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Title: Goethe's Theory of Colours

Author: Johann Wolfgang von Goethe

Translator: Sir Charles Lock Eastlake

Release date: November 29, 2015 [eBook #50572]
Most recently updated: October 22, 2024

Language: English

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*** START OF THE PROJECT GUTENBERG EBOOK GOETHE'S THEORY
OF COLOURS ***

GOETHE'S

THEORY OF COLOURS;
TRANSLATED FROM THE GERMAN:

WITH NOTES BY

CHARLES LOCK EASTLAKE, R.A., F.R.S.

"Cicero varietatem propriè in coloribus nasci, hinc in alienum migrare
existimavit. Certè non alibi natura copiosius aut majore lasciviâ opes suas
commendavit. Metalla, gemmas, marmora, flores, astra, omnia denique quæ
progenuit suis etiam coloribus distinxit; ut venia debeatur si quis in tam
numerosâ rerum sylvâ caligaverit."
CELIO CALCAGNINI.

LONDON:

JOHN MURRAY, ALBEMARLE STREET.

1840

TO

JEREMIAH HARMAN, Esq.

Dear Sir,
I dedicate to you the following translation as a testimony of my
sincere gratitude and respect; in doing so, I but follow the example

of Portius, an Italian writer, who inscribed his translation of
Aristotle's Treatise on Colours to one of the Medici.
I have the honour to be,
Dear Sir,
Your most obliged and obedient Servant,
C. L. EASTLAKE.

THE TRANSLATOR'S PREFACE.

English writers who have spoken of Goethe's "Doctrine of Colours,"[1] have
generally confined their remarks to those parts of the work in which he has
undertaken to account for the colours of the prismatic spectrum, and of refraction
altogether, on principles different from the received theory of Newton. The less
questionable merits of the treatise consisting of a well-arranged mass of
observations and experiments, many of which are important and interesting, have
thus been in a great measure overlooked. The translator, aware of the opposition
which the theoretical views alluded to have met with, intended at first to make a
selection of such of the experiments as seem more directly applicable to the
theory and practice of painting. Finding, however, that the alterations this would
have involved would have been incompatible with a clear and connected view of
the author's statements, he preferred giving the theory itself entire, reflecting, at
the same time, that some scientific readers may be curious to hear the author
speak for himself even on the points at issue.
In reviewing the history and progress of his opinions and researches, Goethe tells
us that he first submitted his views to the public in two short essays entitled
"Contributions to Optics." Among the circumstances which he supposes were
unfavourable to him on that occasion, he mentions the choice of his title,
observing that by a reference to optics he must have appeared to make
pretensions to a knowledge of mathematics, a science with which he admits he
was very imperfectly acquainted. Another cause to which he attributes the severe
treatment he experienced, was his having ventured so openly to question the
truth of the established theory: but this last provocation could not be owing to
mere inadvertence on his part; indeed the larger work, in which he alludes to
these circumstances, is still more remarkable for the violence of his objections to
the Newtonian doctrine.

There can be no doubt, however, that much of the opposition Goethe met with
was to be attributed to the manner as well as to the substance of his statements.
Had he contented himself with merely detailing his experiments and showing
their application to the laws of chromatic harmony, leaving it to others to
reconcile them as they could with the pre-established system, or even to doubt in
consequence, the truth of some of the Newtonian conclusions, he would have
enjoyed the credit he deserved for the accuracy and the utility of his
investigations. As it was, the uncompromising expression of his convictions only
exposed him to the resentment or silent neglect of a great portion of the scientific
world, so that for a time he could not even obtain a fair hearing for the less
objectionable or rather highly valuable communications contained in his book. A
specimen of his manner of alