806
36′ 20′
133° 106°
1·22 ... 117,620 127,494 154,937 1·488 ·820
4′ 45′
128° 104°
1·20 ... 115,692 125,404 152,397 1·440 ·833
55′ 15′
125° 101°
1·18 ... 113,764 123,314 149,857 1·392 ·847
3′ 50′
121°
1·16 ... 99° 29′ 111,835 121,224 147,317 1·346 ·862
26′
118°
1·14 ... 97° 11′ 109,907 119,134 144,777 1·300 ·877
0′
114°
1·12 ... 94° 55′ 107,979 117,044 142,237 1·254 ·893
44′
111°
1·10 ... 92° 43′ 106,051 114,954 139,698 1·210 ·909
36′
108°
1·08 ... 90° 34′ 104,123 112,864 137,158 1·166 ·926
36′
105°
1·06 ... 88° 27′ 102,195 110,774 134,618 1·124 ·943
42′
102°
1·04 ... 86° 21′ 100,266 108,684 132,078 1·082 ·962
53′
100°
1·02 ... 84° 18′ 98,338 106,593 129,538 1·040 ·980
10′
180°
1·00 97° 31′ 82° 17′ 96,410 104,503 126,998 1·000 1·000
0′
157°
0·98 94° 56′ 80° 17′ 94,482 102,413 124,458 ·960 1·020
2′
147°
0·96 92° 24′ 78° 20′ 92,554 100,323 121,918 ·922 1·042
29′
140°
0·94 89° 56′ 76° 24′ 90,625 98,223 119,378 ·884 1·064
6′

133°
0·92 87° 32′ 74° 30′ 88,697 96,143 116,838 ·846 1·087
51′
128°
0·90 85° 10′ 72° 36′ 86,769 94,053 114,298 ·810 1·111
19′
123°
0·88 82° 51′ 70° 44′ 84,841 91,963 111,758 ·774 1·136
17′
118°
0·86 80° 34′ 68° 54′ 82,913 89,873 109,218 ·740 1·163
38′
114°
0·84 78° 20′ 67° 6′ 80,984 87,783 106,678 ·706 1·190
17′
110°
0·82 76° 8′ 65° 18′ 79,056 85,693 104,138 ·672 1·220
10′
106°
0·80 73° 58′ 63° 31′ 77,128 83,603 101,598 ·640 1·250
16′
102°
0·78 71° 49′ 61° 45′ 75,200 81,513 99,058 ·608 1·282
31′
0·76 98° 56′ 69° 42′ 60° 0′ 73,272 79,423 96,518 ·578 1·316
0·74 95° 28′ 67° 37′ 58° 16′ 71,343 77,333 93,979 ·548 1·351
0·72 92° 6′ 65° 32′ 56° 32′ 69,415 75,242 91,439 ·518 1·389
0·70 88° 51′ 63° 31′ 54° 50′ 67,487 73,152 88,899 ·490 1·429
0·68 85° 41′ 61° 30′ 53° 9′ 65,559 71,062 86,359 ·462 1·471
0·66 82° 36′ 59° 30′ 51° 28′ 63,631 68,972 83,819 ·436 1·515
0·64 79° 36′ 57° 31′ 49° 48′ 61,702 66,882 81,279 ·410 1·562
0·62 76° 38′ 55° 34′ 48° 9′ 59,774 64,792 78,739 ·384 1·613
0·60 73° 44′ 53° 38′ 46° 30′ 57,846 62,702 76,199 ·360 1·667
0·58 70° 54′ 51° 42′ 44° 51′ 55,918 60,612 73,659 ·336 1·724
0·56 68° 6′ 49° 48′ 43° 14′ 53,990 58,522 71,119 ·314 1·786
0·54 65° 22′ 47° 54′ 41° 37′ 52,061 56,432 68,579 ·292 1·852
0·52 62° 40′ 46° 2′ 40° 0′ 50,133 54,342 66,039 ·270 1·923
0·50 60° 0′ 44° 10′ 38° 24′ 48,205 52,252 63,499 ·250 2·000
0·45 53° 30′ 39° 33′ 34° 27′ 43,385 47,026 57,149 ·203 2·222
0·40 47° 9′ 35° 0′ 30° 31′ 38,564 41,801 50,799 ·160 2·500
0·35 40° 58′ 30° 30′ 26° 38′ 33,744 36,576 44,449 ·123 2·857
0·30 34° 56′ 26° 4′ 22° 46′ 28,923 31,351 38,099 ·090 3·333
0·25 28° 58′ 21° 40′ 18° 56′ 24,103 26,126 31,749 ·063 4·000
0·20 23° 4′ 17° 18′ 15° 7′ 19,282 20,901 25,400 ·040 5·000

INDEX:
(1) Numerical Aperture. (n sin u = a.)
(2) Air (n = 1·00).
(3) Water (n = 1·33).
(4) Homogeneous Immersion (n = 1·52).
(5) White Light. (λ = 0·5269 μ, Line E.)
(6) Monochromatic (Blue) Light.(λ = 0·4861 μ, Line F.)
(7) Photography. (λ = 0·4000 μ, Near Line hk.)
(8) Illuminating Power (a2.)
(9) Penetrating Power (1/a.)

Abbe’s Apertometer.

Fig. 39.—Abbe’s Apertometer.

The apertometer is an auxiliary piece of apparatus invented by Abbe, for
testing the fundamental properties of objectives and determining their
numerical and angular apertures. This accessory of the microscope involves
the same principles as that of Tolles, which the late Mr. J. Mayall and
myself brought to the notice of the Royal Microscopical Society of London
in 1876. Abbe’s apertometer (Fig. 39) consists of a flat cylinder of glass,
about three inches in diameter, and half an inch thick, with a large chord cut
off, so that the portion left is somewhat more than a semicircle; the part
where the segment is cut is bevelled from above downwards, to an angle of

45°, and it will be seen that there is a small disc with an aperture in it
denoting the centre of the semicircle. To use this instrument the microscope
is placed in a vertical position, and the apertometer is placed upon the stage
with its circular part to the front and the chord to the back. Diffused light,
either from the sun or lamp, is assumed to be in front and on both sides.
Suppose the lens to be measured is a dry one-quarter inch; then with a one-
inch eye-piece having a large field, the centre disc, with its aperture on the
apertometer, is brought into focus. The eye-piece and the draw-tube are now
removed, leaving the focal arrangement undisturbed, and a lens supplied
with the apertometer is screwed into the end of the draw-tube. This lens,
with the eye-piece in the draw-tube, forms a low-power compound
microscope. This is now inserted into the body-tube, and the back lens of
the objective whose aperture we desire to measure is brought into focus. In
the image of the back lens will be seen stretched across, as it were, the
image of the circular part of the apertometer. It will appear as a bright band,
because the light which enters normally at the surface is reflected by the
bevelled part of the chord in a vertical direction, so that in reality a fan of
180° in air is formed. There are two sliding screens seen on either side of
the figure of the apertometer; they slide on the vertical circular portion of
the instrument. The images of these screens can be seen in the image of the
bright bands. These screens should now be moved so that their edges just
touch the periphery of the back lens. They act, as it were, as a diaphragm to
cut the fan and reduce it, so that its angle just equals the aperture of the
objective and no more.

This angle is now determined by the arc of glass between the screens; thus
we get an angle in glass the exact equivalent of the aperture of the
objective. As the numerical apertures of these arcs are engraved on the
apertometer, they can be read off by inspection. A difficulty is not
infrequently experienced from the fact that it is not easy to determine the
exact point at which the edge of the screen touches the periphery of the
back lens, or rather the limit of the aperture. Zeiss, to meet this difficulty,
made a change in the form of the apparatus—furnished a glass disc
mounted on a metal plate, with a slot for the purpose of its more accurate
adjustment.17

Stereoscopic Binocular Vision.

Professor Wheatstone’s remarkable discovery of stereoscopic vision led, at
no distant period, to the application of the principle to the microscope. It
may therefore prove of interest to inquire how stereoscopic binocular vision
is brought about. Indeed, the curious results obtained in the stereoscope
cannot be well understood without a previous knowledge of the
fundamental optical principles involved in this contrivance, whereby two
slightly dissimilar pictures of any object become fused into one image,
having the actual appearance of relief. The invention of the stereoscope by
Sir Charles Wheatstone, F.R.S., 1838, and improved by Brewster, was
characterised by Sir John Herschel as “one of the most curious discoveries,
and beautiful for its simplicity, in the entire range of experimental optics,”
led to a more general appreciation of the value of the conjoint use of both
eyes in conveying to the mind impressions of the relative form and position
of an object, such as the use of either eye singly does not convey with
anything like the same precision. When a near object having three
dimensions is looked at, a different perspective representation is seen with
each eye. Certain parts are seen by the right eye, the left being closed, that
are invisible to the left eye, the right being closed, and the relative positions
of the portions visible to each eye in succession differ. These two visual
impressions are simultaneously perceived by both eyes, and combined in
the brain into one image, producing the effect of perspective and relief. If
truthful right-and-left monocular pictures of an object be so presented to the
two eyes that the optic axes when directed to them shall converge at the
same angle as when directed to the object itself, a solid image will be at
once perceived. The perception of relief referred to is closely connected
with the doubleness of vision which takes place when the images on
corresponding portions of the two retina are not similar. But, if in place of
looking at the solid object itself we look with the right and left eyes
respectively at pictures of the object corresponding to those which would be
formed by it on the retina of the two eyes if it were placed at a moderate
distance in front of them, and these visual pictures brought into
coincidence, the same conception of a solid form is generated in the mind
just as if the object itself were there.

Professor Abbe, however, contended that the method by which dissimilar
images are formed in the binocular microscope differs materially from that
of ordinary stereoscopic vision, and that the pictures are united solely by the
activity of the brain, not by the prisms which ordinarily give rise to
sensations of solidity. This can be only partially true, as binocularity in the
microscope is due to difference of projection exhibited by the different
parallax displacement of the images, and also to the perception of depth
imparted by the instrument.

Wheatstone was firmly convinced that his stereoscopic principle could be
applied to the microscope, and he therefore applied first to Ross and then to
Powell to assist him in its adaptation. But whether either of these opticians
made any attempt to give effect to his wishes and suggestions is not known.
In the year 1851 Professor Riddell, of America, succeeded in constructing a
binocular microscope by employing two rectangular prisms behind the
objective. M. Nachet also constructed a binocular with two body-tubes and
a series of prisms. But neither Riddell’s nor Nachet’s instrument was ever
brought into use; they were either too complicated or too costly.

It will be understood, however, that the binocular stereoscope combines two
dissimilar pictures, while the binocular microscope simply enables the
observer to look with both eyes at images which are essentially identical.
Stereoscopic vision, to be effective, requires that the delineating pencil shall
be equally separated, so that one portion of the admitted cone of light is
conducted to one eye, and the other portion to the other eye.

Select any object lying in an inclined position, and place it in the centre of
the field of view of the microscope; then, with a card held close to the
object-glass, stop off alternately the right or left hand portion of the front
lens: it will then appear that during each alternate change certain parts of
the object will change their relative positions.

To illustrate this, Fig. 40 a, b are enlarged drawings of a portion of the egg
of the common bed-bug (Cimex lecticularis), the operculum which should
cover the opening having been forced off at the time the young was
hatched. The figures exactly represent the two positions that the inclined
orifice will occupy when the right- and left-hand portions of the object-
glass are stopped off. This object is viewed as an opaque object, and drawn

under a two-thirds object-glass of about
28° aperture. If this experiment is repeated,
by holding the card over the eye-piece, and
stopping off alternately the right and left
half of the ultimate emergent pencil,
exactly the same changes and appearances
will be observed in the object under view.
The two different images just produced are
such as are required for obtaining
stereoscopic vision. It is therefore evident
Fig. 40.—Portions of Eggs of Cimex. that if instead of bringing them confusedly
together into one eye we can separate them
so as to bring together a, b into the left and right eye, in the combined effect
of the two projections we obtain at once all that is necessary to enable us to
form a correct judgment of the solidity and distance of the several parts of
the object.

Nearly all objectives from the one inch upwards of any considerable
aperture give images of the object seen from a different point of view with
the two opposite extremes of the margin of the cone of rays; the resulting
effect is that there are a number of dissimilar perspectives of the object
blended together at one and the same time on the retina. For this reason, if
the object under view possesses bulk, a more accurate image will be
obtained by reducing the aperture of the objective.

Fig. 41.

Diagram 3, Fig. 41, represents the method employed by Mr. Wenham for
bringing the two eyes sufficiently close to each other to enable them both to
see through the double eye-piece at the same moment. a a a are rays
converging from the field lens of the eye-piece; after passing the eye-lens b,
if not intercepted, they would come to a focus at c; but they are arrested by
the inclined surfaces, d d, of two solid glass prisms. From the refraction of
the under incident surface of the prisms, the focus of the eye-piece becomes
elongated, and falls within the substance of the glass at e. The rays then
diverge, and after being reflected by the second inclined surface f, emerge
from the upper side of the prism, when their course is rendered still more
divergent, as shown by the figure. The reflecting angle given to the prisms
is 47½°, to accommodate which it is necessary to grind away the contact
edges of the prisms, as represented, otherwise they prevent the extreme
margins of the reflecting surfaces from coming into operation, which are
seldom made quite perfect.

Fig. 42.—Professor Abbe’s Stereoscopic Eye-pieces.

Fig. 42 represents a sectional view of Abbe’s stereoscopic eye-pieces,
which consist of three prisms of crown glass, a, b and b′, placed below the
field-glass of the two eye-pieces; the tube c is slipped into the tube or body
like an ordinary eye-piece. The two prisms a and b are united so as to form
a thick plate with parallel sides, inclined to the axis at an angle of 38·5°.
The cone of rays from the objective is thus divided into two parts, one being
transmitted and the other reflected; that transmitted passing through a b and
forming an image of the object in the axial eye-piece B. Adjustment for
different distances between the eyes is effected by the screw placed to the
right-hand side of the figure, which moves the eye-piece B′, together with
the prism b′, in a parallel direction. The tubes can also be drawn out, if
greater separation is required. The special feature of this instrument is that

on halving the cone of rays by turning the caps, an orthoscopic or
pseudoscopic effect is produced. This double-eyed piece arrangement of
Abbe’s has not been at all brought into use in this country; this is partly
owing to its original adaptation for use with the shorter Continental body-
tube of 160 mm., and not for our 10-inch body.

The most perfect method of securing pleasing satisfactory stereoscopic
vision of objects is that devised by Mr. Wenham. In his binocular
microscope an equal division of the cone of rays, after passing through the
objective is secured and again united in the eye-pieces, which act as one, so
that each eye is furnished with an appropriate and simultaneous view of the
object. The methods contrived by the earlier experimenters not only
materially interfered with the definition of the objective and object, but also
required expensive alterations and adaptations of the microscope, and
sometimes separate stands for their employment. Mr. Wenham’s invention,
on the contrary, offers no such obstacle to its use, and the utility of the
microscope as a monocular is in no way impaired either when using the
higher powers.

The most important improvement, then,
effected by Wenham consists in the
splitting up or dividing the pencil of rays
proceeding from the objective by the
interposition of a prism of the form shown
in Fig. 43. This is placed in the body or
tube of the microscope so as to interrupt
only one-half (a c) of the pencil, the other
half (a b) proceeding continuously to the
field-glass, eye-piece, of the principal
body. The interrupted half of the pencil on
its entrance into the prism is subjected to
very slight refraction, since its axial ray is
perpendicular to the surface it meets.
Within, the prism is subjected to two Fig. 43.
reflections at b and c, which send it forth
again obliquely on the line b towards the eye-piece of the secondary body,
to the left-hand side of the figure; and since at its emergence its axial ray is

again perpendicular to the surface of the glass, it suffers no further
refraction on passing out of the prism than on entering. By this
arrangement, the image sent to the right eye is formed by rays which have
passed through the left half of the objective; whilst the image sent to the left
eye is formed by rays which have passed through the right half, and which
have been subjective to two reflections within the prism, and passing
through two surfaces of glass. The prism is held by the ends only on the
sides of a small brass drawer, so that all the four polished surfaces are
accessible, and should slide in so far that its edge may just reach the central
line of the objective, and be drawn back against a stop, so as to clear the
aperture of the same.

Fig. 44.—Sectional view of the Wenham Binocular.

The binocular, then (Fig. 44), consists of a small prism mounted in a brass
box A, which slides into an opening immediately above the object-glass,
and reflects one-half of the rays which form an image of the object, into an
additional tube B, attached at an inclination to the ordinary body C. One half
of the rays take the usual course with their performance unaltered; and the
remainder, though reflected twice, show no loss of light or definition
worthy of notice, if the prism be well made.

As the eyes of different persons are not the same distance apart, the first and
most important point to observe in using the binocular is that each eye has a
full and clear view of the object. This is easily tried by closing each eye
alternately without moving the head, when it may be found that some
adjustment is necessary by racking out the draw-tubes D, E, of the bodies by
means of the small milled head near the eye-pieces; this will increase the
distance of the centres; and, on the contrary, the tubes, when racked down,
will suit those eyes that are nearer together.

If the prism be drawn back till stopped by the small milled head, the field of
view in the inclined body is darkened, and the rays from the whole aperture
of the object-glass pass into the main body as usual, neither the prism nor
the additional body interfering in any way with the use of the instrument as
a monocular microscope.

The prism can be withdrawn altogether for the purpose of being wiped: this
should be done frequently, and very carefully, on all four surfaces, with a
perfectly clean cambric or silk handkerchief or a piece of wash-leather; but
no hard substance must be used. During this process the small piece of
blackened cork fitted between the prism and the thick end of the brass box
may be removed; but it must be carefully replaced in the same position, as it
serves an important purpose in stopping out extraneous light.

As the binocular microscope gives a real and natural appearance to objects,
this effect is considerably increased by employing those kinds of
illumination to which the naked eye is accustomed. The most suitable are
all the opaque methods where the light is thrown down upon the surface;
but for those objects that are semi-transparent, as sections of bone or teeth,
diatomaceæ, living aquatic animalcules, &c., the dark-field illumination by
means of the parabolic reflector will give an equally good result.

For perfectly transparent illumination, it is much better to diffuse the light
by placing under the object various substances, such as tissue-paper, ground
glass, very thin porcelain, or a film of yellow bees’ wax, run between two
pieces of thin glass.

To ensure the full advantage and relief to both eyes in prolonged
observations with high as well as low powers, and with objectives of large
aperture, Mr. Wenham devised a compound prism for use with his binocular
microscope, the body tubes of which are also made expressly to suit the
prism, as extreme accuracy is necessary to bring them into proper position.
The main prism somewhat resembles in form the ordinary Wenham prism.
Over the first reflecting surface is placed a second smaller prism, the top
plane of which is parallel with the base of the first, so that direct rays pass
through without deviation, but at the two inclined surfaces of the prisms
(nearly in contact) there is a partial reflection from each, which, combined,
give as much light as in the direct tube. The reflected image from these two
surfaces is directed up into the inclined tube as usual. A somewhat later
improvement is that of Dr. Schroeder, the high power prism, by means of
which the whole of the rays emanating from the objective pass through it,
and the full aperture of any power is thereby effectively utilised.
Furthermore, Messrs. Ross have also constructed a right- and left-hand pair
of eye-pieces, which ensure greater perfection of the image. It was, in fact,
noticed that the size of the image in the left-hand field glass slightly
differed from that of the right when examined by the ordinary Huyghenian
eye-pieces. To compensate for this difference, the left-hand eye-piece has
been carefully calculated, and its focus is now so accurately adjusted that
the position of each eye in observing is brought into one plane of the
binocular. The pairs of the several series of eye-pieces A, B and C have also
been altered, and the effect is to greatly improve the image and give
increased comfort to the observer.

Dr. Carpenter, who warmly espoused the binocular, and constantly
employed it in his work, very truly said of it: “The important advantages I
find it to possess are in penetrating power, or focal depth, which is in every
way superior to that of the monocular microscope, so that an object whose
surface presents considerable inequalities is very much more distinctly seen
with the former than with the latter.”

This difference may in part be attributed to the practical modification in the
angle of aperture of the objective, produced by the division of the cone of
rays transmitted through the two halves, so that the picture or image
received through each half of the objective of 60° is formed by rays
diverging at an angle of only about 30°. He confesses, however, that this
does not satisfactorily explain the fact that the binocular brings to the mind’s
eye the solid image of the object, and thus gives to the observer a good idea
of its form and which could hardly be obtained by the monocular
microscope. Carpenter cites in support of his views the wing of a little-
known moth, Zenzera Œsculi, which has an undulating surface, whereon
the scales are set at various angles instead of having the usual imbricated
arrangement, a good object for demonstrating; the general inequality of
surface and the obliquity of its scales, which are at once seen by the
binocular with a completeness not obtained by the monocular instrument.

To one unaccustomed to work with the binocular the views expressed by
Dr. Carpenter as to the extreme value of the instrument for ordinary work
may appear somewhat exaggerated, but from my own experience, having
long had in constant use a Ross-Zentmayer binocular, furnished with a
special prism, constructed for working with a 1⁄8 dry objective or a 1⁄10
immersion, the perfection of picture obtained was in every case quite equal
to that of the monocular microscope. The relief to the eyes can hardly be
over-estimated; the slight inequality of the pencil rays may be regarded
rather as a part of the welcome rest afforded when a prolonged examination
is made; it certainly appears to me to equalise the slight physiological
difference known to exist between the eyes of most people. If one image is
seen a little clearer by the stronger eye, the weaker eye assists rather more
the stereoscopic effect of the object under observation. The advantage
gained by the binocular is perhaps more appreciated when opaque objects
are under examination, as the eggs of insects, and the tongue of the blow-
fly, specimens of mosses, lichens, parasites (vegetable and animal), whose
planes and inequalities of surface require penetration, and which usually
demand more time for their observation.

Fig. 45.—Swift-Stephenson’s Erecting Binocular.

No variation or change of any kind proposed either in the form of the
instrument or the prism has proved of sufficient value or importance to
bring it into use, and therefore Wenham’s instrument is scarcely likely to be
superseded. It must be admitted that the improvement effected in the eye-
piece form by Mr. Tolles, of Boston, U.S., is an exception to the rule laid
down. It consists in mounting the prisms in a light material, vulcanite, made
to fit into the monocular microscope body, thus taking the place of the
ordinary eye-piece. The image transmitted by the objective is brought to a
focus on the face of the first equilateral triangular prism by the intervention
of an erector-eye-piece inserted beneath it. The second set of prisms have a
rack and pinion movement to adjust them to any visual angle. The
illumination of both fields in this eye-piece is nearly equal in brightness.

Mr. Stephenson’s erecting binocular (Fig. 45) has proved to be of some
practical value. It has the advantage of being of equal use with high and low
powers, and with little loss of definition. When used for dissecting purposes
it gives an erect image of the object. It is equally useful as a working
microscope, for arranging diatoms and botanical specimens of every kind.
The sub-stage tube will receive a diaphragm or illuminating apparatus; the
eye-pieces have a sliding adjustment for regulating the widths between
eyes.

Fig. 46.—An early form of the Ross-Wenham Binocular; nose-piece and prism-
holder detached.

CHAPTER II.

Simple and Compound Microscope.

Microscopes are known as simple and compound. The simple microscope
may, for convenience, be divided into two classes; those used in the hand
(hand magnifiers), and those provided with a stand (mounted, as it is
termed) for supporting the object to be viewed, together with an adjustment
for the magnifying power, and a mirror for reflecting the light through the
object.

A simple microscope, mounted, is
preferable to a single lens, being
usually composed of two or more
lenses separated by a small distance
on a common axis; the increase of
the size of an object being the angle
it subtends to the eye of the observer,
or the angle formed by the
combination drawn from the axis of
vision to the extremity of the object,
as in Fig. 47. The lines drawn from
the eye to a and r form an angle,
Fig. 47.—Visual Angle. which, when the distance is small, is
nearly twice as large as the angle
from the eye to o w, formed by lines drawn at twice the distance. This is
called the angle of vision, or the visual angle. Now, the utility of a convex
lens interposed between a near object and the eye consists in its reducing
the divergence of the rays forming the several pencils issuing from it, so
that they enter the eye in a state of moderate divergence, as if they were
issuing from an object beyond the near point of distinct vision, and a well-
defined image is thereby formed upon the retina. In the next Fig. (48), a

double-convex lens illustrates the action of the simple microscope, the small
arrow being the object brought under view, and the large arrow the
magnified image. The rays having first passed through the lens are bent into
nearly parallel lines, or pencils diverging from some point within the limits
of distinct vision. Thus altered, the eye receives rays precisely as if they had
emanated directly from a larger arrow placed about ten inches away from it.
The difference between the real and the imaginary object represents the
magnifying power of the lens. The object in this case is magnified nearly in
the proportion the focal distance of the lens bears to the distance of the
object when viewed by the unassisted eye; and this is due to the object
being more distinctly viewed so much nearer to the eye than it otherwise
could be without the lens.18

Fig. 48.—Virtual Image formed by Convex Lens.

It should be remembered that the shorter the focus and the nearer the eye
the magnifying lens is placed the smaller will be the diameter of the sphere
of which it forms a part, and unless its aperture be proportionally reduced,
the distinctness of the image will be destroyed by the spherical and
chromatic aberrations of its high curvature. Nevertheless, it was by the use
of lenses so constructed that the older microscopists—of whom

Leeuwenhoek was the more eminent—were enabled to do so much
excellent work.

The various kinds of simple pocket lenses for the most part consist of a
double-convex, or a plano-convex, or a combination of both, varying in
focal length from a quarter of an inch to two inches. Sometimes they are set
in pairs with a hole, a small diaphragm, cut in the piece of horn placed
between them. These are extremely useful for carrying in the waistcoat
pocket; to the anatomist and field botanist for examining various objects
and preparations.

Fig. 49.—Wollaston’s Doublet.

Perhaps the most important improvement effected in this form of the simple
microscope was that ascribed to the celebrated Dr. Wollaston, who devised

a doublet of two plano-convex lenses having their focal lengths, in the
proportion of one to three, mounted with their convex side directed towards
the eye of the observer, and the lens of shorter focal length next the object.
The explanation given of the correction thus effected in Dr. Wollaston’s
doublet will be best understood on reference to the annexed diagram, l l′, in
Fig. 49, being the object for a segment of the cornea of the eye, and d d′ the
stop or diaphragm. Now, it will be seen that each pencil of light proceeding
from l l′, the object, is rendered excentrical by the limiting aperture or the
diaphragm d d; consequently, they pass through the lenses on opposite sides
of their common axis o p; thus each becomes affected by opposite errors,
which to some extent balance and correct each other. To take the pencil l,
for instance, as it enters the eye at r b; r b is bent to the right at the first
lens, and to the left at the second; and as each bending alters the direction of
the blue ray more than the red, and as the blue ray falls nearer the margin of
the second lens, where the refraction is greater than that nearer the centre,
and compensates to some extent for the greater focal length of the second
lens, the blue rays will emerge very nearly parallel, and colourless to the
eye. At the same time, its spherical aberration has been diminished, since
the side of the pencil as it proceeds through one lens passes nearer the axis,
and in the other nearer the margin.

This must be taken to apply to pencils farthest from the centre of the object.
Central rays, it is obvious, would pass both lenses symmetrically, the same
portions of rays occupying nearly the same relative places in both lenses.
The blue ray would enter the second lens nearer its axis than the red; and
being thus less refracted than the red by the second lens, some amount of
compensation would take place, differing in principle, and inferior in
degree, to that which is found in the excentrical pencils. In the intermediate
spaces the corrections are still more imperfect and uncertain; and this
explains the cause of aberrations which must of necessity exist even in the
best-made doublet. It is, however, infinitely superior to a single lens, and
will transmit a pencil of an angle of from 35° to 50°.

The next step towards improving the simple microscope was in relation to
the eye-piece, and was effected by Holland. It consisted in substituting two
lenses for the first in the doublet, and placing a stop between them and the
third. The first bending of the pencils of light being effected by two lenses

instead of one, produces less spherical and chromatic aberration, which are
more nearly balanced or corrected at the second bending, and in the
opposite direction, by the third lens.

Another form of simple lens was devised by Dr. Wollaston, the
“Periscopic.” This combination consists of two hemispherical lenses
cemented together by their plane faces, with a stop between them to limit
the aperture. A similar proposal, made by Sir David Brewster in 1820, is
known as the Coddington lens,19 shown at Fig. 50: this has a somewhat
larger field, and is equally balanced in all directions, as is made evident, the
pencils a b and b a passing through under precisely the same circumstances.
Its spherical form has the further advantage of rendering the position in
which it is held of comparatively little consequence. It is still used as a hand
magnifier, although its definition is certainly not so good as that of a well-
made doublet. It is usually set in a folding case, as represented in the figure,
and so contrived as to be admirably adapted for the waistcoat-pocket. It is
usually sold with the small holder, Fig. 50a, for holding and securing small
objects during examination. Browning’s Platyscopic Pocket Lens is a useful
form of pocket lens for the botanist and mineralogist. Its focus is nearly
three times longer than that of the Coddington, and allows of opaque
objects being more easily examined; it has also a magnifying power of 15,
20, and 30 diameters.

Fig. 50.—The Coddington Lens. Fig. 50a.

One of the best combinations of the hand or
pocket form of lens is that known as Steinheil’s
aplanatic lens (Fig. 51); it consists of a bi-
convex lens cemented between two concavo-
convex lenses, giving a relatively long focal
distance and a large flat field. The higher
powers of this lens are much used for
dissecting purposes. This handy magnifier Fig. 51.—Steinheil’s Aplanatic Lens.
appears to have suggested a later combination,
the apochromatic of Zeiss. No hand lens can compare with Steinheil’s
“loups.”

Fig. 52.—Simple Microscope.

When the magnifying power of a lens is considerable, or when its focal
length is short, or it is wished to use it with greater precision and steadiness,
it should be mounted on a short stand with a tubular stem, with rack-work
focussing movement and mirror illumination. Fig. 52 represents a simple
dissecting microscope, with a glass circular stage, 4½ inches in diameter,
supported on three legs—a handy and useful form of instrument for many
purposes.

The Compound Microscope.

The compound microscope differs from the simple, inasmuch as the image
is formed by an object-glass, and further magnified by one or more lenses
forming an eye-glass. For a microscope to be a compound one, its essential
qualification is that it should have an object-glass or objective, and an eye-
glass or eye-piece, so called because they are respectively near the object
and the eye of the observer when the instrument is in use. The microscope

consists of a tube or body, and a stand, an arrangement for carrying the
body, combined with which is a stage for holding the object, and a mirror
for its illumination. To the more modern instrument has been added a
substage, to carry a condenser and other accessories.

The body of a microscope, which carries the system of magnifying lenses,
must be placed at one particular distance from the object, termed the focus,
in order that a clear image may be obtained. For the purpose of focussing
two motions are supplied, the one for coarse adjustment, with lower
powers; the other for higher powers, termed the fine adjustment. It is in this
wise that the magnifying power of the compound microscope is turned to
good account.

There are, however, limits to the use to which lenses can be put with
advantage in the direction of magnifying the object, just as there are in
varying the magnifying power of the eye-glass. Defects in either, although
not first seen, that is, when the image is but moderately enlarged, are
brought into prominence by greater amplification. In practice, therefore, it
is found to be of advantage to vary the power by employing object-glasses
of different values (foci). In whatever way increase of amplification is
brought about, two things will always result from the change: the
proportion of surface of the object of which an image can be formed must
be diminished, and the amount of light spread over the image proportionally
lessened.

In addition to the two lenses mentioned, it was found to be of considerable
advantage to introduce a third lens between the object-glass and the image
formed by it at eye-piece, the purport of which is to change the course of
the rays (bend in the pencil) so that the image may not be found of too great
a dimension for the whole to be brought within the circumference of the
eye-glass. This, it will be readily seen, allows more of the object to be
viewed at the same time by the field-glass, as the eye-piece of the
microscope is termed.

Fig. 53 represents the body of an ordinary compound microscope with its
triplet object-glasses; o is an object, above it is the triple achromatic object-
glass, in connection with the eye-piece e e, f f the plano-convex lenses; e e
being the eye-glass, and f f the field-glass, between which, at b b, the arrow

represents the diaphragm. The course of the light is shown by
three rays drawn from the centre, and three from each end of
the object o; these rays, if not prevented by the lens f f, and
the diaphragm b b, would form an image at a a; but here, as
they meet with the lens f f in their passage, are converged by it
at b b, the diaphragm at b b intercepting a portion of
peripheral rays, permitting only those to pass that are
necessary for the formation of the image, the further
magnification of which is, however, here brought about by the
eye-glass e e, precisely as if it were that of the original object
under examination. It will be apparent, then, that the field-
lens f f belongs in principle to the object-glass, or objective,
taking a share in the image-forming rays, although this is
taken to be a part of the eye-piece.

Fig. 53.

Evolution of the Modern Achromatic Microscope.

The great advances made in the optical arrangements of the modern
microscope necessitated important changes and improvements in its several
mechanical parts. Indeed, as the apertures of objectives became increased,
and focal planes became correspondingly shallower, it was absolutely
necessary to apply a more sensitive system of focussing than that for many
years past commonly in use. The leading manufacturers at once grasped the
situation, and in a short space of time the older model microscopes were
discarded, and replaced by instruments better in workmanship and finish,
and in every way more suitable for the student and the promotion of
original scientific research.

From an early period English amateurs appear to have bestowed greater
attention on the improvement of the microscope than those of any other
country. Between 1820 and 1835 Tully, Pritchard, Dolland, James Smith,
Andrew Ross, and Hugh Powell, encouraged by Wollaston, Brewster,
Goring, Herschel, and Lister, worked out innumerable combinations of

single and compound lenses to be employed as simple microscopes,
explained in a previous chapter.

The theories propounded about this time for the improvement of lenses and
the various combinations for amateurs were not of lasting value.
Nevertheless, they were not wholly made in vain, as during the last twenty
years they have indirectly borne good fruit, inasmuch as by working in
another direction Professor Abbe was led to the discovery of new and better
kinds of glass, by which the secondary spectrum has been so nearly
eliminated, and the optical parts of the microscope so materially improved.
In pursuing this subject I would not have it supposed that Continental
opticians were either idle or supine. On the contrary, Oberhäuser,
Fraunhofer, Chevalier, Nachet, Hartnach, and others took an active part in
the work.

The compound microscope made for anatomists by the first-named optician
about 1825 has not been entirely superseded. He was the first to make a
rotating stage, to apply mechanism to focussing, and to introduce the
system of direct push or pull of the condenser tube within the sub-stage
socket. Nachet made other improvements on the Oberhäuser microscope by
applying under the stage a tail-piece having a dove-tailed groove in which a
slide carrying the sub-stage was moved by a stud-pin. More recently the
lever movement was superseded by American opticians, who made other
changes. Hartnach ultimately very much improved Oberhäuser’s model, and
this remains with us.

The English modern compound microscope, together with the achromatic
objective, we owe to a mind teeming with scientific inventions, Joseph
Jackson Lister, F.R.S., who in 1826 supplied Mr. Tully, a well-known
London optician of that period, with original drawings for the important
improvements in its mechanical details and accessory apparatus which
followed so soon afterwards.

Among the many ingenious novelties enumerated in his published papers
we find the graduated lengthening of the body-tube of the microscope; a
stage-fitting for clamping and rotating the object; a subsidiary stage; a dark-
well, and a large disc to incline and rotate opaque objects; a ground-glass
light moderator; a live-box with bevelled flat-glass plate; an erector-eye-

piece; an adapter for using Wollaston’s camera lucida for microscopical
drawing; and, above all, a combination of lenses to act as a condenser under
the object (evidently the first approach to the present achromatic sub-stage
condenser). The value of the erector-eye-piece for facilitating dissections
under the microscope is not even yet sufficiently appreciated. Tully
published a descriptive account of Lister’s microscope, the first one of
which he made, and acknowledged his indebtedness to “Mr. Lister’s
ingenuity and skill.” Shortly afterwards Lister made known his discovery of
the two aplanatic foci in a double achromatic object glass, and gave verbal
directions to the three principal makers of microscopes in London, James
Smith, Andrew Ross, and Hugh Powell, for the future construction of the
achromatic objective, all of whom were intent on the improvement of their
several models. To the latter the Society of Arts awarded, in 1832, a medal
for his improved mechanical stage movements, on the “Turrell system,”
which Powell first constructed for Edmund Turrell. This stage was made to
rotate completely on its optic axis by means of an obliquely-placed pinion
acting on a bevelled rack on the inner face of the stage-ring supporting the
mechanism. In 1834 Powell once more received a Society of Arts medal,
“the Iris,” for improvements in the application of a new form of fine
adjustment.

About the same date (1835) Andrew Ross introduced the socket-carrier of
the body-tube of the microscope on a strong stem, with rack bent in the
middle, thus affording space for a larger stage. He likewise devised the
hollow cross-bar, placed at right angles to the rack-stem, whereby he was
enabled to use a new system of fine adjustment, consisting of a delicate
screw with large milled head, acting by a point on the long arm of a lever,
the short arm of which ends in a fork in contact with a stud placed on either
side of a cylindrical sliding tube forming the nose-piece of the body-tube,
and into which the objective is screwed. A spiral spring presses down the
nose-piece, and against this the screw and lever act.

This appears to have been the first really sensitive focussing method applied
to the nose-piece; it was, and probably is, one of the most delicate systems
ever applied to the microscope. It has enjoyed a long period of popularity,
and I believe it still survives in Powell and Lealand’s instruments, which are

very generally admitted to be of superior excellence for all purposes where
extreme delicacy of focussing is an essential element.

The rival system of fine adjustment—the short lever and screw applied
externally to the body-tube—known as the Lister-Jackson system, which
appears to have been contrived to allow the body-tube to be supported more
firmly on the limb or stem, has had its merits ably realised in the
microscopes of Smith and Beck and their successors, but, except as
modified by the successors of Andrew Ross (Schrœder’s form), it is, I
believe, admitted that it has been superseded by other modifications lately
introduced into the Ross-Jackson instrument.

The year 1830 was, however, a propitious period in the history of the
modern microscope, as in January of that year Mr. Lister published his
epoch-making paper, “On the Improvement of the Achromatic
Microscope.” This appeared together with certain personal practical
directions (for no man was ever more anxious to communicate his
knowledge than Mr. Lister) to the before-mentioned opticians, which led up
to changes lasting until 1840, when, by the efforts of this gentleman and his
personal friends, “The Microscopical Society of London” came into
existence. Among the more prominent members of the Society was Mr.
George Jackson, a name still well known to microscopists, and who, jointly
with Mr. Lister, gave us the Jackson-Lister form of microscope. This was
forthwith accepted as a perfect model. Soon after Andrew Ross effected a
further change in the instrument, shown in Fig. 54 in its complete form as
left by this optician. It is here represented as having a bar movement, with a
claw foot bolted to two uprights to carry the trunnions with the body and
stage. This base, is insufficiently wide and extended to carry so large an
instrument with its centre of gravity so high. The coarse adjustment bar also
was rectangular, and the fine adjustment a lever, with the milled head in the
middle of the bar, which involved a certain amount of tremor; withal it was
an instrument of excellent workmanship, and its defects were not regarded
as irremediable. Messrs. Ross, however, preferred to construct an entirely
new model designed by Zentmayer, the “Ross-Jackson-Zentmayer,” to
which I shall refer presently. A later model, however, has to some extent
taken its place, “the Histological and Bacteriological Microscope,” Fig. 55.

Fig. 54.—An early Ross-Jackson Microscope.

My reference to the older form of instrument is chiefly with the view of
directing attention to the sensitive focussing system, applied in the first
instance to the nose-piece; now placed below the coarse adjustment. It
certainly is a delicate form of fine adjustment. This model possesses other
points of interest well worth preserving, which fully entitle it to occupy the
prominent place given in the list of the house of Ross. In the Ross-Jackson
“Histological and Bacteriological Microscope” much attention seems to
have been given to eliminate certain weak points in the earlier Ross-Jackson
model—defects still extant in stands of certain English and foreign makers
—while retaining the more practical improvements of both constructions.
Steadiness is secured by an extension of the tripod or claw-foot and the

shorter and more solid uprights that sustain the whole weight of the
instrument.

Fig. 55.—The Ross-Jackson Histological Microscope.

Fig. 56.—Powell and Lealand’s Students’ Microscope, with Amici prism arranged
for oblique illumination, the Sub-stage and Condenser being detached.

The Ross-Jackson, then, survives, together with the original tripod stand of
Hugh Powell’s, upon which he expended all the resources of the practical
optician, and applied the early principles involved in the Lister-Jackson
instrument, but from different points of view. However, there is hardly a
choice between one and the other in workmanship, both opticians having
furnished microscopes of a typical class and very high order. The firm of
Powell and Lealand have but one form of stand, from which they have
never been tempted to deviate. It is supported on a true tripod base, forming
a solid and substantial support to the body, which is of such a length as to
give as nearly as possible the standard optical interval of 10 inches between
the posterior principal focus of the objective and the anterior focus of the
eye-piece; the variation in the optical tube length does not exceed a quarter

of an inch with objectives of ½ inch and upwards. The arm on which the
body is fixed is 5¾ inches long, which not only gives a clearance of 3½
inches from the optic axis, but also permits of the introduction of a long
fine-adjustment lever.

Fig. 57.—Powell’s larger No. 2 Instrument.

Fig. 58.—Powell and Lealand’s Students’ Microscope arranged for direct
illumination. A. Secondary or Sub-stage racked up to bring the Achromatic
Condenser close to the object.

The cross arm encloses the lever mechanism for the fine adjustment, as
originally devised by Andrew Ross. This cross arm is longer than that used
by Ross, and carries the body more forward, so as to provide radial space
for the complete rotation of the stage and the optic axis, and at the same
time the lever of the adjustment is lengthened, and delicacy of motion
secured. The stage retains the mechanical movements invented by E.
Turrell, and first applied by Hugh Powell. It also rotates completely by
means of an obliquely placed pinion acting on a bevelled rack on the inner
face of the stage-ring supporting the mechanism. Finders are engraved on
the plates, and the main support of the stage-ring is graduated for angle
measuring, a pointer on the ring marking the unit of motion in arc.

The sub-stage is carried by rack-work, and has rectangular centring
movements, supporting an inner socket that can be rotated by rack and
pinion, and which carries the several sub-stage accessories. A fine
adjustment, by screw-cone and stud, is applied by means of an extra slide.

The stage is attached to the sheath of the stem by a special arrangement of
screws, by which the rotation in the optic axis can be centred; sliding spring
clips and a movable and a removable and adjustable angle-piece to hold the
slides are applied on the upper surface. The body-tube is pivoted to move
laterally on the top of the stem, and an adjustable steel stud beneath serves
to stop the movement in the axis. Such is Powell’s present instrument, and it
represents the results of sixty years’ steady devotion to secure perfection,
and at the same time embody the best ideas of mechanical design by
Andrew Ross.

A cheaper form of students’ microscope is furnished by Powell and
Lealand, with ¾-inch stage movement, coarse and fine adjustments to body,
plane and concave mirrors, revolving diaphragm, two eye-pieces, and
Lister’s dark wells. These makers also adopt a gauge of tubing, the size
being such that it will take in a binocular body, a Huyghenian 2 inch eye-
piece having the largest field-glass possible. The tube of the sub-stage is the
same size, so as to secure one gauge of tubing throughout. This allows of a
Kellner or other eye-piece to be used as a condenser.

Ross’s Microscopes.

Messrs. Ross have more recently introduced several changes and
modifications in the Zentmayer stand, all tending to improve it, so that the
Ross-Zentmayer model takes its place as a first-class microscope.

Messrs. Ross have lately manufactured other forms of microscopes; one
especially designed for those commencing the study of bacteriology (Fig.
59). This instrument is one of the steadiest among those lately constructed
for high-class work. The circular foot and short stout pillar support the
whole instrument, and a substantial knee-joint sustains the full weight in the
upright or inclined positions, while the centre of gravity is by no means
disturbed, and absolute steadiness secured. The stage is of the horse-shoe
form, which affords convenient space for the fingers to lift the slide up
while the oil is placed in contact with the objective. The fine adjustment is
extremely sensitive, working smoothly and direct; this is entirely covered,
to prevent injury by dust. The micrometer screw works directly in the centre

of its fittings, the milled head being divided to read to 1⁄500 of an inch. The
sub-stage is fitted with a new centring coarse and fine adjustment, so that
when using high powers with the Abbe condenser accurate focus can be
secured with the least amount of trouble.

Fig. 59.—Ross’s “Bacteriological and Histological” Microscope.

The amount of activity shown during the last few years by opticians in the
manufacture of new forms of microscopes renders it somewhat difficult to
keep pace with improvements, some of which are novel. A further source of
congratulation is that economy has all along been studied; so much so, that
the instruments in question are within the reach of persons of moderate
means. Messrs. Ross and Co. have taken a new departure in this respect,
and their “Eclipse” Microscope is an entirely new form of stand with a ring
foot. This microscope has been produced for the especial use of students,

and can be purchased for a moderate sum. It will be seen at a glance (Fig.
60) how steady this form of stand must necessarily be, since the centre of
gravity is secured in every direction and inclination. The body-tube carries
eye-pieces, numbered, of the Continental size and optical tube-length (160
mm.), for which the object glasses are adjusted, and a draw-tube extending
to eight inches.

The fine adjustment is independent
of set screws, and not subject to
derangement. It is extremely
sensitive and direct in action, and
from its construction is equal in
perfection of working to the best that
can be made. Its fitting, by a new
contrivance, is completely covered at
all points, being thus preserved from
disturbance or injury by dust.

The Eclipse is furnished with two
eye-pieces, 1′′ and ¼′′ object glasses
of highest excellence and large
angular aperture, both adjusted to a
double nose-piece, so that they focus
in the same plane; and a swinging
mirror and stage iris diaphragm.

In “Wenham’s Radial” Microscope
the chief aim has been directed
towards providing a very
considerable range of effects, both in
Fig. 60.—Ross’s Rigid Pattern “Eclipse”
Microscope. altitude and azimuth. The leading
principle followed throughout in the
construction of this form of stand is that of facilitating the work of the
microscopist and of obtaining the maximum range of oblique illumination
in all directions. This is fairly well attained by causing all the movements of
inclination and rotation to radiate from the object as a common centre. Thus
it has been found possible to combine seven radial motions, so that when

the instrument is inclined backwards, as in Fig. 61, or placed in the
horizontal, as in Fig. 62 or rotated from in the brass plate, a pencil of light
from a fixed source shall always reach the object and pass to the objective.
The stage is made to rotate completely, and its rectangular motions are
effected by milled heads acting entirely within the circumference. The sub-
stage is mounted on the Zentmayer system, with two centring screws, by
means of which the optic axis is secured. It is also provided with
rectangular and rotating motions. The coarse adjustment is that of the Ross-
Jackson form—a spiral pinion and diagonal rackwork, while the fine is on
an entirely new principle designed by Dr. H. Schrœder.

The “Ross-Zentmayer Microscope”
is a thoroughly substantial and
practical instrument, combining
elegance of appearance with strength
and firmness.

It is a true tripod model, consisting
of a triangular base with two pillars
rising from a cross-piece, which
carries the trunnions. The slow
movement is obtained by a second
slide close behind the first; but to
avoid the friction of rubbing
surfaces, hardened steel rollers are
inserted between them, which give a Fig. 61.—Ross’s Wenham Radial Microscope.
frictionless fine motion, amenable to
the slightest touch of the milled-head
screw situated conveniently at the
back of the limb, through which a
steel lever passes which actuates the
slow motion slide. The body of the
instrument is therefore not touched
during the fine focussing, so that all
lateral movement is avoided. The
mechanical stage rotates axially, and
the outer edge of the lower plate is

divided into degrees, in order to Fig. 62.—The Ross-Wenham Radial Microscope.
register the angles; a simple mode of
adjustment is provided for setting the centre of rotation exactly coincident
with the focal point of the objective. As the plates of the stage have no
screw or rackwork between them (these are placed externally), they are
brought close together, thus affording the advantage of a thin substantial
stage, and ensuring rigidity where most required; phosphor-bronze being
used in its construction. The stage is attached to the limb by a conical stem,
with a screw and clamp nut at the back, so that it can be easily removed for
the substitution of a simple plate or other stage; by turning the stem in the
socket the stage may be tilted sideways at any angle required. A feature in
the Ross-Zentmayer stand is the swinging sub-stage and bar carrying the
mirror, having its axis of rotation situated from an axial point in the plane of
the object, which consequently receives the light without requiring
alteration of focus in any position of the bar; by this means facilities are
afforded for the resolution of objects requiring oblique light and for the
development of their structure. Rays are thus obtained from any angle and
indicated by the graduated circle round the top of the swing-bar, and many
troublesome and expensive pieces of sub-stage apparatus dispensed with.
The value of this arrangement was long ago recognised in Grubb’s “Sector
Stand,” the movement of which was obtained in a far less efficient manner.

Fig. 63.—The Improved Ross-Zentmayer Model.

The base or foot of the Ross-Zentmayer instrument is made in one piece.
Preference must be given to the double pillar support, as this is firmer, and
allows the sub-stage to swing free while the microscope is in a vertical
position, as in working with fluid preparations. The sub-stage is provided
with screws for centring, and, when determined, secured by a clamping
screw.

The sub-stage, with its apparatus in place, can be instantly removed, by
being drawn out sideways, so as to use the mirror alone, which is a great
convenience.

The mechanical movements of this instrument are perfect, and well adapted
to their purpose.

Messrs. Ross have other typical forms of microscopes. Their “New
Industrial” Microscope, for the use of farmers, horticulturists, textile and
other trades, for the examination of produce and raw materials, is a
surprisingly cheap one, and deserving of commendation. The great utility of
microscopical research to purposes of advanced agriculture is fully
recognised, and a less costly instrument than that usually supplied for more
complex investigations was much needed. It is provided with a broad
square stage for the purpose of receiving a glass dish to contain liquids or
manifold objects, and which may be moved on the stage to bring the
various particles under observation. A fitting beneath the stage carries a
plate with diaphragm apertures for modifying the light, and as seeds, textile
fibres, and other opaque objects form a large portion of those to be
examined, this sub-stage plate has a space between the perforations which,
when brought into position, provides a dark ground by preventing the
passage of light from underneath. A condensing lens is, however, provided
for the better lighting of opaque objects. Here we have a microscope which
combines efficiency with stability, while its very simplification allows of a
really good and effective instrument for the small sum of £3 3s.

Fig. 64.—Ross’s “New Industrial” Microscope.

Messrs. Beck’s Microscopes.

Messrs. Beck have adopted what may be termed a rival system of fine
adjustment in their modern microscopes. The short lever and screw applied
externally to the body tube is peculiar, I may say, to the Ross-Jackson
system, and was originally devised to allow of the body tube being
supported somewhat more firmly on the limb. This change had its merits
fully realised in the early microscopes of Smith and Beck. To their
successors, R. & J. Beck, the microscope owes much, and very many
important improvements, while all their instruments and accessories are
excellent examples of good workmanship and finish. In their Pathological

Microscope we have a movement originally found in Tolles’ microscopes: a
vertical disc, by which the centre can be raised or depressed to correspond
with the thickness of the slide. The stage can also be brought into an
inverted position by rack and pinion. Their fine adjustment has been greatly
improved, as we shall presently see, whereby it has been made more
sensitive and delicate of adjustment. The general construction of their
microscopes as a rule possess the following advantages: the stands are
strong, firm, and yet not too light or too heavy, the instruments cannot alter
from the position in which they are placed, as, unfortunately, will
occasionally happen when joints work loose; in every position the heavier
part of the stand maintains the centre of gravity.

Beck’s Pathological Microscope (Fig. 65) is a nearly perfect instrument,
furnished with a firm triangular foot, which ensures great steadiness in any
position. It has a well adapted joint for placing the instrument at any angle
of inclination; coarse adjustment by spiral rack and pinion; fine adjustment
by delicate lever and micrometer screw motion; rack and pinion focussing
and screw centring sub-stage, made to carry all condensers and other sub-
stage apparatus; mechanical stage with horizontal and vertical traversing
motions. The stage is attached to the instrument by two screws and can
therefore be removed at pleasure, leaving a large square flat glass stage for
the culture-plate. It is likewise provided with finder divisions, and as it
always fits on to the same place, any particular portion of the object can be
recorded and found at any moment. The triple nose-piece is a convenient
addition, and a very acceptable one to the student while diligently engaged
in histological research.

Fig. 65.—Beck’s Pathological Microscope, with square and removable stage.

Fig. 66.—Beck’s Large “Continental Model” Microscope.

Beck’s Large “Continental Model” Microscope is of superior finish. It is
provided with a substantial horse-shoe foot, which gives support to the
strong, well-balanced body, jointed for giving the microscope any angle of
inclination. The body is provided with a draw-tube which can be racked
down to the Continental measurement. It has a spiral rack and pinion coarse
adjustment, and a fine adjustment of the most perfect workmanship, which
will be described in detail presently. It has a large square stage with
vulcanite top plate to receive culture preparations. The sub-stage is of the
most approved form for centring, and carries an achromatic or Abbe
condenser, iris diaphragm, &c. The double mirror can be swung out of place
for direct illumination and micro-photography. Altogether, this instrument is
in every way fitted for critical or class-room work.

To return to the fine adjustment of this, as
of other forms of Messrs. Beck’s
microscopes, the applied mechanism of
which is believed to be one of the most
sensitive and delicate character yet
contrived. It is constructed as shown in the
accompanying figure. The body of the
instrument is supported upon the barrel D
D; this barrel is accurately and smoothly
fitted to the triangular core E E. At the top
of barrel D D is screwed the cap G, to
which is attached the rod C; this rod passes
through the centre of the core E E and
connects with the lever arm A at B. The
action of the spring J, which is wrapped
spirally around the rod C, raises the body
of the microscope and holds the lever arm
A tightly against the screw arm F. The
slightest motion, therefore, of the screw F
is communicated through the lever A and
the rod C to the body of the microscope.

Fig. 67.—Beck’s “New Fine Adjustment.” The great delicacy of this arrangement will
be appreciated when it is noticed that the
distance from I H is double the distance of I B, therefore any motion at B is
only half that at H. This adjustment is one of the most delicate made for use
with high powers.

Fig. 68.—Beck’s National Binocular Microscope.

In the construction of Beck’s Binocular National Microscope, the body is
held in a sliding fitting in the limb, and is moved up or down by means of a
rack and pinion motion. This constitutes the coarse focussing adjustment.
The fine adjustment is effected by the milled head, which acts upon the
body by means of a lever inside the limb. The upper circular surface of the
stage is made of glass, and carries the object holder, which is provided with
a ledge and spring to hold the object by means of the pressure of an ivory-
tipped screw, so that it can be moved about readily and smoothly. The
pressure of the screw is adjusted by the milled head, which permits of more
or less pressure being made upon the edge of the object.

Fig. 69.—Beck’s Star Microscope.

When the stage is required for other purposes the object holder can be
unscrewed and removed. Beneath the stage there is a cylindrical fitting for
the reception of a diaphragm, a polariser, or other apparatus. The mirror,
besides swinging in a rotatory semi-circle, is made to slide up or down the
stem. The microscope is supported by a firm pillar on a tripod base, and the
body can be inclined at any angle convenient for working. A sub-stage can
be added at any time for the reception of an achromatic condenser fitted
with concentric screws—a necessity for more delicate microscopical
research work.

Beck’s Star Microscope is in every sense a students’ or class-room
instrument. It is firm and well made, with joint for inclination, large square
stage, sliding coarse adjustment and fine adjustment by micrometer screw,

draw-tube, iris diaphragm, double mirror on swinging crank arm, A or B
eye-piece, a one-inch and quarter-inch objective, the magnifying power of
which ranges from 38·5 to 183.

Fig. 70.—Beck’s Binocular Dissecting Microscope.

An early binocular microscope for dissecting purposes was devised by the
late Mr. R. Beck. (Fig. 70.) This took the form of a simple instrument built
up on a square mahogany base A raised about four inches upon four brass
supports B B, having a large circular stage plate made to revolve on a
second plate, on which the object is placed and brought under the eye for
dissection. On the left hand side is a milled head rack and pinion K, which
acts upon a horizontal bar I for focussing the magnifying lens. Another bar,
R, carries the prism P and a pair of eye-pieces arranged on the principle of
M. Nachet’s binocular microscope. Mr. Beck preferred to adopt Wenham’s
method of arranging these prisms; that is, by allowing half the cone of rays
to proceed to one eye without interruption, while the other half is

intercepted by the prisms and transmitted to the other eye. Beneath the stage
is the ordinary mirror L. The condensing lens M is supported on a separate
brass holder let into one of the supports of the stand. In practice, however,
this arrangement was found inconvenient, and the microscope has therefore
not been brought into general use.

Messrs. Watson’s Microscopes.

Among London opticians, the various microscopes manufactured by
Messrs. Watson, of Holborn, are of high finish and good workmanship.
Those specially designed for the use of students possess merits of their own
in their mechanical construction, and also embody a provision, as indeed do
all their instruments, whether for students or more pretentious work,
whereby wear and tear in their frictional parts can be compensated for by
the user himself. This is effected in a simple but efficient manner. The
fittings are sprung, and screws set just outside the dove-tails. The very
slightest turn of the screws compresses the dove-tails, and a very large
amount of wear can in this way be prevented.

I am glad to notice that Messrs. Watson have adopted certain standard sizes
recommended some time ago by the Royal Microscopical Society for the
diameters of eye-pieces. It would be a great advantage if the same standard
became generally recognised and brought into use, since it is a matter of
much importance to microscopists.

Watson’s Edinburgh Students’ Microscope (Fig. 71) is a thoroughly efficient
one for all practical purposes, great care having been bestowed upon its
smallest details, and it is not difficult to perceive the reason of its popularity
among students. The tripod form of foot ensures great steadiness and
firmness; the body carries the smaller 0·92 eye-piece, and with draw-tube
closed is of the Continental length. The draw-tube is graduated to
millimetres, and when fully extended the body measures 10 inches. The
stage is provided with mechanical and rotary movements; the compound
sub-stage with centring screws, rack and pinion to focus, and a means of
lifting the condenser out of the optical axis when not required for use.
Notwithstanding, none of the movements are at all cramped; a clear

distance is maintained beneath the stage, affording plenty of room for
manipulating the mirror. Both coarse and fine adjustments work with
smoothness, the latter being on Watson’s latest improved principle—one
revolution of the milled head moves the body 1⁄300 of an inch. The stage is of
extra large size, to allow of the use of large culture-plates. No Continental
stand of higher price compares with the Edinburgh microscope. Its height
when placed in the vertical position is 11½ inches.

Fig. 71.—Watson’s Edinburgh Students’ Microscope.

Fig. 72.—Sub-stage of Edinburgh Students’ Microscope. This view of underside of
stage of students’ instrument shows the mirror set at an angle for oblique
illumination, and sub-stage turned aside.

The various sizes of oculars adopted by opticians and at present in vogue
cause considerable confusion. A standard size is specially needed for
students’ and small microscopes. The standard long used by Continental
manufacturers is 0·92 of an inch. The adoption of this size would place the
eye-piece in the same position as that of the universal screw for the
objective, formulated by the Royal Microscopical Society many years ago.
The desirability of using standard sizes has been fully recognised by
Messrs. Watson and they are now adapted to most of their microscopes. The
English diameter, 1·35 of an inch, known as the “Ross” size, is retained in
all their microscopes of large size.

Watson’s Mechanical Draw-tube.

Fig. 73.—Watson’s Mechanical Draw-tube (full-size).

An important feature in connection with the body-tube of Watson’s
Edinburgh Students’ Microscope (as, indeed, in all their fully furnished
instruments) is that they are provided with two draw-tubes; one moved by
rack-work, the other sliding inside the body-tube. The advantage is, that the
body can be made very short or extremely long, while sufficient latitude can
be given to objectives corrected for either Continental or English tube-
lengths, and to adjusting the same for thickness of cover-glass by variation
of tube length. Should the cover-glass be thicker than that for which the
objective is corrected, a shorter tube-length is necessary; if thinner, the body
must be lengthened. This is effected by means of the rackwork draw-tube.
The length of the body when closed is 142 millimetres (55⁄8 inches), and
when the two draw-tubes are extended, 305 millimetres (12 inches), being,

therefore, shorter than the Continental and longer than the English tube
lengths. Both draw-tubes are divided into millimetres, and on the rackwork
draw-tube a double scale is engraved, reading continuously from the sliding
draw-tube when fully drawn out, or giving the body length when the
rackwork draw-tube alone is in use. The utility of this mechanical draw-
tube is that it permits of quick manipulation with perfect results.

Fig. 74.—Watson’s Histological Microscope. Stand “A.”—Height, when placed
vertically and tube pushed home, 9½ inches.

The inside top of the draw-tube is smaller than the remainder, the former
making a fitting for the eye-piece about 1 inch long, permitting of the tube
being blackened inside up to this fitting, thus minimising reflection. The
end of the draw-tube has the universal screw for using the apertometer, &c.

Watson’s Histological Microscope (Fig. 74) is a somewhat cheaper form of
instrument, designed for the student; although of plainer construction it is
quite as well made as the costlier model. It is provided with spiral rack and
pinion coarse adjustment, and with this motion the greatest smoothness is
preserved. There is no backlash, the teeth of the pinion never leaving the
rack; so effective is it that a high power can be perfectly focussed by its
means. It is also furnished with their universal pattern of fine adjustment.
This can be had for £3 3s.

Fig. 75.—Watson’s Semi-Mechanical Stage.

Messrs. Watson have among other accessories of value introduced in
connection with their several microscopes a semi-mechanical stage,
whereby they are enabled to reduce the cost of manufacture. Fig. 75 is an
outline sketch of the same.

This stage is of the horse-shoe shape, with cut-out centre, constructed of ¼-
inch brass plate, and measures over all 5¼ inches wide by 4 inches deep.
Fitting on the edges of the main stage is a frame which is actuated vertically
by means of a double rack and pinion from beneath, giving ¾-inch of
movement, having controlling heads on either side of the stage; on the
edges of this mechanical frame a sliding bar is fitted, consequently
movement may be imparted either by rackwork or by hand. The mechanical
movement, however is in one direction only; but as the bar carries the

object, the worker can easily move the object out horizontally with the
finger. The advantage of this stage is that the whole surface is perfectly
flush, and the pinion heads are below its level, so that culture plates or
continuous sections may be conveniently examined.

Another addition of considerable value
is the centring underfitting for students’
microscopes.

This fitting places in the hands of
student workers a means of accurately
centring the sub-stage condenser, at a
low cost. It consists of the usual
Fig. 76.—New Centring Underfitting for underfitting tube, having a flange at the
Microscope. top which is fitted in a box between two
plates. The centring is effected by
means of two screws, which press the flange against a spring, as in the
ordinary sub-stage centring movement. The fitting can be adapted to any
form of Messrs. Watson’s and most other makers of students’ microscopes.

Watson’s Bacteriological Improved Van Heurck’s Microscope (Fig. 77) is in
every way a superior instrument, and it at once conveys a favourable
impression to the practical worker. When set up for use its many convenient
points—its excellence of workmanship and the precision of its movements—
seem to imply its special adaptation for the bacteriological laboratory and for
other high-class work where absolute reliance has to be placed in the results
obtained. Every detail of the instrument is carried out in the best possible
manner. The coarse adjustment is effected by means of a diagonal rack and
spiral pinion, which ensures the smoothest possible motion; while the fine,
the most important movement in the instrument, is made with an extra long
lever, a specialty of Messrs. Watson’s, and which imparts an extremely slow
action: this is now one of the most delicate and reliable forms of fine
adjustment. By its means the entire body is raised or lowered by means of a
milled head fixed to a screw having a hardened steel point acting on a lever
against a point attached to the body slide, in a dove-tailed fitting about 2½
inches long. Owing to the position of the controlling milled head on the
limb, it can be worked with either hand. Another feature of importance is
that, in using the fine adjustment the distance between the eye-piece and
objective remains unaltered. All the frictional parts of the microscope have
spring slots to the dove-tailed fittings, in which compensating screws are
fitted. These are some few of the more important points, to which much
thought and attention have been given. The body permits also of the use of

objectives of any other optician, since its total length when the draw tubes
are closed up is only 143 mm.; when extended, a total length of 320 mm. is
available. By this means an ample margin is left for the correction for cover-
glass thickness, whether the objective used be intended for the 160 mm. or
250 mm. tube length. The height of the microscope when placed in the
vertical position is 131⁄8 inches.

Fig. 77.—Watson’s Improved “Van Heurck Bacteriological” Microscope.

The Stage.—A somewhat new design has been used in building this up so as
to reduce vibration to a minimum. The bracket carrying the stage, instead of
being screwed on to the front of the limb, as is usually done, is made in a
solid casting, taking the sub-stage beneath, and passing into the joint at the
top of the foot. The joint bolt goes through the whole (limb and stage

bracket), rendering the limb stage and sub-stage as firm as if it were one
piece; a point of considerable importance.

The mirrors, which are plain and concave, are mounted on a swing arm, so
that they may be turned aside when direct illumination of the object is
required. On the right hand side also there is a steel clamping bar for fixing
the microscope at any angle of inclination. The tripod foot, which has
superseded most other forms, is adopted. At the points of contact with the
table the feet are provided with cork pads, which give increased firmness
and prevent vibration to some extent.

The sub-stage is provided with a fine adjustment of similar design to that
employed for the focussing of the objective. It has become needful to
embody such a refinement, in order that sub-stage condensers of large
aperture, such as are in constant use for critical high-power work, may be
adjusted with the same facility and precision as the objective—they, in fact,
require it if the best work is to be got out of them. No pains have been spared
by Messrs. Watson to render it absolutely perfect.

Watson’s Portable Microscope.—This instrument is similar in general detail
to the Histological Microscope, but the foot, mirror stem, &c., are made to
fold up in exceedingly compact form, and when set up for use the stand is
perfectly rigid. Portable microscopes are, as a rule, but makeshifts. This,
however, is a thoroughly sound, practical instrument and capable of best
work with the highest power objectives, having good adjustments and
universal size fittings throughout, so that the objectives and apparatus made
for the larger instruments can be employed with it.

Fig. 78.—Watson’s Portable Microscope. Height of instrument when placed
vertically and racked down is 93⁄8 inches.

Watson’s Petrological Microscope (Fig. 79) is a modification of their
Edinburgh Students’ pattern, and designed specially for petrological and
mineralogical work.

Fig. 79.—Students’ Petrological Microscope.

A polariscope having prisms of large size is supplied with it, the analyser
being fitted in the body, and the polariser in the under-stage fitting. The latter
has a divided circle and a spring catch at every quarter circle. By removing
the polariser and withdrawing the analyser, for which provision is made, the
microscope can be used for purposes of ordinary research. A Klein’s quartz
plate is fitted beneath the analyser, also in the body of the microscope.

The stage, which has a glass surface, rotates concentrically, and has a
divided circumferential edge reading by the verniers. The eye-piece has
cross webs to the diaphragm, and when it is desired, an analyser, having a
divided circle fitted with a calc-spar plate, can be used above the eye-piece,
and condenser lenses attached to the polariser for stereoscopic purposes. All
the fittings have the universal thread, and are interchangeable.

Fig. 80.—Swift’s Histological and Physiological Microscope.

Messrs. Swift’s Microscopes.

Messrs. Swift’s Microscopes have a well-established reputation for quality
and good workmanship, and therefore can in no way suffer by comparison
when placed beside those of other opticians. One of the characteristics of
Messrs. Swift’s microscopes—and this runs through the whole series—is
that they are all made to a standard gauge, so that the several parts of the
instruments, as well as their accessories, are interchangeable; the cheaper
forms, with those of the first quality and finish. Should the student, then,
start with a No. 1 model, he can at any time build it up, as it were, with the
accessories designed for a No. 3 or 4, that is, for an instrument of double the

price he started with. The optical centre is preserved throughout the whole
series of microscopes.

Fig. 81.—Swinging Leg Attachment of Swift & Son’s Four-Legged Microscope
Stand.

The tripod foot has, it appears, taken the place of some of their other forms
of instruments, while their four-legged tripod, if it can be so designated, is a
novelty of quite an unusual character.

The swing leg is attached to the framework of the tripod by the screw (Fig.
A), which is provided with a powerful steel spiral spring, compressed
between two steel collets when the screw is driven home, as shown in Fig.
B.

The expansion of this spring will obviously take up and compensate
automatically any wear and tear that is likely to occur between the bearing
surfaces, and it is therefore impossible for the fitting to get loose.

Swift’s Four-legged Microscope (Fig. 80) is one possessing great stability in
whatever position it may be placed; the body being supported on a horse-
shoe platform, from which its four legs spring, the two front legs being
fixed, while the hind legs are pivoted to the platform. This arrangement of
pivoting the hind legs enables the microscope to adapt itself to any uneven
surface, thus keeping it always in a steady position, while it also reduces the
danger of being upset by any lateral movement of an accidental nature. The
feet are studded with corks, an additional aid to steadiness and fixity for
microphotography. The length of the body from the ocular to the nose-piece
is 6½ inches, and can be extended to 9 or 10 inches by means of the draw-
tube, which has a millimetre graduation. The stage, which is of horse-shoe
shape, is provided with spring clips, to which a movable mechanical stage
can at any time be attached. The sub-stage partakes of two forms, one being
an ordinary fitting, taking an ordinary condenser; the other, the regular rack
and pinion achromatic condenser with centring adjustments. It has a
diagonal rack and pinion coarse adjustment, the fine adjustment being made
by micrometer screw of the finest character.

Fig. 82.—Swift’s Spiral Rack and Pinion Coarse Adjustment.

Fig. 82 is intended to illustrate the advantage of the spiral rack and pinion
which Messrs. Swift fit to their microscopes, in place of the ordinary
conventional horizontal rack and pinion movement. The advantage will at
once be seen, since there is more gearing contact between rack and pinion,
thus ensuring durability and reducing loss of time or back lash to a
minimum, with less wear and tear. The leaves of the pinion also roll into the
teeth of the rack by degrees, ensuring a very much smoother action, which, if
properly made and fitted, prevents the gearing of the two being felt by the
hand whilst focussing.

Fig. 83 is a supplementary draw-tube
with rack and pinion movement, which
can be adapted to any of Swift’s
microscopes in place of the ordinary
draw-tube, the size of the thread being
of the same diameter, so as to render
all draw-tubes, as well as other parts of
these instruments, interchangeable.
The draw-tube being divided into
millimetres can be extended from 160
to 250 millimetres. One advantage of
this arrangement is that the correct
adjustment of any objective with each
eye-piece is easily found and recorded
for future observations with the same
combination.

Messrs. Swift’s Three-legged Tripod
Microscope (Fig. 86). In most respects
the description already given of the
four-legged instrument is applicable to
this stand. Although of an apparently
Fig. 83.—Graduated Supplementary Draw-Tube. different form, it can be built up, as
already explained, into one of a higher
class. It is suitable in every way for histological investigations. The horse-
shoe platform in this, as in the preceding stand, is extremely serviceable, as
it allows the pillar of the instrument to rest firmly upon it, thus rendering the
stand very rigid.

Swift’s Bacteriological Microscope (Fig. 84), designed by Professor Wright,
of the Army Medical School, Netley, a sufficient warranty of its excellency
and perfect adaptation for bacteriological high-class work. One of the
advantages connected with this microscope is the facility with which it can
be adapted for either high or low power investigation, without the necessity
of adding or detaching any part. The objectives, arranged on a triple nose-
piece, are approximately in focus when revolved into position for immediate
use, thus effecting a saving of time in changing the objective. Moreover, the
nose-piece carrying the objectives is of new construction, and fitted in such a
way that the entry of dust is rendered impossible.

Fig. 84.—Swift’s Army Bacteriological Microscope. Fig. 85. Under-Stage of same.

The Abbe condenser, fitted with an iris diaphragm, is mounted on an
eccentric arm, so that it can readily be thrown out of the axis of the
microscope when not required, without having to re-arrange the focus when

again brought into position. The condenser must be turned aside when plate
cultivations and preparations of unstained bacteria are being looked over for
selection of colonies for mounting, in which case an arm carrying a quadrant
with three apertures is brought into position in place of the condenser, the
apertures being severally centred by a spring catch and used with oblique
light. This arrangement, shown in Fig. 85, is seen from the under surface of
the stage. The stage is sufficiently large, so that when Petrie plates are being
examined at the extreme edges there is little fear of their overbalancing.

Fig. 86.—Swift’s Histological Students’ Microscope.

The fine adjustment is the Swift’s Patent Campbell Differential Screw, which
offers great facilities for delicate focussing with the highest power
objectives. The stand is of the most substantial and rigid form, and thus
ensures the microscope from vibration.

The under-stage of microscope (Fig. 85) is seen to be of the most approved
form.

Fig. 87.—Swift’s Advanced Students’ Microscope.

Swift’s Advanced Students’ Microscope.—In this microscope (Fig. 87) we
have a superior instrument for the use of the advanced student, which may
be described as of high mechanical excellence, well suited for every
requirement of work. The stand is the well-known tripod form of their
Challenger Microscope, and admits of the instrument being placed at any
angle of inclination; the body is short enough to work with objectives of
Continental makers, and is provided with a draw-tube, to elongate it to the
standard of 10 inches, with a diameter of 13⁄16 inch to take the same eye-
pieces as the larger stands. The coarse adjustment is by spiral rack and
pinion; the fine, by a carefully made differential screw motion for delicate

focussing. The stage is of the horse-shoe pattern, to which a mechanical
stage can at any time be adapted, as well as an achromatic condenser to the
sub-stage seen beneath. Here the student will find the foundation for a
superior instrument.

Messrs. Baker’s Microscopes.

Of Messrs. Baker’s larger stands, the Improved “Nelson Model,” No. 2 (Fig.
88) stand is selected in preference to their more elaborate No. 1, and their
simpler form, No. 3, as a high-class instrument, and one well suited for fine
critical work; the former being somewhat better, only from having extra
adjustments; the latter possessing no superior advantage over the “Advanced
Students’” Microscope. This microscope is mounted on a solid tripod foot,
which insures stability, whether placed in a vertical, horizontal, or inclined
position; the front toes are slotted, so that they may be clamped to the base
plate of a photo-micrographic apparatus, first introduced for photo-
micrographic work, and will also be found convenient in ordinary work; as
the fine adjustment milled head is placed at the bottom of the pillar, instead
of at the top, the more usual place. For photo-micrographic work the
advantage is that the strain of the pulley in such apparatus actuates the fine
adjustment, and is less liable to cause vibration of the instrument. The
advantage when the instrument is used for ordinary work lies in the fact that
the weight of the hand is rested on the top of the tripod, thus admitting of
steadier movement of the milled head. The fine adjustment is obtained by a
“Campbell” differential screw, each revolution of which is equal to 1⁄200 m.m.
The draw-tubes being graduated in m.m., allow of either short or long tube
objectives being used, closing up to 150 m.m. and extending to 280 m.m.,
the rack and pinion adjustment to the lower tube affording a ready means of
correction for cover-glass thicknesses. The eye-piece gauge, as will be seen
from its dimensions, is of large size, being the same as that adopted by Zeiss
for his long tube compensating oculars; smaller eye-pieces can, however, be
adapted at any time.

Fig. 88.—Baker’s Improved “Nelson Model” Microscope. Dimensions.—Height
when in vertical position and body racked down, 11′′; Height of stage, 41⁄8′′;
Height of optic axis when in horizontal position, 8½′′; Spread of tripod foot, 8 ×
8½′′; Diameter of mirrors, 23⁄8′′; Internal diameter of draw-tube, 13⁄10′′.

The mechanical rotating stage is divided on brass to 1⁄100 inch, with clamping
bars and stop, by which a specimen can always be brought back to a certain
position for registration. The sub-stage has rack-work focussing adjustment,
and centring screws; a fine adjustment is added, if desired. On the whole, the
instrument is suitable for special critical work, and is equally well suited for
photo-micrography.

Fig. 89.—Baker’s Advanced Students’ Microscope. Dimensions.—Height when in
vertical position and body racked down, 11¼′′; Height of stage, 4¾′′; Width of
stage, 4′′; Height of optic axis when in horizontal position, 6½′′; Spread of foot, 6′′
× 6′′; Diameter of mirrors, 1¾′′; Internal diameter of draw-tube, 11⁄12′′.

Explanatory lettering of instrument: A, Huyghenian eye-piece; B, draw-tube
graduated in millimetres; C, nose-piece; D, coarse adjustment; E, fine adjustment
with millimetre screw; F, horse-shoe sliding stage, graduated with sliding bar in
vertical and horizontal directions for use as finder; G, sub-stage rack and pinion
screw; H and I, centring screws to sub-stage; J, carrier for condenser; K, mirror
with movable arm supported on solid tripod foot.

The points of difference between this stand and the No. 1 model are that in
the latter the fine adjustment carries the body only, and not the rack
adjustment; the limb carrying both the body and the sub-stage is in one
piece, giving, if possible, still greater rigidity; the rotation of the mechanical
stage, which is divided on silver, is complete, and can be actuated by hand or
rack work; it has a clamping screw and fine adjustment to sub-stage.

Baker’s Advanced Students’ Microscope (Fig. 89) may be described as a
typical instrument, equally suitable for histological work and that of the
advanced student. The intention of the maker in simplifying the adjustments
and reducing the instrument in size, was to furnish a well-finished portable
instrument at a moderate cost. This object has not been attained by supplying
adjustments of second-rate quality, but by reducing their number to a
minimum.

Fig. 90.—The Mayall Removable Mechanical Stage.

The tripod foot of the “Nelson Model” is replaced by a claw foot, which is in
effect a tripod, as it rests on three points; it has not the same wide spread, but
this, far from being a disadvantage, renders the instrument more portable. It
has rack and pinion coarse and Campbell differential screw fine adjustments,
draw-tube graduated in m.m., extending to 180 m.m., eye-piece gauge the
same as the Continental size, large square open stage to afford the greater
freedom of manipulation; sliding bar with graduations on bar and stage,

which suffice for registering any given field under a low power; holes are
also drilled in the stage ready to receive an attachable mechanical stage
should it be thought advisable to add one at a later date. The sub-stage is of
the universal size with rack-work focussing, adjustment, and centring
screws.

Fig. 91.—Baker’s Model Histological Microscope. Dimensions.—Height when in
vertical position and body racked down, 10½′′; Height of stage, 4′′; Width of stage,
3½′′; Height of optic axis when in horizontal position, 5¼′′; Spread of foot, 5¼′′;
Diameter of mirrors, 15⁄8′′; Internal diameter of draw-tube, 11⁄12′′.

Messrs. Baker have recently introduced a similar instrument with swing-out
sub-stage and adjustments for compensating for wear and tear of rack. The
stage is also somewhat larger from back to front.

These stands are very suitable for bacteriological research, and for amateurs
wishing to obtain a stand which will carry all the apparatus they are likely to
need, without going to the expense of the larger models, no better instrument
could be desired.

Their “Removable Mechanical Stage” (Fig. 90) is a modification of the
pattern designed by the late Mr. J. Mayall. The vertical movement is by rack
and pinion, giving a range of 11⁄8 inch. The horizontal motion of 1½ inch is
accomplished by means of a quick-acting screw. The object is pressed tightly
to the stage of the microscope by means of three points, and the whole of the
mechanical part is firmly clamped by two thumb screws which can be
readily removed. The stage is made to carry slides of any size less than 1¾
inch wide.

Baker’s Histological Microscope (Fig. 91) is of a different type to the
preceding, and is intended to represent one of medium power, affording
magnification of about × 400 as a maximum. It is supplied with a diaphragm
beneath the stage, without other illuminating apparatus than that of the
mirror. But if the adjustments of such a stand are good, there is no reason
why some form of sub-stage condenser should not be added, to make the
instrument somewhat more serviceable. There is, however, a rather too
limited space beneath the stage of an instrument of this kind to admit of a
sub-stage condenser, consequently it cannot be said to be suitable for critical
work. For all ordinary students’ work this microscope is certainly available.

Fig. 92.—Rousselet’s Tank Microscope.

The stand of the Model Histological Microscope has the same form of foot
as the more advanced student’s stand. It is somewhat lighter, and more
portable, a matter of consideration in a student’s microscope, which often
has to be carried to and from a class-room. It is provided with rack and
pinion coarse adjustment, and a Campbell differential screw fine adjustment,
draw-tube, and diaphragm; the diaphragm carrier being of the universal size,
so that it can be replaced by an Abbe condenser at any time. With the
additions suggested, this instrument can be made equal to those of a higher
standard.

Rousselet’s Tank Microscope (Fig. 92), for rapidly looking over pond water
and weeds, consists of a jointed arm moving parallel to the side of the tank
to carry an aplanatic lens; the arm is focussed by means of rack and pinion
fixed to the upright of a mahogany stand, upon which the tank can be placed,
or it can be clamped directly to the tank by means of a screw. This handy
form of pond microscope is made by Messrs. Baker.

Pillischer’s Microscopes.

Mr. Pillischer (New Bond Street) is favourably known for the excellency of
his instruments. He has lately brought out several microscopes of an
improved form. His larger model, the “New International,” consists of a
solid, well-built, firm tripod stand of the Ross-Jackson pattern, which
appears to be quite in the ascendant among London opticians; rack and
pinion coarse adjustment, and a superior micrometer fine adjustment; sub-
stage with centring screws and rack and pinion focussing adjustment; a new
form of sliding pin-hole diaphragm and iris diaphragm; B and C eye-pieces;
5⁄8 and 1⁄7 objectives; Abbe condenser, N.A. 1·20; in every respect a perfect
model, neatly packed in a mahogany case, for a very moderate sum. Mr.
Pillischer’s No. 2 (Fig. 93) “International” Microscope, being the Army
pattern as well as the student’s, is well adapted for clinical work. A firm
tripod stand supports two dark bronze uprights, with rack and pinion coarse
adjustment, e, and fine adjustment, d, the stage, i, is wide and suitable for
clinical work, and large enough for dissecting upon. The whole instrument is
well made; the coarse adjustment is so good that the one-eighth inch can be
focussed with ease, and without using the fine adjustment.

For a few shillings extra, a mechanical stage can be added, consisting of
levers, having an action similar to the movements of a parallel ruler, which is
so easy of adjustment that it can be worked under the eighth-inch objective
with the hands—an advantage in a clinical microscope.

Fig. 93.—Pillischer’s “International” Microscope.

The following reference letters serve to explain the general construction of
the microscope (Fig. 93):—a, the eye-piece; b, the draw-tube; c, the sliding-
tube; d, micrometer or fine adjustment; e e, the coarse adjustment; g, the
mirror arm and mirror; h, sub-stage carrying Abbe condenser; i, the stage
with spring-clips; j, objectives screwed into place and double nose-piece.

The “Kosmos” is Pillischer’s cheaper model. The stand of this somewhat
novel and original microscope is framed entirely of brass and gun-metal. The
fine adjustment is very sensitive and perfectly steady, admitting of the
highest immersion objectives being used. The optical parts are constructed
upon principles consistent with the latest improvements. It has a claw-foot
stand with a semi-circular arm, which carries the body, with sliding-tube
coarse adjustment, and micrometer screw fine adjustment, with a large

square stage diaphragm and mirror. The instrument is neatly packed in a
mahogany box, together with the A or B eye-piece, 1-inch and 1⁄5-inch
objectives of good defining and penetrating power, magnifying from 30 to
380 diameters, in mahogany cabinet, for the moderate sum of £5.

Pillischer’s Binocular Microscope (Fig. 94) is constructed on a plan
somewhat intermediate between that of Beck’s and Ross’s well-known
patterns, and in point of finish is equal to any student’s microscope in use.
The semi-circular form given to the arm carrying the body increases the
strength and solidity of the instrument, although it is doubtful whether it
adds to its steadiness when placed in the horizontal position. The straight
body rests for a great part of its length upon a parallel bar of solid brass
ploughed into which is a groove for the reception of the rack attached to the
body, the groove being of such a form that the rack is held firmly while the
pinion glides smoothly through it. A steady, uniform motion is thus obtained,
which almost renders the fine adjustment unnecessary. The binocular bodies
are inclined at a smaller angle to one another than in most instruments;
nevertheless, the range of motion given to the eye-pieces by the rack and
pinion enables those whose eyes are widely separated to use the instrument
with comfort. The prism is so well set that it illuminates both fields with
equal intensity. The stage is provided with rectangular traversing movements
to the extent of an inch and a quarter in each direction. The milled heads
which effect these are placed on the same axis, instead of side by side, one
of them—the vertical one—being repeated on the left of the stage, so that the
movements may be communicated either by the right hand alone or by both
hands acting in concert. The stage-plate has the ordinary vertical and
rotatory motions, but to a much greater extent than usual; and the platform
which carries the object is provided with a spring clip to secure the object
when the stage is placed in the vertical position. A new form of sub-stage
with centring screws is made to carry the Abbe achromatic condenser,
diaphragm, polarising and other apparatus.

Fig. 94.—Pillischer’s Binocular Microscope.

Continental Microscopes.

Continental Microscopes.—The better known among continental opticians
are Zeiss, Leitz, Seibert, Reichert and Hartnack. All seem to have vied with
each other in the attainment of perfection in the manufacture of the most
useful forms of microscopes. The late Carl Zeiss did more for the modern
microscope than either of the opticians referred to above. I therefore take a
medium typical model of his from a long series of highly-finished
instruments for my illustration. Zeiss’s successors have of late endeavoured
to perfect the mechanical details of their instruments in three or four
directions, i.e., fundamental features of the stand, stage arrangements, means
of focussing, and illumination.

The Stand.—The general form of the stand still partakes too much of the
original sameness of type introduced by Oberhäuser, and modified and
improved by Hartnack; the “Babuchin” stand being still in favour with some
few makers. The greater firmness and steadiness of Zeiss’s stand (Fig. 95) is
secured by the horse-shoe form of foot, which, for the most part, is massive
and well adapted to carry the stout uprights, which support a well-balanced,
substantial body-tube and a graduated draw-tube, circular stage with a
vulcanite disc, 4 inches in diameter; a sub-stage with centring arrangement
for Abbe’s illuminating apparatus, and iris diaphragm and other diaphragms
for use when the condenser is thrown aside. The mirror is full-sized, plane
and concave. The coarse adjustment is regulated by a rack and pillion
movement so perfect that objectives of medium power can be focussed by it
alone. The fine adjustment is made by micrometer screw, the force exercised
by which is transferred to the movable body by a single contact between two
hardened steel surfaces. This ensures extremely delicate and uniform motion
of the body which carries the tube.

The divisions in the milled head of the screw furnish a means for the
registration of the vertical movements of the tube. In the latest stands, each
division corresponds to an elevation or depression of the tube in the direction
of the optic axis of 0·01 mm. By this means measurements of thicknesses
may be made with a considerable degree of accuracy, the upper and lower
surfaces of the object being successively focussed, and the amount read off
on the milled-head, by the fixed index. In doing this, care must be taken to
make both adjustments by a rotation of the screw in the same direction. The
thickness of an object in air is then equal to the difference between the two
readings. By this means the thickness of any other substance may be
measured—that, for instance, of the cover-glass of the object.

Fig. 95.—Zeiss’s Medium Stand Microscope.

The medium tube-length of the microscope is 160 mm. from the attachment
of the objective to the eye-piece end. The draw-tube admits of the length
being increased or diminished, and this may be read off by means of the
millimetre scale engraved on the tube. My description of this model also
applies to the higher class microscopes, which will be found in every way
well finished and adapted to biological and scientific research.

E. Leitz’s of Wetzlar Microscopes.—This optician publishes a series of
twelve high-class forms of instruments. By preference, the horse-shoe form
of stand (Fig. 96) is adopted in the whole of this maker’s models, the body
being supported on a hinge joint and clamped over, and fitted with a circular
revolving centred mechanical stage, attached to the ordinary stage by means
of a set pin, which fixes the stage in position. By removing the screw, the

stage can be detached; in this way, the
stage serves for searching over large
surfaces and registering the results.

Fig. 96.—E. Leitz’s Medium-sized
Microscope.

Fig. 97.—Leitz’s Dissecting Microscope.

The coarse adjustment is made by rack and pinion, and the fine adjustment
by micrometer screw, the head of which is provided with a scale reading 1⁄100
mm. The draw-tube is also cut and ruled to millimetre scale. The sub-stage
has rack and pinion movement, and is arranged for the Abbe condenser and
iris diaphragm. This is attached to the upper stage by means of a set pin,

which fixes and retains it in position after perfect centring. By removing the
pin, the sub-stage can be either detached or swung aside by pressing a
button. In short, this microscope is in all respects well furnished and fitted
with the requisite complex mechanism necessitated by modern high-class
technicological work.

Leitz’s students’ microscope, with sliding body, micrometer screw fine
adjustment, concave mirror, two eye-pieces and two objectives, ¾ inch and
1⁄8 inch, in mahogany case, costs £3 10s. Leitz’s dissecting microscope, with
a heavy foot and rests, is fitted with two aplanatic lenses, magnifying × 10, ×
20 diameters.

Reichert and Seibert adhere to the same model as that of Zeiss, and therefore
require only a brief notice. Their microscopes are characterised by
substantial workmanship, suitable construction, and exact centring. The
coarse adjustment is obtained in the usual way by rack and pinion, the fine
by micrometer screws, which work easily, and are protected against wear
and tear by having their working surfaces hardened. The stands of the better
class instruments have micrometer screws graduated, and draw-tubes cut to
millimetre scale. Their mechanical stages and sub-stages and accessories are
in every way well finished; stage forceps, tests, and an assortment of cover
glasses and slides being added. Their first-class microscopes are sent out in
mahogany boxes.

On going through the continental makers’ catalogues, it will be noticed that
their well-equipped microscopes are rather more costly than that of their
English confreres. It is understood Messrs. Baker and Watson are the
constituted agents for these opticians.

Nachet’s Microscope, a new form of which was first seen at the Antwerp
Exhibition 1892, is very solidly built, and has all the qualities necessary for
histological work. The stage rotates about the optic axis, and carries a
movable slide holder. The coarse adjustment is by rack and pinion
movement, the fine by the new system of micrometer screw (described in the
journal of the Royal Microscopical Society of 1886), with divided head
indicating the 1⁄400 part of a mm. The plane and convex mirror is mounted on
a jointed arm. The draw-tube is divided into millimetres. The illuminating
system, consisting of a wide-angled Abbe condenser (N.A. 1·40) with iris

diaphragm, is raised or lowered by rack and pinion screws. The iris
diaphragm, being mounted on a wheel, is worked by a tangent screw, which
by a very slight movement causes the aperture of the diaphragm to pass from
the centre to the periphery of the condenser. Altogether the arrangement of
the sub-stage is novel, and the instrument is extremely well arranged and
adapted to modern requirements.

Nachet and Hartnack, of Paris, hold an almost equal rank as makers of first-
class microscopes, and in point of excellence of workmanship fairy rival
those of our English makers.

Fig. 98.—Nachet’s Class Demonstrating Microscope.

There are very many other London and Continental makers of microscopes
besides those especially mentioned, who have well-sustained reputations as
opticians, and who, from want of space, I have been obliged to pass over.
Messrs. Newton’s Students’ Microscope must be mentioned with respect. It
is a good and useful instrument, has a firm stand with a reversible (rotatory)
body movement, which seems to ensure steadiness when brought into the
horizontal position for micro-photographic purposes. There are other
opticians whose microscopes have stood the test of time—Messrs. Collins,
Crouch, &c. It may, however, be taken as a well-established fact that those
opticians known to manufacture the more highly-finished models also

produce the more serviceable forms of students’ class-room, and other
microscopes.

The Bacteriological Microscope.

The microscope required for bacteriological studies should be perfect in all
its parts. With regard to the choice of an instrument, it is very much a matter
of price, since the most perfect is usually the most costly; I shall therefore
proceed to give a typical example of the instrument in use in a
bacteriological laboratory. The microscope should possess the following
qualifications, all of which are absolutely necessary for the study of such
minute objects as bacteria and other micro-organisms.

“The typical bacteriological microscope should be well equipped with
objectives of sufficiently high magnifying power, and with a special form of
illuminating apparatus; while the mechanical arrangements for focussing
should act with the greatest smoothness and precision; the stage, also, should
be wide enough to admit of the examination of plate cultivations.”

We will consider these several points and recommendations seriatim,
commencing with the stand.

Messrs. Watson & Sons’ Van Heurck model stand so well answers the
several conditions laid down by an experienced teacher of bacteriology, that
I have no hesitation in presenting it to my readers as a typical instrument,
one in every way worthy of the high praise it has already received from
those who have worked with it, and whose judgment may be relied upon in
every way. The microscope is fully described among Messrs. Watson’s
instruments, page 108.

The Stand.—A good firm stand is undoubtedly of the first importance for all
high-class work. The steadiness of the instrument and its entire freedom
from vibration depends largely upon the form of the stand. I am glad to find
Dr. Crookshank in accord with me as to the Ross-Jackson model, one which,
in my opinion, has not been entirely superseded by models of a more recent
date. Indeed, the latest improvement effected in the Ross-Jackson form, in
which attention has been given to the spreading-out of the feet, has

converted it into as solid and firm a stand as Powell’s; it is equally free from
vibration when placed in the horizontal position.

There are, however, four different forms of stands—the tripod; the plate with
double columns; the single column ending in a plate or a bent claw; and the
horse shoe. The tripod stand, with cork feet, is by far the steadiest form of
model. The single upright pillar support should unquestionably be
condemned, as it admits of considerable vibration, and is most inconvenient
for laboratory work. The heavy horse-shoe form is compact and firm, and
the weight of it can hardly be considered an objection.

The Tubular Body is from eight to ten inches in length, to which is added a
draw-tube with an engraved millimetre scale. By extending the draw-tube
greater magnification is obtained, but since this is at the cost of definition it
should hardly ever be employed in the examination of bacteria. A Triple
Nose-piece is doubtless a convenience, saving time which is otherwise spent
in replacing objectives of different magnifying powers; there is also less risk
of injuring them. Focus should be obtained by means of a rack and pinion
coarse adjustment, together with the most approved kind of fine adjustment.
The sliding tube cannot be recommended, as the motion may be stiff,
encouraging the use of force, which in turn may result in the objective being
brought violently into contact with the specimen, thus doing injury to the
lens or damage to the preparation; or it may get too loose and readily slip out
of focus.

The Stage should be flat and rigid, either rectangular or circular, so long as it
is sufficiently large to accommodate plate cultivation. A removable
mechanical stage is of great advantage for working with high powers, as a
motile bacterium can be constantly kept in view, while one hand is engaged
in working the fine adjustment; it may also be employed as a finder, if
engraved with a longitudinal and vertical scale, and provided with a stop.
The mechanical stage must be removable, so that the stage proper may be
free from any attachments when required for the examination of cultures.

Diaphragms.—The plan of using a series of separate discs of different sizes
should be avoided, as they are easily lost, and bacteriological investigations
may have to be made under conditions in which it is difficult to replace

them. A better plan is a revolving plate with apertures of different sizes, but
the most convenient form is the iris diaphragm.

The Sub-stage Condenser is as necessary in biological work as in the
objective—in fact, the condenser and the objective should be considered as
forming one piece of optical apparatus; the microscope must be regarded as
incomplete without it.

It is by the sub-stage condenser that the rays of light are concentrated at one
point, or on one particular bacterium; for the best definition it is essential
that there should be mechanical arrangements for accurately centring and
focussing the condenser. All this will be explained and enlarged upon under
“Practical Optics.”

In the historical review presented to my readers on the evolution of the
modern microscope, I have for the most part relied upon my long and close
association, extending over a period of upwards of half a century, with
microscopy. I need hardly say I could have very much extended my remarks
with pleasure and profit had space permitted, and thereby much increased
the number of names of manufacturers, who have well-established
reputations for the quality of their work, and whose instruments, more or less
complete in design, realise the wants of students and of that large class of
present-day workers engaged in microscopical pursuits to whom economy of
outlay is almost a first consideration. No valid reason, however, can be
assigned for splitting up, as some writers do, the several forms of
microscopes into some six different classes, which implies inferiority in
mechanical details or finish, whereas the difference wholly consists in
luxurious appliances to save time, and in accessories for special work or
original research. Before bringing these remarks to a close, it is my wish to
direct the student’s attention to one or two points of importance in
connection with the use of the instrument, viz.: variations in body-lengths of
microscopes, especially between those of English and of Continental
manufacture. The optical-standard measurement adopted in this country for
the body-tube-length is 10 inches; and for its mechanical, 8¾ inches. That of
Continental opticians is, optical-tube-length 7·08 inches, or 180 mm.; the
mechanical, 6·3 inches = to 168 mm.

Professor Abbe constructed an apochromatic immersion objective especially
for the English optical tube-length of 10·6 inches (= to 270 m.m.), and
mechanical tube-length somewhat less in measurement. This may be taken
to mean a slight increase in the standard value of the tube, and therefore the
addition of the rack-and-pinion to the draw-tube, now generally made a part
of the microscope, is certainly of some practical value. This difference,
however, when working with the English body-tube of 10 inches, may be
discarded; it is, in fact, only where the shorter Continental body is in use,
that so small a difference of tube-length exercises a disturbing effect over
adjustment. Moreover, an object placed on the stage of the shorter body
microscope will not be seen with the same distinctness by the draughtsman
should he wish to make use of the camera lucida.

The optical tube-length of the body is measured from the back lens of the
objective to the front lens or principal focus of the eye-piece; the mechanical
tube-length from the end of nose-piece of objective to the top lens of the
eye-piece.

The Hartnach Students’ Model Microscope.

CHAPTER III.

Applied Optics:—Eye-pieces; Achromatic Objectives; Condensers.

It is almost unnecessary to say that the eye-piece forms a most important
part of applied optics in the microscope. It is an optical combination
designed to bring the pencil of rays from the objective to assist in the
formation of a real or virtual image before it arrives at the eye of the
observer. Greater attention has been given of late years to the improvement
of the eye-piece, since flatness of field much depends upon it. Opticians
have therefore sought to make it both achromatic and compensatory.

There are several forms of eye-pieces in use, some of which partake of a
special character, and these will receive attention in their proper places. It is,
however, customary among English opticians to denote the value of their
several eye-pieces by Roman capitals, A, B, C, D, and E. Continental
opticians, on the other hand, have a preference for numerals, 1, 2, 3, 4, 5 and
6, or more, and by which they are recognised.

The eye-piece in more general use is that known as the Huyghenian (Fig.
99); this came into use upwards of two centuries ago. It was constructed by
Christian Huyghens, a Dutch philosopher and eminent man of science,
secretary to William III.

It was made for the eye-piece of a telescope he constructed with his own
hands, and it has been in constant use as the eye-piece of the microscope for
nearly two centuries. It consists of two plano-convex lenses, with their plane
surfaces turned towards the eye, and divided at a distance equal to half the
sum of their focal lengths—in other words, at half the sum of the focal
length of the eye-glass and of the distance from the field-glass at which an
image from the object glass would be formed, a stop, or diaphragm, being
placed between the two lenses for the reason about to be explained.

Huyghens himself appears to have been quite unaware of the value of an
eye-piece so cleverly constructed.

It was reserved for Boscovich to
point out that, by this important
arrangement, he had corrected a
portion of the chromatic
aberration incidental to the earlier
form of eye-pieces. Let Fig. 100
represent the Huyghenian eye-
piece of a microscope, f f being
the field-glass, and e e the eye-
glass, and l m n the two extreme
rays of each of the three pencils
emanating from the centre and
ends of the object, of which, but
for the field-glass, a series of
coloured images would be formed
from r r to b b; those near r r
being red, those near b b blue, and
Fig. 99.—Huyghenian Eye-piece A, the dotted lines the intermediate ones green,
show position of lenses. yellow, and so on, corresponding
with the colours of the prismatic
Spectrum.

The effect described, that of projecting the blue image beyond the red, over-
correcting the object-glass as to colour, is purposely produced; it is also seen
that the images b b and r r are curved in the wrong direction to be seen
distinctly by the convex eye-lens; this then is a further defect of the
compound microscope made up of two lenses. But the field-glass, at the
same time that it bends the rays and converges them to foci at b′ b′ and r′ r′,
also reverses the curvature of the images as here shown, giving them the
form best adapted for distinct vision by the eye-glass e e. The field-glass has
at the same time brought the blue and red images closer together, so that they
produce an almost colourless image to the eye. The chromatic aberration of
lenses has been clearly explained in a previous chapter. But let it be
supposed that the object-glass had not been over-corrected, that it had been
perfectly achromatic; the rays would then have appeared coloured as soon as

they had passed the field-glass; the blue rays of the central pencil, for
example, would converge at b′′, and the red rays at r′′, which is just the
reverse of what is required of the eye-lens; for as its blue focus is also
shorter than its red, it would require that the blue image should be at r′′, and
the red at b′′. This effect is due to over-correction of the object-glass, which
removes the blue foci b b as much beyond the red foci r r as the sum of the
distances between the red and the blue foci of the field-lens and eye-lens; so
that the separation b r is exactly taken up in passing through those two
lenses, and the several colours coincide, so far as focal distance is
concerned, as the rays pass the eye-lens. So that while they coincide as to
distance, they differ in another respect—the blue image is rendered smaller
than the red by the greater refractive power of the field-glass upon the
former. In tracing the pencil l, for instance, it will be noticed that, after
passing the field-glass, two sets of lines are drawn, one whole and one
dotted, the former representing the red, and the latter the blue rays. This
accidental effect in the Huyghenian eye-piece was pointed out by Boscovich.
The separation into colours of the field-glass is like the over-correction of
the object-glass—and opens the way to its complete correction. If the
differently-coloured rays were kept together till they reached the eye-glass,
they would still be coloured, and present coloured images to the eye. The
separating effected by the field-glass causes the blue rays to fall so much
nearer the centre of the eye-glass, where, owing to its spherical figure, the
refractive power is less than at the margin, so that spherical error of the eye-
lens may be said to constitute a nearly equal balance to the chromatic
dispersion of the field-lens, and the blue and red rays l′ and l′′ emerge nearly
parallel, presenting a fairly good definition of a single point to the eye. The
same may be said of the intermediate colours of the other pencils. The eye-
glass thus constructed not only brings together the images b′ b′, r′ r′, but it
likewise has the most important effect of rendering them flatter, and assisting
in the correction of chromatic and spherical aberration.

Fig. 100.—Huyghenian Eye-piece.

Fig. 101.—Ramsden’s Eye-piece.

The later form of the Huyghenian eye-piece is that of the late Sir George
Airy, the field-glass of which is a meniscus with the convex side turned
towards the objective, and the eye-lens a crossed convex with its flatter side
towards the eye. Another negative eye-piece is that known as the Kellner, or
orthoscopic eye-piece. It consists of a bi-convex field-glass and an

achromatic doublet eye-lens. This magnifies ten times, but it in no way
compares with the Huyghenian in value. Neither does it afford the same
flatness of field.

The Ramsden, or positive eye-piece, is chiefly employed as a micrometer
eye-piece for the measurement of the values of magnified images. The
construction of this eye-piece is shown in Fig. 101, a divided scale being cut
on a strip of glass in 1⁄100ths of an inch, every fifth of which is cut longer than
the rest to facilitate the reading of the markings, and at the same time that of
the image of the object, both being distinctly seen together, as in the
accompanying reduced micro-photograph of blood corpuscles, Fig. 102.

The value of such measurements in reference to the real object, when once
obtained; is constant for the same objective. It becomes apparent, then, that
the value of the divisions seen in the eye-piece micrometer must be found
with all the objectives used, and carefully tabulated.

It was Mr. Lister who first proposed to place on the stage of the microscope
a divided scale of a certain value. Viewing the scale as a microscopic object,
he observed how many of the divisions on the scale attached to the eye-piece
corresponded with one or more of a magnified image. If, for instance, ten of
those in the eye-piece correspond with one of those in the image, and if the
divisions are known to be equal, then the image is ten times larger than the
object, and the dimensions of the object ten times less than that indicated by
the micrometer. If the divisions on the micrometer and on the magnified
scale are not equal, it becomes a mere rule-of-three sum; but in general this
trouble is taken by the maker of the instrument, who furnishes a table
showing the value of each division of the micrometer for every object-glass
with which it will be employed.

Mr. Jackson’s simple and cheap micrometer is represented in Fig. 103. It
consists of a slip of glass placed in the focus of the eye-glass, with the
divisions sufficiently fine to have the value of the ten-thousandth part of an
inch with the quarter-inch object-glass, and the twenty-thousandth with the
eighth; at the same time the half, or even the quarter of a division may be
estimated, thus affording the means of attaining considerable accuracy, and
may be used to supersede the more complicated and expensive screw-

micrometer, being handier to use, and
not liable to derangement in
inexperienced hands.

The positive eye-piece affords the
best view of the micrometer, the
negative of the object. The former is
quite free from distortion, even to the
edges of the field; but the object is
slightly coloured. The latter is free
from colour, and is slightly distorted
at the edges. In the centre of the field, Fig. 102.—Blood Corpuscles and Micrometer,
magnified 1·3500.
however, to the extent of half its
diameter, there is no perceptible
distortion, and the clearness of the definition gives a precision to the
measurement which is very satisfactory.

Short bold lines are ruled on a piece
of glass, a, Fig. 103, to facilitate
counting, the fifth is drawn longer,
and the tenth still longer, as in the
common rule. Very fine levigated
plumbago is rubbed into the lines to
render them visible; they are then
covered with a piece of thin glass,
cemented by Canada balsam, to
prevent the plumbago from being
wiped out. The slip of glass thus
prepared is secured in a thin brass
frame, so that it may slide freely into
its place.
Fig. 103.—Jackson’s Eye-piece Micrometer.
Slips are cut in the negative eye-piece
on each side, so that the brass frame may be pressed across the field in the
focus of the eye-glass, as at m; the cell of which should have a longer screw
than usual, to admit of adjustment for different eyes. The brass frame is
retained in its place by a spring within the tube of the eye-piece; and in using
it the object is brought to the centre of the field by the stage movements; the

coincidence between one side of it and one of the long lines is made with
great accuracy by means of the small screw acting upon the slip of glass.
The divisions are then read off as easily as the inches and tenths on a
common rule. The operation, indeed, is nothing more than the laying of a
rule across the body to be measured; and it matters not whether the object be
transparent or opaque, mounted or unmounted, if its edges can be distinctly
seen, its diameter can be taken.

Previously, however, to using the micrometer, the value of its divisions
should be ascertained with each object-glass; the method of doing this is as
follows:—

Place a slip of ruled glass on the stage; and having turned the eye-piece so
that the lines on the two glasses are parallel, read off the number of divisions
in the eye-piece which cover one on the stage. Repeat this process with
different portions of the stage-micrometer, and if there be a difference, take
the mean. Suppose the hundredth of an inch on the stage requires eighteen
divisions in the eye-piece to cover it; it is plain that an inch would require
eighteen hundred, and an object which occupied nine of these divisions
would measure the two-hundredth of an inch. Take the instance supposed,
and let the microscope be furnished with a draw-tube, marked on the side
with inches and tenths. By drawing this out a short distance, the image of the
stage micrometer will be expanded until one division is covered by twenty in
the eye-piece. These will then have the value of two-thousandths of an inch,
and the object which before measured nine will then measure ten; which,
divided by 2,000, gives the decimal fraction ·005.

Enter in a table the length to which the tube is drawn out, and the number of
divisions on the eye-piece micrometer equivalent to an inch on the stage; and
any measurements afterwards taken with the same micrometer and object-
glass may, by a short process of mental arithmetic, be reduced to the decimal
parts of an inch, if not actually observed in them.

In ascertaining the value of the micrometer with a deep objective, if the
hundredth of an inch on the stage occupies too much of the field, then the
two-hundredth or five-hundredth should be used and the number of the
divisions corresponding to that quantity be multiplied by two hundred or five
hundred, as the case may be.

The micrometer should not be fitted into too deep an eye-piece, as it is
essential to preserve good definition. A middle-power Kellner or
Huyghenian is frequently employed; at all events, use the eye-piece of lower
power rather than impair the image.

The eye-lens above the micrometer should not be of shorter focus than three-
quarters of an inch, even with high-power objectives.

The Ramsden Eye-piece.—The cobweb micrometer is the most efficient
piece of apparatus yet brought into use for measuring the magnified image.
It is made by stretching across the field of the eye-piece two extremely fine
parallel wires or cobwebs, one or both of which can be separated by the
action of a micrometer screw, the trap head of which is divided into a
hundred or more equal parts, which successively pass by an index as the
milled head is turned, shown in Fig. 104. A portion of the field of view is cut
off at right angles to the filaments by a scale formed of a thin plate of brass
having notches at its edges, the distances between which correspond to the
threads of the screw, every fifth notch (as in the previous case) being made
deeper than the rest, to make the work of enumeration easier. The number of
entire divisions on the scale shows then how many complete turns of the
screw have been made in the separation of the wires, while the number of
index points on the milled head shows the value to the fraction of a turn, that
may have been made in addition. A screw with one hundred threads to the
inch is that usually employed; this gives to each division in the scale in the
eye-piece the value of 1⁄100th of an inch. The edge of the milled head is also
divided into the same number of parts.

Micrometer
scale to drop
into Eye-piece.

Fig. 104.—Ramsden Screw Micrometer Eye-piece.

In Watson’s Ramsden screw micrometer, Fig. 104, the micrometer scale
(seen detached) is ruled on a circular piece of glass, and this, by unscrewing
the top, is dropped into its place, and one of the wires, both being fixed, is
set a little to the side of the field, the teeth of the screw being cut to 1⁄100ths,
and the drum giving the fractional space between the teeth to 1⁄100ths, so that
the 1⁄10000th of an inch can be read off. This micrometer eye-piece is
constructed entirely of aluminium, a decided advantage, being so much
lighter than brass to handle.

In the screw micrometer of other makers, other modifications are found. An
iris diaphragm being placed below the web to suit the power of the eye-piece
employed, a guiding line at right angles to the web is sometimes added. Care
should be taken to see that when the movable web coincides exactly with the
fixed web, the indicator on the graduated head stands at zero.

The Compensating Eye-piece.—The very important improvements effected
in the construction of the objective naturally led up to an equally useful
change for the better in the eye-piece.

All objectives of wide aperture, from the curvature of their hemispherical
front lenses, show a certain amount of colour defect in the extra-axial
portion of the field, even if perfectly achromatic in the centre. Whether an
image be directly projected by the objective, or whether it be examined with
an aplanatic eye-piece, colour fringes may be detected, possibly in an
increasing degree towards the periphery. This residual chromatic aberration
has at length been very nearly eliminated by the aid of the compensating
eye-piece.

The construction of compensating eye-pieces is somewhat remarkable, since
they have an equivalent error in an opposite direction—that is, the image
formed by the red rays is greater than that corresponding to the blue rays;
consequently, eye-pieces so constructed serve to compensate for the unequal
magnification produced by different coloured rays, and images appear free
from colour up to the margin of the field.

Zeiss’s compensating eye-pieces are so arranged that the lower focal points
of each series lie in the same plane when inserted in the body-tube of the
microscope; no alteration of focus is therefore required on changing one eye-
piece for another. This of itself is not only an advantage but also a saving of
time, while the distance between the upper focal point of the objective and
the lower one of the eye-piece, which is the determining element of
magnification, remains constant.

Fig. 105.—A sectional view of Zeiss’s Compensating series of Eye-pieces, ½ the
full size.

A.—Plane of the upper edge of the tube.

B.—Lower focal plane of eye-pieces, with their lenses in situ.

The ordinary working eye-pieces, Huyghenian and others, commencing with
a magnification of four diameters, are so constructed that they can be
conveniently used, as we are accustomed to use them in England, with high
powers, Zeiss’s Nos. 12 and 18 compensating eye-pieces being adapted for
use with his lower power apochromatic lenses of 16 and 8 mm. The
numbering of the eye-pieces is carried out on the plan originally proposed by
Professor Abbe—that is, the number denotes how many times an eye-piece,
when employed with a given tube-length, increases the initial magnifying
power of the objective, and at the same time furnishes figures for their
rational enumeration. It is on this basis that the German compensating eye-
pieces have been arranged in series, and in agreement with their magnifying
power and distinctive numberings of 2, 4, 6, 8, 12, 18. Of these several eye-
pieces, 12 is found to be the most useful. The magnification obtained by
combining a compensating eye-piece with any apochromatic objective is
found by multiplying its number by the initial magnification of the objective,
as given in the following proof:—An objective of 3·0 mm. focus, for
example, gives in itself a magnification of 83·3 (calculated, for the
conventional distance of vision, 250 mm.); eye-piece 12 therefore gives with
this objective a magnification of 12 × 83·3 = 1000 diameters. The
classification, however, of these eye-pieces, as furnished by Abbe, is
dependent upon increase in the total magnifying power of the microscope
obtained by means of the eye-piece as compared with that given by the
objective alone. The numbering, then, denotes how many times an eye-piece
increases the magnifying power of the objective when used with a given
body-tube; the proper measure of the eye-piece magnification; and, at the
same time, the figures for rational enumeration.

Fig. 106.—B and C Achromatic Eye-pieces.

Compensating eye-pieces have been introduced for the correction of certain
errors in high-power objectives—those made with hemispherical fronts. All
such lenses, whether apochromatic or not, are greatly improved by the
compensating eye-piece, but the dry objective and the lower powers are
certainly deteriorated. The lower power compensating eye-pieces are
Huyghenian, the higher are combinations, with no field-lens, and therefore
in working act as a single or positive eye-piece. This is of importance to
those who work with low powers—the older forms of objectives.

Messrs. Watson and Swift have adopted a new formula for their series of
achromatic eye-pieces, whereby their magnification and flatness of field are
improved. These also bear a constant ratio to the initial power of their
objectives.

The compensating eye-pieces of these makers are constructed on the same
principle as those of Zeiss’s for the correction of errors of colour in the
marginal portion of the field, and consequently are in every way as effective
as those of Continental manufacture. Figs. 106, 107, and 108 show in dotted
outline the form and position of the several lenses combined in these eye-
pieces.

Projection Eye-pieces are chiefly used in micro-photography, and for screen
demonstrations. The cap of this eye-piece is provided with a spiral
adjustment for focussing, the diaphragm being placed in front of the eye-

lens, an essential arrangement for obtaining an accurate focus. The ring seen
below the cap, Fig. 108, is graduated so that the rotation for distance of
screen may be carefully recorded.

Fig. 107.—The Compensating Eye-piece. Fig. 108.—Projection Eye-piece.

Schmidt’s goniometer positive eye-piece, for measuring the angles of
crystals, is so arranged as to be easily rotated within a large and accurately
graduated circle. In the focus of the eye-piece a single cobweb is drawn
across, and to the upper part is attached a vernier. The crystals being placed
in the field of the microscope, care being taken that they lie perfectly flat, the
vernier is brought to zero, and then the whole apparatus turned until the line
is parallel with one face of the crystal; the frame-work bearing the cobweb,
with the vernier, is now rotated until the cobweb becomes parallel with the
next face of the crystal, and the number of degrees which it has traversed
may then be accurately read off.

Goniometer.—If a higher degree of precision is required, then, the double-
refracting goniometer invented by the late Dr. Leeson must be substituted.
With this goniometer (Fig. 109) the angles of crystals, whether microscopic
or otherwise, can be measured. It has removed the earlier difficulties
incident to similar instruments formerly in use. Among other advantages, it
is capable of measuring opaque and even imperfect crystals, beside
microscopic crystals and those in the interior of other transparent media. It is
equally applicable to the largest crystals, and will measure angles without
removing the crystal from a specimen, provided only the whole is placed on
a suitable adjusting stage. The value of the goniometer depends on the
application of a doubly refracting prism, either of Iceland spar or of quartz,
cut of such a thickness as will partially separate the two images of the angle
it is proposed to measure.

Dr. Leeson strongly insisted on the importance of the microscope in the
examination of the planes of crystals subjected to measurement, as obliquity
in many cases arises from not only conchoidal fractures, but also from
imperfect laminæ elevating one portion of a plane, and yet allowing a very
tolerable reflection when measured by the double refracting goniometer.

Microscopes for crystallographic and
petrological research are now
specially constructed for measuring
the angles of crystals.

Erector eye-pieces and erecting
prisms are employed for the purpose
of causing the image presented to the
eye to correspond with that of the Fig. 109.—Leeson’s Goniometer.
object. They are also helpful in
making minute dissections of structure; the loss of light, however, by
sending it through two additional surfaces is a drawback, and impairs the
sharpness of the image. Nachet designed an extremely ingenious
arrangement whereby the inverted image became erect; he adapted a simple
rectangular prism to the eye-piece. The obliquity which a prism gives to the
visual rays when the microscope is used in the erect position, as for
dissecting, is an advantage, as it brings the image to the eye at an angle very

nearly corresponding to that of the inclined position in which the microscope
is ordinarily used.

The Achromatic Objective.

Fig. 110.—Pan-aplanatic Achromatic Objectives.

The Achromatic Objective, of all the optical and mechanical adjuncts to the
microscope, is in every way the most necessary, as well as the most
important. The ideal of perfection aimed at by the optician is a combination
of lenses that shall produce a perfect image—that is, one absolutely perfect
in definition and almost free from colour. The method resorted to for the
elimination of spherical and chromatic aberration in the lens has been fully
explained in a former chapter. It will now be my endeavour to show the
progressive stages of achromatism and evolution of the microscope
throughout the present century.

It is almost as difficult to assign the date of the earliest application of
achromatism to the microscope as to that of the inception and many
modifications of the instrument in past ages; indeed, the question of priority
in every step taken in its improvement has been the subject of controversy.

Among the earlier workers in the first decade of this century will be found
the name of Bernardo Marzoni, who was curator of the Physical Laboratory
of the Lyceum of Brescia. He, an amateur optician, it has come to light, in
1808 constructed an achromatic objective, and exhibited it at Milan in 1811,
when he obtained the award of a silver medal for its merits, under the
authority of the “Institute Reale delli Scienzo.” Through the good offices of
the late Mr. John Mayall one of Marzoni’s objectives, which had been
carefully preserved, was presented to the Royal Microscopical Society of
London in 1890.20 This objective is a cemented combination, with the plane
side of the flint-lens presented to the object. This was an improvement of a
practical kind, and of which Chevalier subsequently availed himself. In 1823
Selligue, a French optician, is credited with having first suggested the plan
of combining two, three, or four plano-convex achromatic doublets of
similar foci, one above the other, to increase the power and the aperture of
the microscope. Fresnel, who reported upon this invention, preferred on the
whole Adam’s arrangement, because it gave a larger field. Selligue
subsequently improved his objective by placing a small diaphragm between
the mirror and the object.

In this country, Tully was induced by Dr. Goring to work at the achromatic
objective, and his first efforts were attended with a success quite equal to
that of Chevalier’s. Lister on examining these lenses said:—“The French
optician knows nothing of the value of aperture, but he has shown us that
fine performance is not confined to triple objectives.” Amici, the amateur
optician of Modena, visited this country in 1827 and brought his achromatic
microscope and objectives, which were seen to give increase of aperture by
combining doublets with triplets. The most lasting improvement in the
achromatic objective was that of Joseph Jackson Lister, F.R.S., the father of
Lord Lister, and one of the founders of the Royal Microscopical Society of
London.

Lister’s discoveries at this period (1829) in the history of the optics of the
microscope were of greater importance than they have been represented to
be. That he was an enthusiast is manifest, for, being unable to find an
optician to carry out his formula for grinding lenses, he at once set to work
to grind his own, and in a short time was able to make a lens which was said
to be the best of the day.

Lister, in a paper contributed to the proceedings of the Royal Society the
same year, pointed out how the aberrations of one doublet could be
neutralised by a second. He further demonstrated that the flint lens should be
a plano-concave joined by a permanent cement to the convex crown-glass.
The first condition, he states, “obviates the risk of error in centring the two
curves, and the second diminishes by one half the loss of light from
reflection, which is very great at the numerous surfaces of every
combination.” These two conditions then—that the flint lens shall be plano-
concave, and that it shall be joined by some cement (Canada balsam) to the
convex—may be taken as the basis for the microscopic objective, provided
they can be reconciled with the correction of spherical and chromatic
aberration of a large pencil.

Andrew Ross was not slow to perceive the value of Lister’s suggestions and
in 1831 he had constructed an object-glass on the lines laid down by Lister,
Fig. 112; a a′ representing the anterior pair, m the middle, and p the
posterior, the three sets combined forming the achromatic objective,
consisting of three pairs of lenses, a double-convex crown-glass, and a
plano-concave of flint.

Fig. 111.—Lister’s double-convex crown and plano-concave Fig. 112.—Andrew Ross’s ¼-
flint cemented combination. inch Objective.

Lister proposed other combinations, and himself made an object-glass
consisting of a meniscus pair with a triple middle, and a back plano-convex
doublet. This had a working distance of ·11 and proved to be so great a

success that other opticians—Hugh Powell, 1834; James Smith, 1839—
made objectives after the same formula.

The publication of Lister’s data proved of value in another direction: it
stimulated opticians to apply themselves to the further improvement of the
achromatic objective. Andrew Ross was one of the more earnest workers in
giving effect to Lister’s principles and a short time afterwards found that a
triple combination, with the lenses separated by short intervals, gave better
results. In the accompanying diagram the changes made in the combination
of the objective from 1831, and extending over a period of about twenty
years from this date, are shown.

Each objective, from the ½-inch to the 1⁄12-inch, is seen to be built up of at
least six or eight different fronts, the back combinations being a triplet
formed of two double-convex lenses of crown glass with an intermediary
double concave lens of flint-glass.

Fig. 113.—Combinations of Early Dry Objectives.

A, Double-convex lens; B, Plano-concave; C, Bi-convex and plano-concave united;
shown in their various combinations, as at D, form the 3-in., 2-in. or 1½-in.; at E,
1-in. and 2⁄3-in.; and at F, the ½-in., 4⁄10-in., ¼-in. and 1⁄25-in. objectives.

Combination D was for many years known as the Norfolk Objective.

No sooner had Ross constructed ¼-
inch achromatic objectives on Lister’s
formula than he discovered an error
which had hitherto escaped attention,
viz., that the thinnest cover-glass of an
object produced a considerable
amount of refractive disturbance. A
marked difference was observed in the
image when viewed with or without a
cover-glass. This difficulty was first
met by the addition of a draw-tube to Fig. 114.—Lister’s CorrectionCollar, (in section).
the microscope body. But as this also
impaired the image, Lister overcame the difficulty by mounting the front
lens of the objective in a separate tube made to fit over a second tube
carrying the two pairs of lenses. This arrangement led up to his invention of
the screw-collar adjustment, the mechanism for applying which is shown in
Fig. 114. The anterior lens a at the end of the tube is enclosed in a brass-
piece b containing the combination; the tube a, holding the lens nearest the
object, is then made to move up or down the cylinder b, thus varying the
distance, according to the thickness of the glass covering the object, by
turning the screw ring c, thus causing the one tube to slide over the other,
and clamping them together when properly adjusted. An aperture is made in
the tube a, within which is seen a mark engraved on the cylinder, on the edge
of which are two marks, a longer and a shorter, engraved upon the tube.
When the mark on the cylinder coincides with the longer mark on the tube,
the adjustment is made for an uncovered object; and when the coincidence is
with the shorter mark, the proper distance is obtained to balance the
aberrations produced by a cover-glass the hundredth of an inch thick; such
glass covers are now supplied. The adjustment should be tested
experimentally by moving the milled edge which separates or closes the
combinations, and at the same time using the fine adjusting screw of the
microscope. The difficulty associated with the cover-glass of old has, by the
introduction of the homogeneous immersion system, been very nearly
eliminated. There still remains, however, a disturbing amount of residual
colour aberration in the achromatic dry objective, and for the correction of
which Zeiss proposed mounting the several lenses on a method somewhat
different to that so long in use in this country. Fig. 115 shows an objective in

which the screw-collar ring b b is made to adjust the exact distance between
the two back lenses placed at a a. The value of the screw-collar is not
questioned. It is difficult to obtain at all times cover-glasses of a perfectly
uniform thickness; they will vary, and therefore perfect definition must be
obtained, as heretofore, by adjusting for each separate preparation while the
object is under examination.

As early as 1842 the excellence of
Andrew Ross’s achromatic objectives
were acknowledged, and his formula
for their construction was generally
followed. No doubt many of these early
objectives of his manufacture are still
regarded as treasures. I possess a ½-
inch and a ¼-inch, which I believe to be
comparable with any achromatic
objectives of the same apertures of the
present day. These I have always found
most serviceable for histological work.

In 1850 Mr. Wenham produced an
Fig. 115.—The Continental Screw-collar
achromatic objective of considerable
Adjustment.
achromatic value. This consisted of a
single hemispherical front combination,
shown in the accompanying enlarged diagram, Fig. 116. Wenham’s formula
seems to have been generally adopted by Continental opticians, who sold
these lenses at a reduction of price. In Paris, Prazmowski and Hartnack—I
have had one of Hartnack’s earliest immersions in use for many years—
brought this form of objective to greater perfection, and in 1867 Powell and
Lealand adopted the single front combination system in their early water-
immersion objective, whereby the focal distance was said to be “practically a
constant quantity, while reduction of aperture by making the front lens
thinner ensures a much greater working distance without affecting the
aberrations, since the first refraction takes place at the posterior or curved
surface of the front lens, the removal of any portion of thickness at the
anterior or plane surface simply cuts off zones of peripheral rays without
altering the distance—any space being filled by the homogeneous immersion
fluid, or by an extra thickness of cover-glass.”21

Great improvements were brought
about by R. B. Tolles, of Boston,
1874, in the objective, as well as in
the optical and mechanical parts of
the microscope, most of which,
however, must be ascribed to the
criticisms and suggestions of amateur
workers skilled in the exhibition of
test-objects—the late Dr. Woodward
of Washington, for example, whose
series of photographs of the more
difficult frustules of diatoms have
rarely been surpassed. Such results
were due to improvements made in Fig. 116.—A Single Front Combination
the optical part of the microscope at formulated by Wenham for Messrs. Ross
his suggestion. He came to the (enlarged).
conclusion, arrived at about the same
time by mathematical scientists, that increase of power in the microscope
was only possible in two directions, the qualitative and the quantitative.

It was now that microscopists turned to the late Professor Abbe for
assistance in perfecting the objective in the dioptric direction. This, he
pointed out, must be looked for in further improvements in the art of glass-
making.

A series of experiments ultimately brought to light a mineral substance,
Fluorite, which, when combined in the proper proportion, one part to two of
German crown and flint glass, was found to have the qualities looked for,
and to possess different relations of a dispersive and refractive power. From
Professor Abbe’s researches, begun in 1876, we have had the aperture of the
objective greatly enlarged, and the homogeneous system brought into
general use.

Previous to this date the best made objective merely approximated to colour
correction. Undoubtedly the chief object to be obtained was the removal or
diminution of the secondary colour aberration. This, together with other
residual errors Abbe pointed out in 1880, led to the improvement of the
optical quality of the glass used in the manufacture of all optical instruments,

the chief difficulties being surmounted in the Jena glass factory, whereby a
complete revolution was effected in the microscopic objective. The
apochromatic glasses of Zeiss, Powell, Beck, Ross, Watson, Swift, and other
makers, in which the secondary spectrum has been totally eliminated, or
only a negligible tertiary spectrum remains—that is to say, the objectives of
these makers—are now corrected for three spectrum rays, and not two, as in
the older objectives; and only those who look forward for making further
discoveries in the intimate structure of bacilli or for resolving the finest
diatom markings can be said to fully appreciate the importance and value of
the investigations of the late Professor Abbe, and which have, so to speak,
entirely changed old empirical views as to the value of high aperture, and
demonstrated that high amplification, unless associated by proportionally
high aperture, necessarily produces untrue images of minute structures. It
was he also who introduced a practically perfect system of estimating
apertures, known as the “numerical aperture notation,” by which not only
can an accurate comparison be made of the relative apertures of any series of
objectives, whether dry or immersion, but their resolving power under the
various conditions of the kind of light employed. Their penetrating power
and their illuminating power can now be estimated with mathematical
exactness.

The practical advantages, then,
secured by the adoption of the
homogeneous system were, on the
whole, greater than any before made
or believed to be possible, and when
taken into account in connection with
the improvement of the eye-piece
(also due to Abbe), almost perfect
achromatism and homogeneity
between objective, object, and eye-
piece is secured, together with a sharp
definition of the image over the whole
visual field. These, with an increase
Fig. 117.—Diagram of an Apochromatic
of working distance between the Combination.
object and the objective, and other
important results, have been placed within the reach of the microscopist by
men of science, and the outcome is the general adoption of the homogeneous

system, termed by Carl Zeiss, a fellow-worker with Abbe, the22
apochromatic system of constructing objectives.

Relative Merits of the English and German Objectives.

As to the relative merits of German-made objectives, no superiority can be
claimed for them over those made by English opticians.

The Continental form of the 1⁄12-inch oil-immersion objective, shown in Fig.
118, on the scale of 6 to 1, consists of four systems of lenses, namely, the
front, a deep hemispherical crown lens of high refractive index; the second
front of the system, an achromatic lens of such a form that it gathers the light
from the hemispherical front; the middle lens, a single meniscus; and the
back an achromatised lens, the second front of the back being connected in
such a way as to compensate for the spherical and chromatic aberrations of
the front lens.

The first homogeneous immersion objective which came under my
observation was manufactured in the well-known Jena workshop of Carl
Zeiss, December, 1877. This had a very considerable increase of numerical
aperture, upwards of 50 per cent.; a clear gain, as an oil angle of even 110°
proved to be of greater value than an angle of 180° in air, while the resolving
power of the objective was increased in like proportion. There does not at
present appear to be a bar to the construction of objectives of yet higher
power, with increase of aperture. The available course open in this direction
is the further discovery of another vitreous material and a suitable immersion
fluid with an index of 1·8 or 1·9, and glass with a corresponding index, so as
to ensure homogeneity of the combination. Zeiss asserts that in the more
difficult departments of microscopical research the apochromatic lenses will
supplant the older objectives, yet there are many problems in microscopy
awaiting solution which do not demand the highest attainable degree of
perfection in the objective, and in the majority of cases the older achromatic
objective is all that is needful, provided it is good of its kind. The achromatic
objectives and eye-pieces of the older type have still an advantage, as, owing
to their simpler construction, really good lenses of the class required can be
purchased at considerably lower prices than the objectives of the new series.

These, from being more complicated in construction, involve a greater
amount of skilled manual labour.

The German glasses of to-day
afford satisfactory evidence both
of skill and workmanship
displayed in their production.
Their cost is greater, then, for the
reason given, as will be seen on
reference to Continental
catalogues. The dry series of
objectives cost somewhat less, a
½-inch (numerical aperture 0·30)
can be had for £1 10s., and a 1⁄6-
inch (numerical aperture 0·65) for
£2. On the other hand, the
apochromatic series rapidly
increase in price as the numerical
Fig. 118.—The Continental 1⁄12-in. Oil-immersion
Combination (enlarged diagram). aperture approaches the limit of
numerical aperture 0·40. The best
of Zeiss’s series are the 12 mm. (½-inch) and the 3 mm. (1⁄8-inch), numerical
aperture 1·4, both of which possess the optical capacity assigned to them.
These objectives are undoubtedly the finest to be met with in the workshop
of any optician. Achromatic objectives of Continental manufacture have
been as much improved as those of English make by the introduction of the
newer varieties of glass, as already explained, while a new nomenclature has
sprung up in consequence. We now have semi-apochromatic and
parachromatic. The German opticians have followed Zeiss’s lead, since
almost the same series of objectives are given in the catalogues of Leitz,
Reichert, and Seibert, while the quality of both dry and immersion objectives
is found to be much the same. The low price of Reichert’s immersion
objectives should be noted, as their performance is quite perfect. A 1⁄12-inch
(numerical aperture 1·30) of Leitz’s, with which I have worked at bacteria,
has given me much satisfaction; supplied by Watson and Baker at £5. A 1⁄12-
inch dry objective by the same maker (numerical aperture 0·87) costs £3,
and a water immersion 1⁄12-inch (numerical aperture 1·10) £3 5s. Leitz
reminds me that it requires a good lens of from six to seven hundred
magnifying power for the examination of bacteria. For this reason he has

constructed a new form of lens, a 1⁄10-inch oil-immersion of 2·5 mm. focus,
for the purpose of adding to the resources of bacteriology. This lens
necessarily has a lower magnification than his former 1⁄12-inch oil-lens, but as
it is less costly to manufacture it is sold at a smaller price. The before-
mentioned 1⁄12-inch, with a No. 3 compensating eye-piece, gives a
magnification of over seven hundred or eight hundred diameters. To secure
the best results in using the higher powers of Leitz’s, from No. 5 upwards, a
cover-glass of 0·17 mm. in thickness should be used, and care taken to make
the length of the draw-tube equal to 170 mm. This length of tube should be
adhered to in the use of this optician’s oil-immersion lenses. If the
microscope be provided with a nose-piece, the draw-tube should be drawn
out to 160 mm.; in its absence it should be set at 170 mm., a deviation of 10
mm. or more from the correct tube-length deteriorates from the value of
Leitz’s oil-immersion objectives as of other opticians. It is suggested that the
German apochromatic combination of three cemented lenses is that adopted
by Steinheil long before, in the construction of his well-known hand-
magnifier (see page 77, Fig. 51). Zeiss’s 3 mm. objective has a triple front,
balanced by two triple backs—in all nine lenses—a somewhat amplified
diagram of which is represented in Fig. 118. The formula for this
combination was furnished by Tolles, of Boston, America, and it at once
secured increase of aperture (the value of this optician’s many contributions
to microscopy has since his death been generally acknowledged). The
metrical equivalent focus assigned by Zeiss to his series of dry achromatic
objectives is given in somewhat ambiguous terms, which tend to confuse
rather than classify them; for instance, two lenses of the same aperture—24
mm. and 16 mm.—corresponding to the English 1-inch and 2⁄3-inch, each
have assigned to them an aperture of 0·30; a 12 mm. and 8 mm.,
corresponding to the English ½-inch and 1⁄3-inch, have an aperture of 0·65;
while a 6 mm. = ¼-inch, and a 4 mm. = ¼-inch and 1⁄6-inch, have each an
aperture of 0·95.

Nachet exhibited at the Antwerp Exhibition a fine 1⁄10-inch oil-immersion,
which was highly praised by the jurors.

It is necessary, to make the fact perfectly clear, that dry and immersion
lenses having the same angular aperture have also a similar defining power.
The pencil of rays, however, differs in intensity and density as the rays
emerging from the cover-glass of the object into air are very considerably

deflected, and the cone suffers a corresponding loss of brightness. On this
important point, then, I believe it will prove of value to interpolate a clear
and full exposition of the change brought about by the cover-glass.

It is not difficult, then, to perceive the importance of Amici’s discovery as to
the value of a drop of water inserted between the object and the objective,
and it now seems somewhat surprising it should have been so long neglected
by opticians, since it is at once seen to diminish the reflection which takes
place in the incidence of oblique light. The film of water not only gives
increased aperture, but also greater cleanness and sharpness to the image.
The film, then, as already shown, collects the straying away of peripheral
rays of light, and sends them on to the eye-piece, and greatly assists in
rendering the image more perfect, and materially aids in the removal of
residuary secondary aberrations; while with air, or dry objectives, a certain
amount of aberration takes place, sufficient to affect the pencils on their
passage from the radiant to the medium of the front lens, adding a
considerable ratio to the total spherical aberration with the objective, which,
in the case of wide angles, increases disproportionately from the axis
outwards. This can only be corrected by a rough method of balancing; that
is, by introducing an excess of opposite aberration in the posterior lens. An
uncorrected residuum, rapidly increasing with larger apertures, is then left,
and this appears in the image amplified by the total power of the objective,
so that with a non-homogeneous medium there is a maximum angular
aperture which cannot be surpassed without undergoing a perceptible loss of
definition, provided working distance is required. If we abolish the anterior
aberration for all colours, by an immersion fluid which is equal to cover-
glass in refractive and dispersive power, the difficulty is at once overcome.
If, for instance, we have an objective of 140° in glass (= 1·25 N.A.) and water
as the immersion fluid, the aberration in front would affect a pencil of 140°.
Substituting a homogeneous medium, the same pencil, contracted to the
equivalent angle in that medium of 112°, will be admitted to the front lens
without any aberration, and may be made to emerge from the curved surface
also without any disturbing aberration, but contracted to an angle varying
from 70° to 90°. The first considerable spherical aberration of the pencil then
occurs at the anterior surface of the second lens, where the maximum
obliquity of the rays is already considerably diminished.

Fig. 119. Fig. 119a.

Figs. 119 and 119a will doubtless make this clearer. If the objective of 140°
works with water (Fig. 119), there would be a cone of rays extending up to
70° on both sides of the axis, and this large cone would be submitted to
spherical aberration at the front surface a. But with homogeneous
immersion Fig. 119a) the whole cone of 112° is admitted to the front lens
without any aberration, there being no refraction at the plane surface; and as
the spherical surface of the front lens is without notable spherical aberration,
the incident pencil is brought from the focus F to the conjugate focus F′, and
contracted to an angle of divergence of 70°-90° without having undergone
any spherical aberration at all.

The problem of correcting a very wide-angled objective has thus been
reduced by the homogeneous oil-immersion system, both in theory and
practice.23

Abbe’s Test-plate.

Abbe designed the test-plate (Fig. 120) for testing the spherical and
chromatic aberrations of objectives, and estimating the thickness of cover-
glasses corresponding to the most perfect correction: six glasses, having the
exact thickness marked on each, 0·09 to 0·24 mm., cemented in succession
on a slip, their lower surface silvered and engraved with parallel lines, the
contours of which form the test. These being coarsely ruled are easily
resolved by the lowest powers; yet, from the extreme thinness of the silver,
they form also a delicate test for objectives of the highest power and widest

aperture. The test-plate in its original size is seen in Fig. 120, with one of the
circles enlarged.

Fig. 120.—Abbe’s Test-plate for estimating thickness of glass-covers.

To examine an objective of large aperture, the discs must be focussed in
succession, observing in each case the quality of the image in the centre of
the field, and the variation produced by using, alternately, central and very
oblique illumination.

When the objective is perfectly corrected for spherical aberration, the
outlines of the lines in the centre of the field will be perfectly sharp by
oblique illumination, and without any nebulous doubling or indistinctness of
the edges. If, after exactly adjusting the objective for oblique light, central
illumination is used, no alteration of the focus should be necessary to show
the outlines with equal sharpness.

If an objective fulfils these conditions with any one of the discs, it is free
from spherical aberration when used with cover-glasses of that thickness. On
the other hand, if every disc shows nebulous doubling, or an indistinct
appearance of the edges of the line with oblique illumination, or, if the
objective requires a different focal adjustment to get equal sharpness with
central as with oblique light, the spherical correction of the objective is more
or less imperfect.

Nebulous doubling with oblique illumination indicates over-correction of the
marginal zone; indistinctness of the edges without marked nebulosity
indicates under-correction of the zone; an alteration of the focus for oblique

and central illumination points to an absence of concurrent action of the
separate zones, which may be due to either an average under or over
correction, or to irregularity in the convergence of the rays.

Fig. 121.—Zeiss’s Cover-glass Gauge.

COVER-GLASS GAUGE.

Zeiss has gone a step further to lay the microscopist’s ghost of the cover-
glass. He invented a measurer (Fig. 121) whereby the precise determination
of thickness of glass-covers can be obtained. This measurement is effected
by a clip projecting from a circular box; the reading is given by an indicator
moving over a divided circle on the lid of the box. The divisions seen cut
round the circumference show 1⁄100ths of a millimeter. This ingenious gauge
measures upwards of 5 mm.

This necessary and important digression has led me away from the
consideration of the achromatic objective, and to which I shall now return.

English Immersion and Dry Objectives.

The homogeneous immersion system met with its earliest as well as its
staunchest advocates among English opticians. Among its more energetic
supporters were Messrs. Powell and Lealand, who were the first to construct
a 1⁄8-inch immersion objective on a formula of their own, and which was
found to resolve test-objects not before capable of resolution by their dry
objectives. This encouraged them to make a 1⁄16-inch, acquired by Dr.
Woodward for the Army Medical Department, Washington, and
subsequently a 1⁄25-inch; neither of which surpassed their 1⁄8-inch in aperture,
and a new formula was tried in the construction of their first oil-immersion
objective. This had a duplex front, and two double backs; but even this did
not quite accomplish what was expected of it, and another change was
subsequently made; the anterior front combination became greater than a
hemisphere—a balloon-lens. This at once gave an increase of aperture to a
1⁄12-inch objective of 1·43 numerical aperture. After some few more trials a
more important change of the formula took place. The front lens was made
of flint-glass, and the combination took the form represented in diagram
(Fig. 122). This, on an enlarged scale, represents Powell’s 1⁄12-inch numerical
aperture 1·50. It is a homogeneous apochromatic immersion of high quality
and very flat field. It will be noticed that in this combination the four curves
of the lenses are very deep compared with those of other opticians.

Messrs. Ross have made many
important improvements and changes
in the construction of their several
series of achromatic objectives; the
calculations and formulæ for which
were made exclusively for them by
Dr. Schrœder. The list is too long to
quote, but most of these lenses are of
a high-class character, and work with
admirable precision. Among the best
of their objectives, I can commend a
1-inch of 30° and two oil-immersions,
a 1⁄8-inch of 1·20 and a 1⁄12-inch of
1·25 numerical aperture, each of
Fig. 122.—Powell and Lealand’s 1⁄12-in. Oil- which bear the highest oculars equally
immersion Objective, drawn on a scale of 6-1. well; a good test, as I have always
maintained, of excellence. Their 1⁄10-

inch has a somewhat larger aperture, and therefore shows a fine image of the
podura scale. The finish of Ross’s several series of objectives fully maintains
the high character and reputation of this old-established firm of opticians.

Messrs. R. and J. Beck have bestowed great attention upon the improvement
of their dry-objective series, much in demand for histological work,
especially among the students of city hospitals, who usually commence their
pathological work with the cheaper forms of objectives. In that case an inch
objective of about 25° air angle, a ½-inch of not less than 40°, and a ¼-inch
or 1⁄5-inch magnifying from 50 to 250 diameters, is quite sufficient for most
of their work. For bacteriological research, Messrs. Beck supply a 1⁄6-inch
immersion taken from a series, having a high aperture and a better finish at a
moderate price. Their 1⁄10-inch immersion has in my hands proved a
serviceable power for bacteriological research; it requires a good sub-stage
illuminating achromatic condenser to obtain the best results.

Messrs. Watson and Sons have much
enhanced their reputation by the
marked improvement lately brought
about in the manufacture of their
whole series of objectives. This
probably is chiefly due to the
introduction of the Jena glass into
their manufacture, and which has
enabled them to give increase of
aperture to one series in particular,
that of the para-chromatic, all of
which in consequence are of very
high quality. It is difficult to
particularise their several objectives,
the whole having special features in Fig. 123.—1⁄6-in. English Combination, largely
proportion to their magnifying used.
powers, while much care seems to
have been bestowed on them for the elimination of residual colour. A 1⁄8-inch
with correction collar is comprised of a single deep and rather thick front
lens, plano-concave flint, and double convex-crown for the middle and triple
combination for the back, the latter consisting of two crown lenses cemented
to a dense flint (Fig. 124) drawn to scale of 5-1, with lined portions intended

to represent the flint, and white the crown glass lenses of the combination.
The initial magnification of this objective is 83 diameters, and the numerical
aperture ·94. This superior objective can be had for the small sum of £2.
Another remarkably useful and cheap objective, their 1-inch numerical
aperture 0·21, consists of two achromatic systems forming the front and
back with the separation between them of about half an inch, and may also
be especially recommended for students’ work.

In the accompanying diagram the lenses are drawn on too large a scale, and
therefore the distance between the two combinations should be much greater.

Among the more useful of Watson’s series, the 1-inch, the ½-inch, and the
1⁄6-inch, together with the 1⁄8-inch dry-objective, and a 1⁄9-inch, will be found
the most serviceable.

Messrs. Baker have their own series of
objectives, most of which are so very
nearly allied to those of the continental
opticians; and what has been said of
Zeiss’s and Leitz’s objectives may be
taken to apply also to Baker’s, who
have an established reputation for their
histological series, all of which are well
suited for students’ and class-room
work.

Messrs. Swift and Son have a new series
of objectives, semi-apochromatic and
pan-aplanatic, most of which are
excellent in quality and show increased
Fig. 124.—Watson’s 1⁄8-in. Objective Para-
chromatic Combination, scale 5-1. flatness of field together with that of
achromatism; the index of refraction in
each series having been correctly determined together with exact radial focal
distance, thus affording more available aperture. I may select for special
commendation their 1⁄12-inch £5 5s. homogeneous immersion objective,
which is in every way suitable for bacteriological work; its definition is very
good, as is seen in a micro-photograph of podura scale, given further on.
Their dry 1⁄6-inch can be had for £1 16s.—a marvel of cheapness. Of their

general series the most useful for
histological work are the ½-inch, the
1⁄3-inch at £1 12s., and their 1⁄5-inch of
numerical aperture 0·87 at £3.

Mr. Pillischer, of Bond Street, has
manufactured many excellent
objectives. A fine homogeneous oil-
immersion 1⁄12-inch numerical
aperture 1·25 is worthy of special
notice; it will be found suitable for
bacteriological work; it has fine
definition with a considerable amount
of penetration.
Fig. 125.—Watson’s 1-in. Achromatic
Combination. A more intelligent idea of the
magnifying power of the objective
combined with the eye-piece will be gained by consulting the table given
below; precision in this respect has long been a desideratum with
microscopists.

Magnifying Powers of Eye-Pieces and Objectives.

A TYPICAL AND INITIAL SELECTION OF POWERS OF EYE-PIECES
CALCULATED FOR THE 10-INCH TUBE-LENGTH.

Huyghenian Eye-pieces.
Name A B C D E F
of Maker. 0 or No. 1 2 3 4 5 6
Baker 6 8 12 15 — — Diameters.
Beck, R. & J. 4 8 15 20 25 not made. "
Leitz 5 6 7 8 10 12 "
Powell & Lealand 5 7·5 10 20 40 " "
24
Reichert 2·5 3·5 4 5 6·5 " "

25
Ross 3 8 12½ 20 25 40 "
Swift & Son 6 9 12 15 18 21 "
Watson & Sons 4 6 8 10 12 15 "
Zeiss 3 4 5·5 7 9 not made. "

Compensating Eye-pieces for use with Apochromatic Objectives.

Zeiss 2 4 8 12 18 27 Diameters.

This may be taken as a typical set, further treated of among Eye-pieces.

Initial Powers of Objectives calculated for the 10-inch Tube-
length.

This is ascertained by dividing the distance of distinct vision 10 inches by
the focus of the objective, thus—

Focus-
4 3 2 1½ 1 2⁄3 ½ 4⁄10 ¼ 1⁄5 1⁄6 1⁄8 1⁄12
inches
Initial
magnifying 2·5 3·3 5 7·5 10 15 20 25 40 50 60 80 120 diameters.
power

A reference to the above table will at once show that the nomenclature of
objectives expresses at once the initial magnifying powers, but as makers
have great difficulty in so calculating their formulæ so as to obtain the exact
power, these figures must be taken as approximate. Thus a ¼-inch, which
should magnify 40 diameters if true to its description, might actually
magnify a little more or less.

The magnifying powers of Zeiss’s and other apochromatic objectives can be
ascertained by dividing the focal length of the objective in millimeters into
250 mm. (the distance of distinct vision), thus

Focus millimetres 24 16 12 4 3 2 1·5

Initial magnifying power 10·5 15·5 21 63 83 125 167 diameters.

The total magnification, when any eye-piece is working in conjunction with
an objective, is ascertained by multiplying the initial power of the objective
by that of the eye-piece.

The above calculations are all for a 10-inch tube-length. Should, however, a
shorter or longer length of body be employed, the magnification can at once
be ascertained by a proportion sum. If the magnification be 180 with 10-inch
tube-length, what would it be with a 6-inch body—10 : 6 :: 180 = 108
diameters.

Abbe designed three different forms of eye-pieces: 1, the searcher eye-piece;
2, the working eye-piece; and 3, the projecting eye-piece. The Searcher is a
negative form of low power. The working is both negative and positive, the
positive form of which is constructed on a newer principle; while the
projection is chiefly intended for microphotography, its field being small and
its definition superlatively sharp. These are severally explained among eye-
pieces.

High-Power Objectives.

Points of Importance for securing the best results with High-power
Objectives.—Always give to the body-tube of the microscope the length for
which the objective is corrected, 0·160 mm. for the short continental tube,
and 0·250 mm. for the English tube (10-inch). Employ both dry and
immersion objectives mounted for correction, commencing with a numerical
aperture of 0·75 (that is about 100° in air). If the graduation is not given in
thickness of cover-glass apply to the maker to correct this omission.

With the homogeneous oil-immersion objective it is highly necessary to
utilise all marginal pencils of light, to optically unite the upper lens of the
condenser with the preparation as well as the front lens of the objective by
means of a liquid having the same index of refraction or at least equal to that
of the immersion. Cedar Oil has been generally adopted for the purpose
mentioned, the better way of using which is as follows: place a drop on the
centre of the front objective, or on the top of the cover-glass, and then lower

the objective by means of the coarse adjustment until it comes in contact
with the oil, and carefully bring into focus by the fine adjustment. If the slide
is held between the finger and thumb of one hand and moved from side to
side, while the other hand is working the fine adjustment, there can be no
danger of injuring either the objective or the specimen. Before putting the
microscope away, take a fine camel-hair brush dipped in ether, alcohol, or
methylated spirit, and carefully remove the oil from the objective and the
glass cover of the object; a soft chamois leather or cambric pocket
handkerchief will dry it off, or a piece of fine white blotting paper answers
equally well. Should the lens come accidentally into contact with the Canada
balsam, it must be very carefully removed either by ether or alcohol. The
former is by far the safest, as alcohol, if not very carefully used, quickly
dissolves out the balsam and loosens the cover-glass of the object.

Achromatic Condensers.

The Achromatic Condenser can no longer be classed among the accessories
of the microscope, since it is an absolutely indispensable part of its optical
arrangements. Its value, then, cannot be overrated, and the corrections of the
lenses which enter into the construction of the condenser should be made as
perfect as they can be made—in fact, as nearly approaching that of the
objective as it is possible to make them. It may therefore be of interest to
know something of the rise and progress of the achromatic condenser. In my
first chapter I have noticed the earlier attempts made by Dr. Wollaston,
whose experiments led him to fit to the underside of the stage of his
microscope a short tube, in which a plano-convex lens of about three-
quarters of an inch focal length was made to slide up and down (afterwards
moved up and down by two knobs); to improve definition he placed a stop
between the mirror and the lens. The stop was found to act better when
placed between the lens and the object. From this improvement Dr.
Wollaston enunciated that “the intensity of illumination will depend upon the
diameter of the illuminating lens and the proportion of the image to the
perforation, and may be regulated according to the wish of the observer.”
Dujardin in France and Tully in England were at work in the same direction.
The former a year or two later on contrived an instrument, which he termed
an eclairage, to remedy the defects of Wollaston’s, and for illuminating

objects with achromatic light. This was submitted for approval to Sir David
Brewster, who, when the use of the achromatic condenser was first
broached, used these encouraging words:—“I have no hesitation in saying
that the apparatus for illumination requires to be as perfect as the apparatus
for vision, and on this account I would recommend that the illuminating lens
should be perfectly free from chromatic and spherical aberration, and that
the greatest care be taken to exclude all extraneous light both from the object
and eye of the observer.” This far-seeing observer in optical science has
borne good fruit, and the outcome of his views is seen in the great
development and improvement of the achromatic condenser. In 1839
Andrew Ross made his first useful form of condenser, and gave rules for the
illumination of objects in an article written for the “Penny Cyclopædia.”
These, epitomised, read as follows: 1. That the illuminating cone should
equal the aperture of the objective, and no more. 2. With daylight, a white
cloud being in focus, the object has to be placed nearly at the apex of the
cone. The object is seen better sometimes above and sometimes below the
apex of the cone. 3. With lamplight a bull’s-eye lens is to be used, to
parallelise the rays, so that they may be similar to those coming from the
white cloud. It has been seen that Mr. Lister foreshadowed the sub-stage
condenser.

The early form of Ross’s condenser consists of two small brass tubes made
to slide one in the other. To the outer one is attached a flat brass plate which
slides underneath the stage of the microscope, and by means of a screw the
adjustment of the axis of the illuminator is effected. The upper portion of the
apparatus carries the achromatic combination, which by a rack and pinion
movement is brought nearer to, or removed further from the object on the
stage. The several parts of the illuminator unscrew, so that the lenses may be
used either combined for high powers, or separated for low powers.

Messrs. Smith & Beck greatly improved upon Ross’s condenser by adding
another achromatic lens to the combination, three being employed when
used with high-power objectives and two or even one with the lower, the
adjustment and focussing being made by rack and pinion arrangement
beneath the stage. Some further changes for the better were made in the
condenser by Powell, and in 1850 an amateur microscopist, Mr. Gillett, fully
grasping the value of controlling the cone of rays passing into the
microscope, devised a new form of condenser, in connection with which a

revolving series of diaphragms of
different values were made to pass
between the achromatic lenses and the
source of light.

Andrew Ross constructed the first
condenser on Gillett’s principle, and
this proved to be one of the most
successful pieces of apparatus
contrived. Gillett’s Condenser
consists of an achromatic lens c,
about equal to an object-glass of one
quarter of an inch focal length, with
an aperture of 80°. This lens is
Fig. 126.—Original form of Gillett’s Achromatic
screwed into the top of a brass tube, Condenser.
and intersecting which, at an angle of
about 25°, is a circular rotating brass plate a b, provided with a conical
diaphragm, having a series of circular apertures of different sizes h g, each
of which in succession, as the diaphragm is rotated, proportionally limits the
light transmitted through the illuminating lens. The circular plate in which
the conical diaphragm is fixed is provided with a spring and catch e f, the
latter indicating when an aperture is central with the illuminating lens, also
the number of the aperture as marked on the graduated circular plate. Three
of these apertures have central discs for circularly oblique illumination,
allowing only the passage of a hollow cone of light to illuminate the object.
The illuminator above described is placed in the secondary stage i i, which is
situated below the general stage of the microscope, and consists of a
cylindrical tube having a rotatory motion, also a rectangular adjustment,
which is effected by means of two screws l m, one in front, and the other on
the left side of its frame. This tube receives and supports all the various
illuminating and polarising apparatus, and other auxiliaries.

Directions for using Gillett’s Condenser.—In the adjustment of the
compound body of the microscope for using with Gillett’s illuminator, one
or two important points should be observed—first, centricity; and secondly,
the fittest compensation of the light to be employed. With regard to the first,
place the illuminator in the cylindrical tube, and press upwards the sliding
bar k in its place, until checked by the stop; move the microscope body

either vertically or inclined for convenient use; and, with the rack and pinion
which regulates the sliding bar, bring the illuminating lens to a level with the
upper surface of the object-stage; then move the arm which holds the
microscope body to the right, until it meets the stop, whereby its central
position is attained; adjust the reflecting mirror so as to throw light up the
illuminator, and place upon the mirror a piece of clean white paper to obtain
a uniform disc of light. Then put on the low eye-piece, and a low power (the
half-inch), as more convenient for the mere adjustment of the instrument;
place a transparent object on the stage, adjust the microscope-tube, until
vision is obtained of the object; then remove the object, and take off the cap
of the eye-piece, and in its place fix on the eye-glass called the “centring
eye-glass,”26 which will be found greatly to facilitate the adjustment now
under consideration, namely, the centring of the compound body of the
microscope with the illuminating apparatus of whatever description. The
centring-glass, being thus affixed to the top of the eye-piece, is adjusted by
its sliding-tube (without disturbing the microscope-tube) until the images of
the diaphragms in the object-glass and centring lens are distinctly seen. The
illuminator should now be moved by means of the left-hand screw on the
secondary stage while looking through the microscope, to enable the
observer to recognize the diaphragm belonging to the illuminator, and by
means of the two adjusting screws to place this diaphragm central with the
others: thus the first condition, that of centricity, will be accomplished.
Remove the white paper from the mirror, and also the centring-glass, and
replace the cap on the eye-piece, also the object on the stage, of which
distinct vision should then be obtained by the rack and pinion, or fine screw
adjustment, should it have become deranged.

The re-publication of the original directions is given with the view of
showing what a clear conception Gillett had of the value of his invention.
The careful directions given for centring must be regarded with interest,
although nearly superseded by the centring screw arrangement in connection
with the sub-stage. The best results, he goes on to say, will be secured by
using the plain mirror and focussing the window-bar on the object, while a
white-cloud illuminator will afford as much light as may be required. It is a
mistake to suppose that direct light is more critical than indirect. As a rule,
the student is given to over-illuminate the object. These questions will,
however, be discussed further on.

Very many modifications of Gillett’s
condenser have, since 1850, become
known to microscopists. Ross’s
present improved form (Fig. 127) is
made to drop into the sub-stage of the
microscope, and when adjusted, is an
extremely efficient instrument. The
optical part is similar to a 4⁄10-inch
objective. It has two sets of revolving
diaphragms, with apertures and stops
for showing surface markings in a
perfect manner.

Fig. 127.—The Ross Improved Achromatic
Condenser, with diaphragm stops.

Abbe’s Condenser.

The essential feature of this condenser is its short focus, which collects the
light reflected by the mirror, so as to form a cone of rays of very large
aperture, having its focus in the plane of the object.

The full aperture of the illuminating
cone should only be used when
finely granular and deeply stained
particles (protoplasm, bacteria, &c.)
are being examined with objectives
of large aperture. In all cases the
cone must be suitably reduced, either
by an iris, or other form of
diaphragm (central illumination). By
placing the diaphragm excentrically,
by means of rack-work attached to
Fig. 128.—The Iris Diaphragm, and carrier for the carrier, the central rays are
Stops.
excluded and a certain extra-axial
portion of the illuminating pencil
falls upon the object (oblique illumination). When the diaphragm is thus
excentrically placed, this oblique pencil can be directed from all sides by
rotating the carrier round the optic axis. The central stop diaphragm shuts
off all the axial and transmits only the marginal rays, thus producing dark-
ground illumination. The iris diaphragm (Fig. 128) is so shaped that the
edge of its smallest opening closely approximates the object-slide on the
stage.

The Abbe condenser is the most
popular form in use, for all purposes.
Owing to the large aperture of the
cone of light which it projects, it can
be employed with the highest
powers; by removing the top lens it
can also be used with low powers.
Dark ground illumination may be
obtained with it up to a ¼-inch
objective.

Fig. 129.—The Abbe Condenser, detached from
the Sub-stage of the Microscope.

Fig. 130.—Optical Arrangement of Abbe Fig. 131.—Optical Arrangement of Abbe
Illuminator, 1·2 N.A. Illuminator, 1·4 N.A.

The condenser is made in two forms of 1·2 and 1·4 numerical aperture by
Messrs. Watson. The lenses are mounted in aluminium. Fig. 130 is in more
general use, but by workers with high powers Fig. 131 is preferred, as it
ensures the most oblique illumination with objectives of largest aperture. It
is preferred for photo-micrographic purposes.

Watson’s Achromatic Condenser (Fig.
132), 1·0 numerical aperture, shown in
section, although originally designed
for use with the micro-spectroscope, is
equally efficient for ordinary purposes.
This condenser transmits a larger
aplanatic cone of light than Abbe’s. It
may therefore be employed with
higher power objectives, and by
removing the top lens it is just as
Fig. 132.—The Optical Arrangement of
Watson’s Achromatic Condenser.
useful a condenser for lower powers.
Being constructed with lenses of an
unusually large size, it is well adapted for use with the micro-spectroscope.
It is certainly one of the best all-round condensers in use. The new Schott
glass enters into the construction of the lenses, and these are mounted in
aluminium.

Fig. 133.—Powell’s Achromatic Condenser.

Many microscopists consider on the whole that Powell’s sub-stage
apochromatic condenser with collar correction (Fig. 133) surpasses that of
Abbe. The mechanical arrangement of Powell’s is very simple: the
correction collar is similar to that of an ordinary objective, it has a steeper
spiral slot and only half a revolution of movement; a long arc is fixed to the
collar so that it may conveniently be reached by the finger. It is so
constructed as to turn easily and smoothly at the slightest touch. The collar
moves only the back lens of the combination, leaving the mount rigid. The
diaphragms are regulated by A and B.

Fig. 134. Fig. 134a.

Fig. 134b.—Powell’s Apochromatic Oil Immersion Condenser, N.A. 1·40.

The object of the correctional movement is to increase the maximum
aplanatic aperture of the condenser by separating the lenses. If the back of a
wide-angled objective be examined when an object is illuminated by the
full aperture of the condenser, the edge of the flame being in focus, it will
be noticed that the illuminated portion of the back lens will be oval and
pointed instead of circular. Also that when the condenser is racked up,
although the external shape of the illuminated portion becomes more
circular, two dark patches will appear on either side of the centre, showing
the operation of the spherical aberration of the condenser. If under these
circumstances the lenses are separated by means of the collar adjustment,
the black spots will be closed up, and a circular and evenly-illuminated disc
of illumination of a larger size will result. The wheel of diaphragms, or a
series of graduated diaphragm discs to drop into a holder, is intended for
critical work; the diaphragm can always be recorded, and the identical
illuminating cone reproduced.

Hence we have a simple method of graduating apertures between any two
contiguous diaphragms; if, for example, we place the lever to the left, so
that the lens may be separated as far as possible, and use a No. 6
diaphragm, and if, on examining the object, it is thought that the
illuminating cone is not large enough, and if when No. 7 is turned on it is
found too much, we can go back to No. 6, and by turning the lever 60°
towards the right, closing the lenses and increasing the power a little, we
shall obtain an aperture somewhere between Nos. 6 and 7 diaphragm. Thus
we can by means of the correction collar graduate the aperture with the
facility as with an iris, and we can record any particular aperture with a
degree of accuracy foreign to the iris. It must be admitted, however, that the
cone of light transmitted by the condenser is a very small one.

Powell also supplies an apochromatic oil-immersion condenser, numerical
aperture 1·40, but without collar correction; Fig. 134 shows the sliding tube
lowered by arm A and cell B withdrawn for changing stops, which can be
done without altering the focus of the condenser. Fig. 134a shows the cell B
closed and raised by arm A close to the back lens of optical combination. In
Fig. 134b six of the principal stops are shown. Powell’s dry apochromatic
condenser, of nearly 0·9 aplanatic cone, is also very good; but the high price
of all is a bar to their more general use. The speciality of these is the
conversion of axis light into condensed oblique incident light by the
refraction of the condenser.

Messrs. R. & J. Beck have various forms of achromatic condensers, some
of which partake of a somewhat elaborate arrangement; others are simple
and inexpensive, to suit the students’ microscope; as when the light of the
concave mirror proves insufficient for any object requiring intense
transmitted light, an achromatic condenser must be adapted to even the
students’ form of microscope. The latest form of condenser (Fig. 135) is
fitted with revolving stops and iris diaphragm, and other appliances for
obtaining satisfactory results.

Beck’s Compound Illuminating Apparatus (Fig. 136).—It is useful in
working with the microscope to be enabled to rapidly change the
illumination, and for this reason this compound form of condenser has been
constructed. It consists of an upper portion A, a wide-angle condenser, the

aperture of which can be reduced at
will by an iris diaphragm, moved by
the lever B. This can be used for all
other purposes. Below this diaphragm
is a plate C, which can be swung back
out of position at will, as shown in
outline. Into a cell in this plate the
stops D can be dropped, and the
condenser can be used for dark field
illumination, or for high powers as an
oblique illuminator. A large-size
polarising prism E, fastens to the plate
Fig. 135.—Beck’s newer form of Achromatic
Condenser. C, and can be removed when not
required. In this way any of the various
modes of illumination may be separately or conjointly obtained.

Fig. 136.—Beck’s Compound Condenser.

Their condenser (Fig. 137) has a large aperture, and facilities for rotating
the series of diaphragms. It is available for either dry or immersion
objectives up to 1·3 numerical aperture on diatoms, and wet or dry
histological objects. The spherical form of the front is worked by a milled-
head that rotates a series of lenses and diaphragms. It also avoids the
inconvenience of having the connecting fluid drawn away by capillary

attraction, as would be the case if
mounted on a flat surface. It is also
less in the way of the sub-stage
movements.

Fig. 137.—Beck’s Spherical Achromatic
Condenser.

Fig. 138.—Watson’s Parachromatic Condenser.

The Parachromatic Condenser of Messrs. Watson (Fig. 138) was made to
meet a demand for a condenser giving a large solid cone of illumination
free from colour. The optical part of this condenser consists of a full
hemispherical front lens, and the middle and back combinations of such
forms as to produce the necessary corrections. The Jena phosphate crown
and silicate flints are used in its manufacture, and to these are due its special
qualities. The total aperture of the condenser is 1·0, and it yields an
aplanatic aperture of ·90 numerical aperture. The magnifying power is 2⁄7ths

of an inch. From this it will be seen that it is especially suitable for use with
high-power objectives.

It can also be employed without the front lens, when the magnifying power
is 4⁄10ths of an inch, and the numerical aperture ·35. It is mounted in an
exceedingly convenient manner, the iris diaphragm being fitted in such a
way as to be absolutely central with the optical system.

The arc through which the handle controlling the iris travels is divided, and
indicates the aperture at which the condenser may be working at any time.
An important feature in this condenser is that it is almost wholly free from
colour. As a rule condensers of the same form are found difficult to work
with, because of the small diameter of the field or back lens. This difficulty
has been successfully overcome by increasing the size of this lens, and the
whole of which is fully utilised.

Most London opticians have their own especial form of achromatic
condenser, designed for and fitted to their several stands and objectives,
varying from a small price to the more expensively-fitted accessories.

Fig. 139.—Swift’s Illuminating Polarising Apparatus.

Fig. 139a.—Swift’s Diaphragms and Central Stops.

Messrs. Swift’s illuminating apparatus (Fig. 139) is conveniently supplied
with numerous useful appliances. The optical combination A is computed to
be used as an effective spot lens from a 3-inch objective up to a sixth. C C
are two small milled heads by means of which the optical combination A is
centred to the axis of the objective. The revolving diaphragm E has four
apertures for the purpose of receiving central stops, oblique light discs, and
selenite films. D is a frame carrying two revolving cells, into one of which a
mica film is placed, which can be revolved with ease over either of the
selenites below, whereby changes of colour can be obtained in
experimenting with polarised light. The darts and P A’s indicate the position
of the positive axis of the mica and selenite films, and by this means results
can be recorded, etc. Either of the revolving cells can be thrown into the
centre of the condenser, and there stopped by means of a spring catch; when
so arranged the mica film, &c., may be revolved in its place by turning the
cell D, as both cells are geared together with fine racked teeth. F is a
polarising prism mounted on an eccentric arm, rendered central when in
use, or thrown out, as seen, when out of use. G is the rack dove-tail slide for
indicating and focussing the condenser on the object. The advantages
associated with this condenser consist in having the polarising prism,
selenite films, dark-ground, and oblique light stops, so that they may be
brought close under the optical combination.

Baker’s Nelson Condenser, shown in
Fig. 140, is intended for use with their
medium instruments. It has, however,
many pieces of apparatus essential to
those of a higher class. It is applicable,
indeed, to all instruments having
sufficient depth beneath the stage to
receive it. It comprises an achromatic
combination of 90° aperture, available
with all powers up to 1⁄8-inch tinted
Fig. 140.—Baker’s Nelson Achromatic glass for neutralising the yellow rays
Condenser. of artificial light, focussing adjustment,
dark-ground illuminator, large
diaphragm with rotating tube to carry oblique light stops, small wheel of
apertures, polarising prism with two selenite films, clear aperture, and
oblique light-shutter for low powers.

Baker’s Students’ Condenser (Fig. 141) is designed to take the place of
Abbe’s, and costs much less. It transmits a larger aplanatic cone of light,
and can be used either with high or low powers by removing the front lens.
It is equally useful for photo-micrographic work.

Mr. J. Mayall’s semi-cylinder or prism for oblique illumination (Fig. 142) is
a convenient form, as it permits of the semi-cylinder being tilted and placed
excentrically; in this manner, without immersion contact, and by suitable
adjustment, a dry object can be viewed with any colour of monochromatic
light. If placed in immersion contact with the slide, the utmost obliquity of
incident light can be obtained. Objects in fluid may be placed on the plane-
surface of the semi-cylinder, and illuminated by ordinary transmitted light,
or rendered “self-luminous” in a dark field, as with the hemispherical
illuminator or Wenham’s immersion paraboloid. A concave mirror with a
double arm is quite sufficient to direct the illuminating pencil. This semi-
cylinder was originally made by Tolles, of Boston, for measuring apertures,
but, at Mr. Mayall’s suggestion, Messrs. Ross mounted it as an illuminator.

Fig. 141.—Optical Arrangement of Baker’s Abbe Condenser.

The spiral slot should be fixed close beneath the larger lens of the
condenser, and when properly arranged will be found a convenient mode of
obtaining oblique light.

Fig. 142.—Mayall’s Semi-Cylinder Illuminator and Spiral Diaphragms.

The Webster-Collins Universal Condenser (Fig. 143) is so well known that
it scarcely calls for any lengthy description. It is an inexpensive form of

condenser, designed in the first instance for use with the students’
microscope. It is fitted into the sub-stage; has an iris diaphragm as well as a
series of revolving diaphragms moved by a milled head screw arrangement.

Fig. 143.—The Webster-Collins Universal Condenser.

Oblique Illumination.

Wenham’s Parabolic Condenser.—Mr. Wenham’s many useful additions to
the microscope and its accessories demand especial notice. When mention
is made of the various immersion condensers (illuminators, as he preferred
to call them), his original right-angled prism, his truncated hemispherical
lens, his immersion paraboloid, and his reflex illuminator, in which rays
beyond the angle of total reflexion are utilised by reflex action from cover-
glass on to the surface of the object, every one of these well-devised
inventions will always be spoken of in terms of praise. All in their turn
conferred a great service upon the microscope, and enabled the student to
clear up difficulties that stood in the way of developing structure when
achromatic lenses and dry-objectives were considered perfect. The superior
illumination of the object was wholly due to, and effected by, reflected rays
from the object to the aperture of the objective, and obviously, reflex action
could only take place with dry-objectives. This reflex action must be
regarded as Mr. Wenham’s special discovery. It must be observed, however,
that it is not the same as the more modern achromatic appliances used for
throwing direct rays upon the object, and which proved the existence of

apertures capable of direct transmission up to 27° measured in the body of
the front lens.

The most practical of Mr. Wenham’s
inventions is probably the hemispherical
lens, since adopted by Messrs. Ross in
connection with their excellent Zentmayer
stand, and which has proved eminently
serviceable. But the fact is that devices of
the kind for obtaining direct oblique light
require a thin stage, and therefore most of
those who possess the earlier-made
microscope stand would doubtless hail the
appearance of any appliance which will
convert axial light into oblique light; as by
so doing the possessors of such
instruments, in which the stage is generally
of considerable thickness, would enjoy the
pleasure of seeing the best resolution it is
possible to get with their dry-objectives.27
Fig. 144.—Wenham’s Parabolic Reflector.
Wenham’s Parabolic Reflector.—This will
be better understood by reference to Fig. 145, which represents it in section
A B C, and shows that the rays of light r r′ r′′, entering perpendicularly at
its surface C, and then reflected by its parabolic surface A B to a focus at F,
can form no part of the largest pencil of light admitted by the object-glass
and represented by G F H; but an object placed at F will interrupt the rays
and be strongly illuminated. A stop at S prevents any light from passing
through direct from the mirror.

In the microscope the parabolic reflector fits into the cylindrical fitting
under the stage, and the adjustment of its focus upon the object is made by
giving it a spiral motion when fitted in—that is, carefully pushing it up or
down at the same time that it is turned round by the milled edge B B. It
must then be focussed by the rack and pinion motion. As the rays of light
must be parallel when they enter it, a flat mirror, which in this case should
be added to the instrument, is generally used; daylight will then require only

direct reflection, but the rays from an artificial source will have to be made
parallel by placing a side condenser between the light and the mirror, about
1¾ inch from the former and 4½ inches from the latter. Nearly the whole
surface of the mirror should be equally illuminated; this may be tested by
temporarily placing upon it a card or piece of white paper. Parallel rays can
also be obtained from the concave mirror, if the light is placed about 2½
inches from it. Dark-ground illumination is not suitable for very transparent
objects—that is, unless there is a considerable difference in their index of
refraction, or they are pervaded by air-cells.

Fig. 145.—Parabolic Reflector.

One very remarkable example of this may be seen in the tracheal system of
insects. If any of the transparent larvæ of the various kinds of gnat be
mounted in gelatine and glycerine jelly, slightly warmed but not enough to

kill the insect outright, about the third day the fluids circulating in the body
will be absorbed and replaced by air. Illuminated by the parabolic
condenser, and viewed with a binocular microscope, and a low power, the
gnat-larva becomes a superb object. The body of the insect is but faintly
visible, and in its place is displayed a marvellous tracheal skeleton, with the
tubes standing out in perspective, shining brilliantly, like a structure of
burnished silver. Unfortunately, such objects are not permanent, for when
the whole of the water dries up, the tracheal tubes either collapse or become
refilled with fluid.

As to the blackness of field, and luminosity of the object, this depends upon
excess of light from the paraboloid received beyond the angle of aperture of
the object-glass. It is found in practice that more and more of the inner
annulus of rays from the paraboloid has to be stopped off, until at last, with
high-angled objectives, it is scarcely possible to obtain a black field.

The light, on the whole, most suitable for this method of illumination is
lamp, the rays of which should in all cases be rendered more parallel by
means of a large plano-convex lens, or condenser.

Wenham’s Immersion Condenser.—Mr.
Wenham, in the year 1856, described
various forms of oblique illuminators, one
of which was an immersion; a simple Fig. 146.—Wenham’s Hemispherical Lens.
right-angled prism, connected by a fluid
medium of oil of cloves. This, however, was abandoned for a nearly
hemispherical lens connected with the slide, and although an improvement,
did not touch the point of excellence Mr. Wenham was looking for.
Ultimately he adopted a semi-circular disc of glass of the exact form and
size represented in the drawing, Fig. 146, having a quarter-inch radius, with
a well-polished rounded edge, the sides being grasped by a simple kind of
open clip attached to the sub-stage, the fluid medium used for connecting
the upper surface with the slide being either water, glycerine, or oil; an
increase of oblique illumination being obtained by swinging the ordinary
mirror sideways. By means of an illuminator of the kind difficult objects
mounted in balsam are resolved. This simple piece of glass collects and
concentrates light in a marvellous manner, and is by no means a bad

substitute for some of the more costly forms of achromatic condenser. It can
be used either in fluid contact with the slide, or dry, as an ordinary
condenser.

Mr. Wenham subsequently contrived a small truncated glass paraboloid, for
use in fluid contact with the slide; water, glycerine, oil, or other substance
being employed as a contact medium. The rays of light in this illuminator,
being internally reflected from a convex surface of glass, impinge obliquely
on the under surface of the slide, and are transmitted by the fluid uniting
medium, and internally reflected from the upper surface of the cover-glass
to the objective. To use the reflex illuminator efficiently it must be racked
up to a level with the stage. The centre of rotation is then set true by a dot
on the fitting, seen with a low power, a drop of water is then placed on the
top, and upon this the slide is laid. Minute objects on the slide must be
found either by the aid of a low power, by their greater brilliancy, or by
rotating the illuminator; the effect on the podura scale is superb, the whole
scale appearing dotted with bright blue spots in a zig-zag direction. Objects
for this illuminator should be especially selected and mounted.

The Amici Prism, originally designed for
oblique illumination, consists of a flattened
triangular glass prism, the two narrower
sides of which are slightly convex, while
the third or broadest side forms the
reflecting surface. When properly used, it is
capable of transmitting a very oblique
pencil of light. The prism is either
mounted, as in Fig. 147, for slipping into
Fig. 147.—The Amici Prism. the fitting of the sub-stage, or on an
independent stand, as arranged for Powell’s
microscope, page 85, Fig. 56.

Method of Employing the Achromatic Condenser to the Greatest
Advantage.

Its Illumination.—Good daylight is the best for general work. The
microscope should be placed near a window with a northern aspect. Direct
sunlight should never be utilised; the best light is that reflected from a white
cloud. A good paraffin lamp is the most serviceable artificial source of light,
and it is quite under control. As an illuminant more often brought into
requisition in the smoky atmosphere of towns, the paraffin lamp is on the
whole the handiest and the most useful. If gas-light can be brought into use
as suggested for micro-photography, with the incandescent mantle, it will
be found to be the purest and best form of artificial illumination for the
microscope. Among paraffin lamps those constructed by Baker and Swift
are all that can be desired.28

Sectional view of the
Optical Arrangement of the
Aplanatic Bull’s-eye Lens,
fitted in gymbal on the front
of the lamp. Fig. 148.—Baker’s Microscope Lamp.

As the chimneys of these lamps are made of metal, and blackened, no
reflected light disturbs the eye. Care must be taken to have the wick evenly

trimmed; the metal chimney has a glazed front, giving exit to the rays of
light, the flat of the flame being used with low powers, and the image of the
flame being reflected by a plane mirror to give equal illumination of the
whole field. In working with high powers, the lamp is turned with the flame
edge-wise, and at the same time the mirror must be dispensed with. By
working, as it is termed, directly on the edge of the flame, the illumination
is greatly increased, and a band of light can be concentrated on any part of
the preparation it is desired to make a careful study of.

To obtain the best results, time and care must be given to the illumination of
the object. The lamp and microscope having been placed in position, a low
power is first used and the smallest diaphragm. On looking through the
microscope it will probably be observed that the image of the diaphragm is
not in the centre of the field; by moving the centring screw of the condenser
this may be adjusted. The low power is then replaced by a high power, the
largest diaphragm used, and the bacteria or diatom brought into focus. The
diaphragm must now be replaced by one of medium size, and by racking
the condenser up and down, a point will be arrived at when the image of the
edge of the flame appears as an intensely bright band of light. If this is not
exactly in the centre of the field the centring screw of the condenser must
again be adjusted. With regard to the use of diaphragms, various sizes
should be tried while focussing with the fine adjustment, at the same time
using the correction colour; in this way we obtain the sharpest possible
image. When the condenser has been accurately centred, it will still be
necessary to focus it for each individual specimen, so as to correct for
difference in the thickness of slides and the layers of mounting medium.
Correction for different thickness of cover-glasses must be made by the aid
of the collar adjustment in the following way: a high-power eye-piece is
substituted for the ordinary eye-piece, and the faults in the image will
thereby be intensified. By moving the collar completely round, first in one
direction and then in the other, while carefully observing the effect of the
image, it will be seen to become obviously worse whichever way the collar
is turned. The collar must then be turned through gradually diminishing
distances until an intermediate point is reached at which the best image
results with the high-power eye-piece, and on replacing this by the low-
power eye-piece the sharpest possible image will be obtained.

Effect of the Sub-stage Condenser.—The sub-stage condenser gives the
most powerful illumination when it has been racked up until it almost
touches the specimen. It produces a cone of rays of very short focus, and
the apex of the cone should correspond with the particular bacterium or
group of bacterias under observation. The effect of the condenser without a
diaphragm is to obliterate what Koch has termed a structure picture. If the
component parts of a tissue section were colourless and of the same
refractive power as the medium in which the section is mounted, nothing
would be visible under the microscope. As, however, the cells and their
nuclei and the tissues do not differ in this respect, the rays which pass
through them are diffracted, and an image of lines and shadows is
developed. If in such a tissue there were minute coloured objects, and if it
were possible to mount the tissue in a medium of exactly the same
refractive power, the tissue being then invisible, the detection of the
coloured objects would be much facilitated. This is exactly what is required
in dealing with bacteria which has been stained with aniline dyes, and the
desired result can be obtained by the use of the sub-stage condenser.

If we use the full aperture of the condenser the greatly converged rays play
on the component parts of the tissue, light enters from all sides, the shadows
disappear, and the structure picture is lost. If now a diaphragm is inserted,
so that we are practically only dealing with parallel rays, the structure
picture reappears. As the diaphragm is gradually increased in size the
structure picture gradually becomes less and less distinct, while the colour
picture, the image of the stained bacteria, becomes more and more intense.
When, therefore, bacteria in the living condition and unstained tissues are
examined, a diaphragm must be used, and when the attention is to be
concentrated upon the stained bacteria in a section or in a cover-glass
preparation the diaphragm must be removed and the field flooded with light
—(Crookshank).

The wide-angle condenser, it will be understood, consists of a combination
of lenses, which concentrate all the light entering them to a small point, and
the condenser must be so accurately focussed that this brilliant cone of
light, when it emerges from the upper lens of the condenser, falls upon the
object from all directions, forming a wide-angle cone of light, at the apex of

which the object must be placed (see Fig. 149). That is to say, the object is
illuminated by a cone of rays passing through it in all directions.

There are, however, objects which
require a fully illuminated field,
when the lamp should be turned
round and the Herschel lens
condenser (shown in section, Fig.
148) should be used to collect the
Fig. 149.—Front Lens of Condenser. light and throw it upon the mirror.
For moderate powers, as a four-tenth
or one-fifth, the condenser should be used a little below the focus to give an
even illumination over the whole field. Moreover, as to the use of the
condenser for defining general objects, it must be borne in mind that to
show different kinds of structure different apertures in the iris diaphragm
are necessary, and that whereas some objects show their structure better
with a large angle of light cut down in intensity by the use of blue glass,
others show better with a small pencil of direct rays. For the resolution of
diatoms it is often necessary to use oblique light only, and for this purpose
diaphragms with central patches are used, the iris diaphragm being opened
to its full extent. An annular ring of oblique light emerges from the
condenser upon the object, and it is in this manner also that dark-ground
illumination is obtained with moderate and low powers.

THE DIAPHRAGM.

Fig. 150.—The Diaphragm.

The early form of diaphragm in use was that shown in Fig. 150.

It consists simply of a circular brass plate
with a series of circular openings of
different sizes, arranged to revolve upon
another plate by a central pin or axis, the
last being also provided with an opening as
large as the largest in the diaphragm-plate,
and corresponding in situation to the axis
of the microscope body. The holes in the
diaphragm-plate are centred and retained in
place by a bent spring in the second plate,
which rubs against the edge of the
diaphragm-plate and catches in a notch. Fig. 151.—Shutter Diaphragm.
The blank space shuts off the light from the
mirror when condensed light is about to be used. It is usually made to fit in
under the stage of the microscope. This has been almost superseded by the
iris diaphragm, originally designed by Wales, of America. It was made by
this optician for his working students’ microscope. An early form of the iris
diaphragm is seen in Fig. 151. By pressing upon the lever handle at the side
the aperture gradually closes up, and without for a moment losing sight of
the object under examination.

The Mirror.

The mode in which an object is illuminated is, in the words of the late
Andrew Ross, “second only in importance to the excellence of the glass
through which it is seen.” To ensure good illumination the mirror should be
in direct co-ordination with the objective and eye-piece; it must be regarded
as a part of the same system, and tending by a combined series of acts to a
perfect result. Illumination of the object is recognised as of three kinds or
qualities—reflected, transmitted, and refracted light. For the illumination of
transparent objects, transmitted light is brought into use; for opaque objects,
reflected light is needed.

The mirror should be about 2½ or 3 inches in diameter, and it must not be
fixed, but made to slide up and down the stem under the stage, so that the
rays of light emanating from it may be brought to a focus. The utility of the

mirror is so obvious that it is occasionally passed over in silence by writers.
To myself it appears to be an important accessory of the microscope, and I
shall therefore proceed to combine theory with practice in what I have to
say with regard to the mirror.

Fig. 152.—Principal Focus of Mirror.

The microscope mirror should be the segment of a true sphere, and its
centre that of a true curvature. If the mirror has a true circular boundary, the
central point on line A (Fig. 152) of the reflecting surface, is the pole of the
same. The line A C is known as its principal axis, and any other straight line
through C, which meets the mirror, is its secondary axis. When the incident
axis is perfectly parallel to the principal axis, the reflected rays converge to
a point F, its principal focus. So much for the theory of the mirror. Now we
come to its practical use.

Simple as the mirror of the microscope may appear to be, if the curve of the
surface is not perfect, it will yield a secondary reflection or double pencil of
rays. The plane mirror will occasionally be found to emit more than one
reflection of the lamp-flame; this we find may be corrected by rotating the
mirror in its cell. Many years ago I proposed to meet a difficulty of the kind
by arranging a rectangular prism on a separate stand, shown in Fig. 153,
consisting of a prism A B, mounted in gimbal C, D, and E, secured to a
brass tube G, fitted to the stem, and thus made to take the place of the
mirror.

The direct method of employing the mirror, that more generally resorted to,
is by reflecting rays from the concave surface; the plane surface is preferred
when the condenser is used. Whichever is employed, it should not be
forgotten that the optic axis must be preserved throughout, and so brought
to the centre of the open tube of the microscope. Another method is to
interpose a bull’s-eye lens, and in this way supply the mirror with a beam of
parallel rays of light. The plane side of the bull’s-eye lens should be turned
towards the lamp, so that lamp, bull’s-eye, sub-stage condenser, and
objective, are brought into an exact line, the bull’s-eye being set at right-
angles to the line. A piece of thin white paper held across the bottom of the
sub-stage will serve to show whether the rays of light are fairly parallel. The
next care is to focus the object on the stage, and then the sub-stage
condenser on the slide; further correction should be made by means of the
centring screws of the sub-stage, or by moving the bull’s-eye lens or lamp
slightly, thus perfecting the arrangements for working with parallel rays of
light.

Fig. 153.—Rectangular Prism.

Accessories of the Microscope.

The accessories and appliances of the microscope have become so very
numerous, that any attempt to describe them and explain the uses to which
they are put would demand more space than I find myself in a position to
bestow upon them. I must therefore confine my remarks to those
accessories in more general use.

Having described the method of employing
transmitted light, I have a few words to add
with regard to the illumination of opaque
objects by reflected light. A very early and
efficient form of opaque illumination is the
well-known Lieberkühn. This has not been
entirely surpassed by more recent
inventions. The concave speculum termed a
Lieberkühn, so named after its celebrated
inventor, directly reflects down upon the
object the light received either from the
mirror or bull’s-eye lens. It consists of a
silver cap, which slides over the objective
(Fig. 154), a indicating the lower part of
the compound body, and b the objective
Fig. 154.—The Lieberkühn. over which slides the Lieberkühn, c; the
rays of light are collected to a focus upon
the object at d. The object may either be mounted on a slip of glass, or held
by the stage-forceps, f; if very small, or transparent, it may be gummed to
the dark well, e, or mounted on a Beck’s opaque disc-revolver.

Fig. 155.—Stage Forceps, for holding objects while under examination.

This holder will be found useful for the examination of opaque or other
objects that cannot be conveniently held by the stage forceps, the specimen

being temporarily attached to it by
gum or gold size. The holder is
intended to rotate, so that every
portion of the object can be brought
into view. In this way it will be found
Fig. 156.—Beck’s Disc-holder. useful in the study of insects,
foraminifera, &c.

With the Lieberkühn, however, the illumination of opaque objects must be
more or less one-sided, and therefore, the silver side-reflector has
superseded it for general use (Fig. 157). To ensure a more perfect
illumination of the object, the bull’s-eye lens should also be used. Mr. Sorby
devised a reflector to fit over the objective. It consists of a semi-circular
cap; is, in short, a modification of the parabolic reflector. The light from the
mirror can, by slightly varying its inclination, be brought into use with this
reflector.

The silver side-reflector is usually made with a ball-and-socket joint, so that
it can be turned in any direction. It is secured to the stage of the microscope
by the pin, which drops into a hole purposely drilled to receive it, and
facility given for turning up and down, or in any position. If daylight is used
the microscope should be placed in such a position that the light from a
white cloud falls upon the speculum, but the light of the lamp is far more
manageable for use with the reflector.

Fig. 157.—Silver Side-reflector.

The Lieberkühn is only intended to be used with low powers—a 2-inch, ½-
inch and a 2⁄3-inch. Such objects as the elytra of the diamond and other
beetles are well suited for examination.

While experimenting with a
parabolic reflector (Fig. 158), Mr.
Sorby saw the value of examining
objects under every kind of
illumination. As on viewing
specimens of iron and steel with this Fig. 158.—Sorby’s Modification of the Parabolic
reflector he found that, from the Reflector.
great obliquity of the illumination
obtained, the more brilliantly polished parts of the specimen reflected the
light beyond the aperture of the objective, and these could not be
distinguished from those parts which absorbed light, he thereupon
proceeded to place a small flat mirror in front of the objective, and cover
half its aperture, and at the same time stop off by means of a semi-
cylindrical tube the light from the parabolic reflector. This arrangement
produced the reverse appearance of that first employed, and it proved to be
a useful aid in determining structure.

The Bull’s-eye Condensing Lens.

This accessory is brought into constant use for the purpose of converging
rays from a lamp or mirror; or, for reducing the diverging rays of the lamp
to parallelism with the parabolic illuminator, or silver side-reflector. The
form in use is a plano-convex lens of about three or four inches in focal
length (Fig. 159). It is usually mounted on a brass stand, so that it may be
placed and turned in any direction, and at any height. When used by
daylight, its plane side should be turned towards the object, and the same
position maintained when used for converging the rays of light from the
lamp; but when used with the side-reflector the plane side must be towards
the lamp. Much attention has been paid to this very necessary accessory, the
bull’s-eye lens. A doublet has been brought into use which has increased the
value of the bull’s-eye condenser in bacteriological research, and in micro-
photography generally.

Fig. 159.—Bull’s-eye Lens.

“During a recent investigation of the spherical aberration in doublets, it was
believed to be impossible to construct a doublet of the form known as
‘Herschel’s doublet’ free from aberration, although these doublets figure in
many books on optics. In a condenser made by Baker the aberration is
reduced to a minimum, 27 per cent. less than Sir John Herschel’s. This
doublet, it appears, differs from Herschel’s both in the ratio of the radii of
the meniscus, and also in the ratio of the foci of the two lenses; indeed, the
only point of similarity is in the first lens, which is crossed. To test this,
project the image of the flat lamp-flame on a piece of white card with a
plano-convex lens (the field-lens of the Huyghenian eye-piece), use first the
convex side and then the plane side towards the card, the lamp being placed
about 6 feet from the lens. Focus the lamp-flame as sharply as possible, and

a circular halo of misty light will be seen to surround the lamp-flame; but
when the plane side of the lens is made to face the card this halo of misty
light will be seen to be greatly reduced, and the brightness of the image of
the flame proportionately increased. If the lens, then, were strictly aplanatic
there should be no misty halo, all the light being concentrated in the image
of the lamp-flame, and the image of maximum brightness. In short, the
diameter of the halo or misty light is the measure of the spherical
aberration. If the condenser referred to above, having the form of minimum
aberration for two planes, be compared in the same manner with an
ordinary single bull’s-eye of the same focus, the diameter of the misty halo
will be found reduced to a radius of about 1⁄5-inch, but, with this new
condenser there is a further reduction, so that the radius of the misty halo
measures only 1⁄20-inch. These experiments are instructive, because the
brightness, or the mistiness of the microscopical image is an associated
phenomenon.”29

A sectional view of the optical arrangement of Baker’s aplanatic bull’s-eye
doublet is shown, together with lamp, in Fig. 148.

The Microscope Lamp.—The introduction of paraffin into household use
has somewhat modified our views with regard to the most suitable artificial
source of illumination. Good paraffin burns with a whiter and purer flame
than colza oil, and consequently is less liable to fatigue the eyes. The first
cost of the lamp is trifling; for a moderate sum a handy form of lamp can be
had, mounted on an adjustable sliding ring stand, and with a porcelain,
metal or paper shade, to protect the eyes from scattered rays of light. All
opticians supply accepted forms of lamps.

To give the increased effect of whiteness to the light (“white cloud
illumination” as it is termed), take a piece of tissue paper, dip it into a hot
bath of spermaceti, and, when nearly cold, cut out a circular piece and
secure it over the largest opening in the diaphragm plate. This will be found
to materially moderate and soften the light.

Beck’s Complete Lamp is constructed especially for delicate microscopical
work. It has a burner giving a flat flame; this can be rotated to enable the
edge or the flat of the flame to be used; likewise a metal chimney with two
apertures, in which 3 × 1 glass slips slide; either white or coloured glasses

may be used. A Herschel aplanatic
condenser is carried on a swinging
arm, which rotates around the lamp
flame as a centre, and can be
clamped in any position. The whole
lamp has a raising and lowering
motion, with a spring clamp to hold
it in any position. The lamp is so
designed that at its lowest position
the flame is only three inches from
the table. Here the microscopist is
furnished with a lamp which will
accomplish all he may require with
regard to illumination.

Fig. 160.—Beck’s Complete Lamp.

Fig. 162.—Glass Holder for carrying
Fig. 161.—Watson’s Microscope Lamp. Coloured Glasses.

Watson’s lamp (Fig. 161) has a metal chimney, and is somewhat simpler in
structure than those already referred to. For the student, the simpler and
cheaper form will answer every purpose. A glass holder for carrying various
tinted slips of coloured glass to act as a screen or modifier of the light is
much employed, and assists in determining fine structures (Fig. 162).

Nose-pieces and Objective Changers.

A convenient appendage to the microscope is the rotating nose-piece,
invented by Mr. Charles Brooke, F.R.S., and intended to carry two or more
objectives, whereby a saving of time is effected, and the trouble of
repeatedly screwing and unscrewing is avoided. In the application of the
nose-piece attention should be given to centring. Messrs. Baker’s objective

changer is intended to facilitate the placing and replacing the nose-piece in
position. This adaptation consists of a milled head, acting on three jaws,
having a universal screw thread, a decided improvement on the screw. Zeiss
has adopted a tube-sliding objective changer with centring adjustments.
Messrs. Watson met the difficulty of centring by making the nose-piece a
part of the body-tube of their microscopes (Fig. 163). This, when adapted to
the shorter body of the students’ microscope, fully compensates for want of
length.

Their triple nose-piece is constructed
with much care, and when in use is
found very effective. It is
manufactured of that very light metal
aluminium, and which minimises the
Fig. 163.—Watson’s Centring Nose-piece of strain produced by the heavier brass
Microscope. nose-piece.

Finders.—The finder affords a necessary and useful means of registering
the position of any particular object, so that it may be readily found again at
any subsequent period. In the work of examination the finder will save time
when making a special research, extending over a considerable surface.

That the finder has been of use may be
surmised from the number invented and
figured in the “Journal of the Royal
Microscopical Society.” By far the most
useful form is that of graduating the plates
of the mechanical stage, dividing a certain
portion into 100 parts. Powell and Lealand
have adopted this system in their No. 1
stands, while Baker and Watson have added
a graduated scale on silver to 1⁄100th mm. as
a finder, and also a stage micrometer in
1⁄10th and 1⁄100th of a millimetre, together

Fig. 164.—Triple Nose-pieces. with a Maltwood finder for lodging the
position of any desired portion of a
specimen under examination.

The Maltwood finder (Fig. 165) can be used with any microscope, and
without a mechanical stage. This useful finder continues to occupy a
permanent place among the accessories of the microscope. It consists of a
glass slide, 3 × 1¼ inches, on which is photographed a scale occupying a
square inch; this is divided by horizontal and vertical lines into 2,500
squares, each of which contains two numbers marking its “latitude,” or
place in the vertical series, and its “longitude,” or place in the horizontal
series. The scale is in each instance an exact distance from the bottom and
left-hand end of the glass slide; and the slide, when in use, should rest upon
the ledge of the stage of the microscope, and be made to abut against a stop,
a simple pin, about an inch and a half from the centre of the stage.

Dr. Pantacsek’s finder appears to have
some advantage over Maltwood’s, but it
cannot be used with the same facility, and
therefore will not displace an old favourite.
The Amyot finder I have long had in use; it
is efficient and inexpensive—can indeed, if Fig. 165.—Maltwood’s Finder.
misplaced or lost, be replaced by the aid of
the square and compasses.

Fig. 166.—Amyot’s Object Finder.

The Okeden finder consists of two graduated scales, one vertical, attached
to the fixed stage-plate, the other horizontal, attached to an arm carried by
the intermediate plate; the first of these scales enables the worker to “set”
the vertically-sliding plate to any determinate position in relation to the
fixed plate, while the second gives the power of setting the horizontally-
sliding plate by that of the intermediate.

Micrometers.—It is of the utmost importance to have a means of measuring
with accuracy the objects, or part of objects, under observation. The most
efficient piece of apparatus for the purpose is the micrometer eye-piece, the
earlier form of which, Jackson’s, has been described under the heading Eye-
pieces (p. 144). In the case of micrometers, as in that of most other
accessories, every optician has his own adaptation and method of
employing the same.

For the measurement of bacteria, a stage micrometer should be used with a
camera lucida. The stage micrometer consists of a slip of thin glass ruled
with a scale consisting of tenths and hundredths of a millimetre. The image
is projected on to a piece of paper placed on the table, and the drawing
made, and the object to be measured can be readily compared with the
scale.

In the Ramsden micrometer eye-
piece, as previously explained, two
fine wires are stretched across the
field of an eye-piece, one of which
can be moved by a micrometer
screw. In the field there is also a
scale with teeth, and the interval
between them corresponds to that of
the threads of the screw.

The circumference of the brass head
is usually divided into one hundred
Fig. 167.—The Ramsden Micrometer Eye- parts, and a screw with one hundred
piece.
threads to the inch is used. The
bacterium to be measured is brought
into a position in which an edge appears to be in contact with the fixed

wire, and the micrometer screw is turned until the travelling wire appears to
be in contact with the other edge. The scale in the field and scale on the
milled head, together, give the number of complete turns of the screw and
the value of a fraction of a turn in separating the wires.

In the micrometer eye-piece constructed by Zeiss, the eye-piece with a glass
plate with crossed lines is carried across the field by means of a micrometer
screw. Each division on the edge of a drum corresponds to ·01 mm.
Complete revolutions of the drum are counted by means of a figured scale
in the visual field.

In the micrometer used with Zeiss’s apochromatic objectives and
compensating eye-pieces the divisions are so computed, that, with a tube-
length of 160 mm., the value of one interval represents, with each objective,
just as many micra (·0001 mm.) as there are millimetres in its focal length.
A value of tables is therefore not required for these eye-pieces, since the
focus of the lenses indicates their micrometer values within 5 per cent.

Fig. 168.—The Wollaston Camera Lucida.

The Camera Lucida will prove an extremely useful adjunct to the
micrometer, and a large number of contrivances have been devised for its
employment. There are those which project the image on to the surface of a
sheet of paper provided for the drawing, and those which project the pencil
and paper into the field of the microscope. The former method is that
usually adopted. To draw an object, with either a Wollaston camera lucida
or a neutral tint reflector, such as that of Beale’s, both of which are made to
slide on and take the place of the cap of the eye-piece, as shown in Fig. 168,
with its flat side uppermost, the whole instrument must be raised until the
edge of the prism is exactly 10 inches from a piece of paper placed upon the
table; with the latter the instrument retains its vertical position, and the
image of the object is thrown on the paper placed in front of the stand. The

light must be so regulated that no more than is really necessary is upon the
object, whilst a full light should be thrown upon the paper. Only one eye is
to be used; and if one half of the pupil be directed over the edge of the
prism, the object will appear upon the paper, and can be traced on it by a
pencil, the point of which will also be seen. Should any blueness be visible
in the field, the prism is pushed too far on, and should be drawn back till the
colour disappears.

Fig. 169.—Microscope in position for drawing.

The position in which the microscope must
be placed is shown in the accompanying
illustration (Fig. 169).

Beale’s neutral tint reflector (Fig. 170) is
much in use, and its advantages are utility,
simplicity, and inexpensiveness.

Fig. 170.—Beale’s Neutral Tint Reflector.

Fig. 171.—The Abbe Model Camera Lucida.

The Abbe model of camera lucida has been brought into use because the
projected image can be better illuminated, and is consequently so much
brighter. This form is now made in aluminium by Messrs. Watson & Sons.
In place of the image being traced by projection on paper, the reverse is the
case, both the paper and pencil are projected into the field of view. The
mirror reflects the paper on to the silvered surface of a prism placed over
the eye-lens of the eye-piece of the microscope, and it is thereby conveyed
to the eye. There is a central opening in the silvering through which
microscopic vision is obtained. It is fitted in a new manner by means of a
cloth-lined adapter, fitting over the outside of the microscope tube; this
saves all trouble in centring and ensures concentricity. Where the
instrument has capped eye-pieces, the camera lucida must be adapted to the
eye-piece, the cap being removed. The apparatus can be disconnected from
the fitting adapter by means of a sliding pin, and readily replaced, or can be
lifted over out of the way, as shown in the drawing. Being made almost
entirely in aluminium it is very much lighter than other forms of apparatus,
and does not cause vibration. It can be used with the microscope at any

angle, the only necessity being that the paper on which the sketch is made
should be kept at the same angle as the instrument.

Micro-Photography.

Micro-photography or photo-micrography, as it is indifferently termed, has,
to a very considerable extent, superseded the use of the camera lucida for
the delineation of images seen under the microscope. I may claim to be
among the first workers with the microscope (1841) to prove beyond a
doubt that the camera could be made to render invaluable aid to the
microscopist, whereby a great saving of time might be effected, and a
drawing obtained with greater accuracy than that of the pencil of the
draughtsman.

It was about 1864-5 that Dr. Woodward’s earlier micro-photographs were
first seen in London. His skill in the manipulation of the microscope had
been long known. His first series of photographs of test diatoms created, I
remember, quite a sensation; they have probably never been surpassed.
These were taken by sun-light, magnesium, and electric-light. I was the
recipient of a series taken at a later date (1870), and which, bound in quarto
volume, are almost as perfect in definition as any of a later date taken by
oil-immersion objectives.

The objectives used by Dr. Woodward, throughout, were a 1⁄8-inch of
Wales’s (new series), and a 1⁄16-inch immersion, of Powell & Lealand’s,
especially produced for work with the camera. The magnification varied
from 800 to 3,000 diameters, a frustule of Grammatophora Marina
magnified 2,500, and a scale of podura, marked 3,000 in my collection, are
equal in definition to those taken by a high-angle 1⁄12-inch oil-immersion.
Pathological specimens taken with lower powers are equally instructive, a
section of epithelial cancer showing both nuclei and cells with distinctness.

Dr. Maddox in 1864 was also experimentally engaged in the improvement
of the processes of photography for the purpose of promoting the work of
microscopists. His labours were attended with great success. To him we are
indebted for the gelatine dry-plate process, which gave a remarkable

impetus to photography in general. Dr. Maddox has, for a period extending
over forty years, diligently and successfully cultivated and promoted micro-
photography. Among other workers to whom we are indebted for
improvements in micro-photography I may mention Wenham, Draper,
Shadbolt, Highley, Koch, Sternberg, Pringle, Leitz, and Pfeiffer.

Dr. Koch justly claims the credit of having extended the application of
micro-photography to the delineation of bacteria. A series of instructive
micro-photographs were published by him in 1877.

The importance of the camera has become more manifest as the work of the
bacteriologist has progressed. Koch strongly advocated micro-photography
on the ground that illustrations, especially of bacteria, should be as true to
nature as possible. Dr. Edgar Crookshank holds the same opinion, and in
support of his views we have numerous illustrations of the bacteria given in
his valuable “Text-book of Bacteriology.” But he does not disguise the truth
that there are difficulties to be encountered, the first of which is owing to
the fact that the smallest and most interesting bacteria can only be made
visible in animal tissues by staining. This drawback has been very nearly
overcome by the use of eosin-collodion. With this medium, and by shutting
off portions of the spectrum by coloured glasses, Koch succeeded in
obtaining photographs of bacteria, which were stained with blue and red
aniline dyes. This method, however, introduced a disturbing element of
another kind. Owing to the longer exposure required, the results were
wanting in definition, attributable, it was thought, to vibrations of the
apparatus produced by passing traffic, or by assistants moving about over
the floor of the laboratory.

Koch nevertheless showed, at the great meeting of the International Medical
Association in London, 1881, a series of micro-photographs of bacteria and
tissue sections, which were the admiration of all who saw them. To meet a
difficulty occasioned by the aniline dyes, Koch recommended that the
preparations should be stained brown; other experimenters found that
preparations stained either yellow or yellowish-brown gave good
photographic representations; but it is by no means an easy matter to find a
good differential stain of bacteria in the tissues, as even Bismarck brown is
not entirely successful. Other bacteriologists have encountered similar

difficulties at the outset. Hauser succeeded in showing the value of micro-
photography in the production of pictures of impression preparations and
colonies of bacteria in nutrient-gelatine. But to give the general effect, as
well as faithfully reproduce the minute details in these preparations of
bacteria by the aid of the pencil, would in most cases create insurmountable
difficulties, except in the hand of the most accomplished draughtsman.
Hauser employed Gerlach’s apparatus, and Schleusser’s dry-plates, and
obtained his illumination by means of a small incandescent lamp, which
gave a strong white light. The preparations so photographed were for the
most part stained brown, and mounted in the ordinary way in Canada
balsam.

In 1884, Van Ermengen succeeded in photographing preparations of
comma-bacilli stained with fuchsine and methyl violet. These pictures
afforded the first practical illustration of the value of iso-chromatic plates in
micro-photography, and their introduction marks a distinct era in the
progress of micro-photography. The iso-chromatic, or more properly the
ortho-chromatic, dry-plate process was introduced because in photography
blue or violet comes out almost or quite white, while other colours, yellow
and red, are represented by a sombre shade or even by black. This is due to
the want of equality of strength between the luminous and the actinic or
chemical rays of light. In other words, the violet and blue rays are more
chemically active than any other portion of the spectrum. It was found,
then, that if plates were coloured yellow with turmeric, the blue and violet
rays were intercepted, and their actinism proportionately reduced.

“In 1881, the so-called iso-chromatic plates were introduced. The emulsion
of bromide of silver and gelatine was stained with eosin, and it was claimed
that colours could be represented with their relative intensity; chlorophyll
and other stains have also been tried, and by such methods the ordinary
gelatine dry-plates can be so treated that they will reproduce various
colours, according to their relative light intensity, and thus be rendered iso-,
or what is now known as ortho-chromatic.”

Apparatus and Material.

Apparatus and Material used in micro-photography have, from time to
time, been greatly varied by different workers, some preferring to use the
microscope in the vertical position with the camera superimposed or fitted
on the eye-piece of the microscope tube; others, again, prefer that both the
microscope and the camera should be arrayed horizontally. In another form
the ordinary microscope is dispensed with and the objective stage and
mirror are adapted to the front of the camera, together with a suitable
arrangement for holding the object. Lastly, the camera is lain aside, and an
operating-room rendered impervious to light, takes its place, and the image
is projected and focussed upon a ground glass screen held in its place by a
separate support. This method has been made practical since the
introduction into microscopy by Zeiss of the projection eye-piece. It is well
known that micro-photographs can be produced by employing these
projection eye-pieces, as well as for screen illustrations in the lecture-room.

Fig. 172.—Swift’s Horizontal Apparatus.

With regard to the position of the microscope and camera, the horizontal
affords greater stability than the vertical, and is on this account to be
preferred. The simplest apparatus consists of a camera fixed upon a base
board, four or five feet in length, upon which the microscope can be
clamped, and which also carries the lamp and bull’s-eye lens (Fig. 172).
This arrangement I have found economical and useful. No more elaborate
arrangement is actually necessary. Sunlight is no doubt the best, but a good
paraffin lamp is a handy and available illuminant.

With the former, and rapid plates, a short exposure of three or four seconds,
even when high powers are used, is found sufficient; whereas, with the
paraffin lamp it will vary from three to ten minutes.

Walmsley gives the following table for exposures with the lamp:—

1½-inch objective 3 to 45 seconds.
2⁄3-inch " 7 to 90 "
4⁄10-inch " ½ to 3 minutes.
1⁄5-inch " 2 to 7 "
1⁄10-inch " 4 to 10 "

For micro-photography the following practical rules must be observed. The
sub-stage condenser may be dispensed with when low powers are used, as
well as the mirror, and the lamp so placed that the image of the flat of the
flame appears accurately adjusted in the centre of the field of the
microscope. The bull’s-eye lens is so interposed, that the image of the flame
disappears, and the whole field becomes equally illuminated with high
powers; the sub-stage achromatic condenser must be used, and a greater
intensity of illumination is obtained by placing the lamp-flame edgeways. It
is advisable to begin the practice of micro-photography with low powers,
and a trial experiment should be made with some well-known object as the
blow-fly’s tongue.

Dr. Crookshank is of opinion that, in the case of micro-organisms when
their biological characters are studied under low powers of the microscope,
photographs are preferable, because they give a more faithful representation
of the object. A micro-organism, even under the highest powers of the
microscope, is so minute an object, that to represent it in a drawing requires
a very delicate touch, and it is only too easy to make a picture which gives
an erroneous impression to those who have not seen the original.
Photography enables the scientific worker to record rapid changes, and it is
quite possible as the art advances we may find the film more sensitive than
the human retina, and that it will bring out details in bacteria which would
be otherwise unrecognised. The result, therefore, of experience is that in
research laboratories it will come into more general use as a faithful and
graphic method. I cannot better bring these observations to a close than by
giving a quotation from Dr. Piersoll’s practical method of obtaining micro-
photographs.

The three essential conditions to ensure success in micro-photography are:
—(1) Satisfactory apparatus; (2) good illumination; (3) suitable
preparations. With high amplifications (1,000 diameters and over), the
conditions are greatly changed by the approach to the limit both of the
shortness of the focus of the objective and of the length of the camera
which can be advantageously used; for the first experience leads to the
adoption of the 1⁄12-inch, for the second four feet is the limit, since a given
high amplification, say 2,000 diameters, can be more satisfactorily and
more conveniently obtained with a superior 1⁄12-inch connection with
suitable optical means to increase the initial magnifying power of the
objective, than with an unaided 1⁄25-inch lens, and the plate removed to a
greater distance. Until quite recently the various amplifiers offered the best
means of increasing the power of an objective, but the introduction of the
projection-oculars of Zeiss is an accessory piece of apparatus, far superior
to any older device. These projection-oculars resemble ordinary
microscopical oculars or eye-pieces only in general form and name, being
optically a projection-objective in connection with a collecting lens. The
new oil-immersion apochromatic lenses, in combination with these
projection-oculars, form undoubtedly the more efficient equipment for
high-power work; it is as true for high-power photography as for
microscopical observation in general, that the best results are obtained with
fine and necessarily expensive, optical appliances. If for the satisfactory
study of the intimate structure of a cell, or of a micro-organism, the most
improved immersion lenses are necessary, it is to be expected that, for the
successful photography of the same, tools at least as good are needed.
Sunlight certainly affords the most satisfactory illumination whereby good
micro-photographs can be obtained, as well as for recording microscopical
images. That by good lamp-light fair impressions of objects under extreme
magnification can be secured is encouraging, but the negatives produced by
such illumination seldom, if ever, possess the characteristics of a really
good sunlight negative, where the sharpest details are combined with an
exquisite softness and harmony of half-tones.

If the mirror of the microscope be of good size, it will only be necessary to
make an arm on which to support the removed mirror outside some
southerly exposed window, since it is desirable to have a greater distance
between the mirror and the stage than would be possible were the mirror

attached in its usual place. Where the microscope mirror is too small to be
satisfactorily used, a rectangular wood-framed looking-glass is readily
mounted, with the aid of a few strips of wood, so as to turn about both axes.

The rays from the plane side of the mirror should pass through a condensing
lens (of 8-10-inch focus, if possible), so placed that they are brought to a
focus before reaching the plane of the object. The exact position of the
condensing lens is a matter of experience; usually, however, the most
favourable illumination is obtained at that point where the field is brilliantly
and uniformly illuminated, just before the rays form the image source of
light; the nearer the focus the less disturbance from diffraction rings.
Ordinary objectives will require the employment of monochromatic light—
produced either by a deep blue solution of ammonia-sulphate of copper, or
by the green glass screen—since the optical and actinic foci do not usually
perfectly coincide. Powers up to the ¾-inch will require no further
condenser; with the ¼ or 1⁄6-inch objectives, the low power (1 or ¾-inch)
serves with advantage as an achromatic condenser, when attached to the
sub-stage. The Abbe condenser, although so important for fine
microscopical investigation, is not adapted to photography unless a very
wide cone of light is desired, which, for the majority of preparations, is
some advantage; a low-power objective, used as a condenser, is found to be
more satisfactory than the Abbe with a small diaphragm.30

The greatest delicacy in manipulation is necessary, as in working with a 1⁄12-
inch objective a turn too much of the fine adjustment will cause the image
to vanish. With fine preparations of bacteria it is not easy to trace the image,
and hence the advantage of commencing with a well-marked object, as that
of the fly’s tongue. The development and fixation of the image must be
proceeded with as in the ordinary photographic process. In the text-books of
photography full accounts of failures will be found, their causes and
prevention. Numerous papers and suggestions for micro-photographic work
will also be found scattered throughout the “Journal of the Royal
Microscopical Society.”

The Projection Eye-piece has become an essential part of micro-
photography, and it is so arranged that it may be employed with advantage
with objectives of either the apochromatic or ordinary series for

photographic purposes, projecting an exquisitely sharp image of the object
on the plate. A diaphragm between the lenses limits the field, and a sharp
image of it should appear on the screen when the eye-piece is adjusted. The
adjustment may be effected by revolving the eye-piece cap in a spiral slot,
so that the eye or top lens is either brought closer or removed farther away
from the diaphragm, as may be required, and divisions and a reader are
usually provided for registering positions. Such eye-pieces are made to fit
any size microscope body.

Initial magnifying powers:—
English length of tube 10-in. 3 and 6.
Continental " " 6-in. 2 and 4.

The microscope and camera (Fig.
173) are here seen to be part of the
same instrument. The bellows of the
camera have an extension varying
from 6 in. to 30 in. The board on
which the microscope and limelight
jet are fixed is made to turn out of
the line of the camera to facilitate
adjusting the instrument and radiant,
either limelight, electric light or
paraffin lamp; when this is done the
board carrying the same is turned
back to a stop which brings the
microscope into a central position
with the focussing screen. An
adjustment is supplied at the side of
the camera, geared to the slow
movement, for finely focussing the
object upon the screen. A light-
excluding connection is fitted to the
front of the camera and microscope;
immediately behind this, in the Fig. 173.—Baker’s Pringle Vertical Micro-
bellows, is an exposing shutter which photographic Apparatus.
is manipulated by means of a small

milled head. Two focussing screens are usually supplied, one grey, and one
patent plate, together with a double dark slide.

Mr. Andrew Pringle’s vertical micro-photographic apparatus is an excellent
form; it consists of a heavy base and brass support, carrying a quarter-plate
camera, grey and plain glass focussing screen, double dark back, camera
extending to 24 inches, and turning aside as shown in Fig 173. It is light-
tight in all its connections.

To secure uniform results in micro-photography, only thin preparations,
which lie as nearly as possible in one plane, can be relied upon for good and
perfect negatives.

An electric arc lamp specially designed for
micro-photographic work, wherever the
electric current is available, is that known
as “the Ross-Hepworth projection arc
lamp.” The advantage gained by this form
of lamp is not only on account of the ease
with which it may be employed, but also on
account of its superior power and quality. It
is of primary importance that the lamp
employed to convert the electricity into
light should be of a good and reliable
pattern. It is not essential that it should be
automatic in its working—many
Fig. 174.—Ross’s Arc Lamp.
experienced micro-photographers
preferring a simple hand-feed lamp to the
one of a more complicated kind, being so much less difficult to keep in
order. A good hand-feed microscope-lamp has the advantage of greater
simplicity and portability.

The argand gas-light arranged for me many years ago for micro-
photography may be employed with advantage. It is clean, and always
ready for use when brought down to the table attached by a piece of india-
rubber tubing. The incandescent form of burner enhances its value, since
the light is thereby rendered whiter. The arrangement is shown in the
diagrammatic drawing, Fig. 175.

Over the argand burner B, is a pale-blue glass chimney, resting on a wire
gauze, stage A; this secures a uniform current of air. The colour of the flame
may be still more influenced by a disc of neutral tint, or other coloured
glass, inserted into the circular opening at E, in a half-cylinder of metal, G,
used to cut off all extraneous light; can be rotated on the stage by the ivory
nob at H, a metallic reflector I, attached to the standard rod, on being
brought parallel to F serves to concentrate the light and send it on to the
bull’s-eye, and through it to the mirror, or directly to the photo-microscopic
camera.

Fig. 175.—Table Incandescent Gas-lamp.

By removing the shield G, and bringing the shade M over the burner, it is at
once converted into a useful microscopical lamp, for all ordinary purposes.
The screw R clamps the lamp-flame at any height, while the support N

carries a water-bath O, or a plate P, both of which will be found useful in
preparing and mounting objects.

A special incandescent gas-lamp is made by Messrs. R. & J. Beck.

Polarisation of Light.

Common light moves in two planes at right angles to each other, while
polarised light moves in one plane only. Common light may be turned into
polarised light either by transmission or reflection; in the first instance, one
of the planes of common light is got rid of by reflection; in the other, by
absorption. Huyghens was one of the first physicists to notice that a ray of
light has not the same properties in every part of its circumference, and he
compared it to a magnet or a collection of magnets; and supposed that the
minute particles of which it was said to be composed had different poles,
which, when acted on in certain ways, arranged themselves in particular
positions; and thence the term polarisation, a term having neither reference
to cause nor effect. It is to Malus, however, who, in 1808, discovered
polarisation by reflection, that we are indebted for the series of splendid
phenomena which have since that period been developed; phenomena of
such surpassing beauty as to exceed most ordinary objects presented to the
eye under the microscope.

Certainly no more misleading name could well have been found to describe
the causation, in one particular direction, of small displacements in the
medium, through which the light waves are made to pass.

The effect of “polarising” light is simply to alter the directions of the
vibrations of light, and allow of certain waves to pass which are vibrating in
one direction only, vertical, horizontal, or oblique, as the case may be. The
most efficient agent discovered for the polarisation of light is that of Iceland
spar, cut and mounted as a “Nicol” prism.

By cutting crystals of Iceland spar into two parts, at a particular angle, and
cementing them together again in the reverse way, Nicol succeeded in
showing that one of the two polarising pencils could be totally deflected to

one side, while the other is directly transmitted through the Nicol prism,
and thereby the beam of light becomes at once “polarised” in one plane
only. No apparent difference can be seen in the prism on holding it up to the
light, except it be in a very slight loss of brightness; but if another similarly
heated crystal be held before, and made to revolve around, a quarter of the
circle just where the two cross each other, total darkness results. This
phenomenon alternately recurs at every quadrature of the circle. A pair of
Nicol prisms, when appropriately mounted, constitute “a polarising
apparatus” for the microscope, one being fitted into the sub-stage, and the
other either immediately above the objective or eye-piece, where it can be
easily rotated, the object to be examined being placed on the stage of the
microscope, that is, between the polarising and analysing prisms.

Polariscope Objects.

Tuffen West, del. Edmund Evans.

Plate VIII.

The significance of polarised light centres in the fact that it affords a wider
insight into the structure of crystals, minerals, and a number of other
substances, and which could not otherwise be obtained without its aid. Its
usefulness is multifold, as even glass itself, when not properly annealed,
exhibits points of fracture, by a display of Newton’s rings. The knowledge
thus acquired is turned to account by glass manufacturers.

Double refraction.—When an incident ray of light is refracted into a crystal
of any other than the cubic system, or into compressed or unannealed glass,
it gives rise to two refracted rays which take different paths; this
phenomenon is termed double refraction. Attention was called to this in
1670, by Bartolin, who first observed it in Iceland spar; and the laws for
this substance were accurately determined by Huyghens.

Iceland spar or calc spar is a form of crystallized carbonate of lime. It is
composed of fifty-six parts of lime and forty-four parts of carbonic acid,
and is usually found in rhombohedral forms of crystallization.

To observe the phenomenon of double refraction, a rhomb of Iceland spar
may be laid on a page of a printed book, when all the letters seen through it
will appear double; the depth of the blackness of the letters is seen to be
considerably less than that of the originals, except where the two images
overlap.

In order to state the laws of the phenomena with precision, it is necessary to
attend to the crystalline form of Iceland spar, which has equal obtuse
angles. If a line be drawn through one of these corners, making equal angles
with the three edges which meet there, it, or any line parallel to it, is called
the axis of the crystal; the axis being, properly speaking, not a definite line
but a definite direction.

The angles of the crystals are the same in all specimens. If the crystal is of
such proportions that these three edges spoken of are equal, as in the
smaller crystal (Fig. 176), the axis is the direction of one of its diagonals, as
represented.

Any plane containing (or parallel to) the axis is called the principal plane of
the crystal.

In the next diagram, Fig. 177, the line appears double, as a b and c d, or the
dot, as e and f. Or allow a ray of light, g h, to fall thus on the crystal, it will
in its passage through be separated into two rays, h f, h e; and on coming to
the opposite surface of the crystal, will pass out at e f in the direction of i k,
parallel to g h. The plane l m n o is designated the principal section of the
crystal, and the line drawn from the solid angle l to the angle o is where the
axis of the crystal will be found; this is its optic axis. Now when a ray of
light passes along this axis, it is undivided, and there is only one image; but
in all other directions there are two images.

Fig. 176.—Axis of Crystals of Iceland Spar.

Mr. Nicol, of Edinburgh first
succeeded in making a rhomb of
Iceland spar into a single-image
prism. His method of splitting up the
crystal into two equal parts was as
follows:—

A rhomb of Iceland spar of one-
fourth of an inch in length, and about
four-eighths of an inch in breadth

Fig. 177.—A Rhomb showing the passage of
and thickness, is divided into two
Rays of Light.
equal portions in a plane, passing
through the acute lateral angle, and
nearly touching the obtuse side angle. The sectional plane of each of these
halves must be carefully polished, and the two portions cemented firmly
together with Canada balsam, so as to form a rhomb similar to that before
division; by this management the ordinary and extraordinary rays are so
separated that only one is transmitted: the cause of this great divergence of
the rays is considered to be owing to the action of the Canada balsam, the
refractive index of which (1·549) is that between the ordinary (1·6543) and
the extraordinary (1·4833) refraction of calcareous spar, and which will
change the direction of both rays in an opposite manner before they enter
the posterior half of the combination. The direction of rays passing through
such a prism is indicated by the arrow, Fig. 178.

Polarised light cannot be distinguished
from common light, as already said, by the
naked eye; and for all experimental
purposes in polarisation, two pieces of
apparatus must be employed, one to Fig. 178.
produce polarisation, and the other to show
or an analyse it. The former is called the polariser, the latter the analyser;
and every apparatus that serves for one of these purposes will also serve for
the other.

Fig. 179.—Polariser. Fig. 179a.—Analyser.

Polarising Apparatus for Students’ Microscope.

In all cases there are two positions, differing by 180°, which give a
minimum of light, and the two positions intermediate between these give a
maximum of light. The extent of the changes thus observed is a measure of
the completeness of the polarisation of light.

The two prisms mounted as shown in Figs. 179 and 179a constitute the
apparatus adapted to the microscope. The polariser slips into place below
the stage, and the analyser, with the prism fixed in a tube, is screwed in
above the objective.

The definition is considered by some experimenters as somewhat better if
the analyser be used above the eye-piece, and is certainly more easily
rotated.

Method of employing the Polarising Prism (Fig. 179).
—After having adapted it to slide into a groove on the
under-surface of the stage, where it is secured and kept
in place by the small milled-head screw, the other prism
Fig. 179a) is screwed on above the object-glass, and
thus passes directly into the body of the microscope.
The light from the mirror having been reflected through
them the axes of the two prisms must be made to
coincide; this is done by regulating the milled-head
screw until, by revolving the polarising prism, the field
of view is entirely darkened twice during its revolution.
If very minute salts or crystals are submitted for
examination then it will be found preferable to place the
analyser above the eye-piece, as in Fig. 180. Thus the
Fig. 180.—Prism
mounted as an Eye- polariscope is seen to consist of two parts; one for
piece. polarising, the other for analysing or testing the light.
There is no essential difference between the two parts,
except what convenience or economy may lead us to adopt; and either part,
therefore, may be used as polariser or analyser; but whichever is used as the
polariser, the other becomes the analyser.

Fig. 181.—More Modern Polariser and Analyser.

Opticians have their own methods of adapting the polariser and analyser to
their several microscopes. Watson’s special form of apparatus is represented
in Fig. 181, the polariser being adapted to the sub-stage, and the analyser to
screw into the objective.

Tourmaline.—A semi-transparent mineral, of a neutral or bluish tint, called
tourmaline, when cut into thin slices (about 1⁄20-inch thick) with their faces
parallel to their axes exhibit the same phenomena as the Nicol prism. The
only objection to which is that the transmitted polarised beam is more or
less coloured. The tourmaline to be preferred stops the most light when its
axis is at right-angles to that of the polariser, and yet admits the most when
in the same plane. Make choice of a tourmaline as perfect as possible; size
is of less importance when intended for use with the microscope.

Transmission of rays through tourmaline is only one of several ways in
which light can be polarised. When a beam of light is reflected from a
polished surface of glass, wood, ivory, leather, or any other non-metallic
substance, at an angle of 50° to 60° with the normal, it is more or less
polarised, and in like manner a reflector composed of any of these
substances may be employed as an analyser. In so using it, it should be
rotated about an axis parallel to the incident rays which are to be tested, and
the observation consists in noting whether this rotation produces changes in
the amount of reflected light.

For every reflected substance there is a particular angle of incidence, which
gives a maximum of polarisation in reflected light. It is called the polarising

angle for the substance, and its tangent is always equal to the index of
refraction of the substance; or, what amounts to the same thing, it is that
particular angle of incidence which is the complement of the angle of
refraction, so that the refracted rays are at right angles. This important law
was discovered experimentally by Sir David Brewster.

Tourmaline, like Iceland spar, is a negative uniaxial crystal; and its use as a
polariser depends on the property which it possesses of absorbing the
ordinary much more rapidly than the extraordinary ray, so that a thickness
which is tolerably transparent to the latter is almost completely opaque to
the former. Its pale cobalt blue colour enhances the beauty of certain crystal
and mineral substances, but like Iceland spar, the paler and more perfect
crystals are becoming scarce.

Selenite is another mineral of value in polarisation experiments. It is a
native crystalline hydrated sulphate of lime. A beautiful fibrous variety
called satin-gypsum is found in Derbyshire. The form of the crystal most
frequently met with is that of an oblique rectangular prism, with ten
rhomboidal faces, two of which are much larger than the rest. It is usually
split up into thin laminæ parallel to their lateral faces; each film should have
a thickness of from one-twentieth to one-sixtieth of an inch. In the two
rectangular directions these films allow perpendicular rays of polarised light
to traverse them unchanged, termed their neutral axes. In two other
directions, however, which form respectively angles of 45° with the neutral
axes, these films have the property of double refraction, a direction known
as the depolarising axis.

The thickness of the film of selenite
determines the particular tint. If,
therefore, we use a film of irregular
thickness, different colours are
presented by the different thicknesses.
These facts admit of very curious and
beautiful illustration, when used under
the object placed on the stage of the
microscope. The films employed should
Fig. 182.—Darker’s Selenite Films and Stage. be mounted between two glasses for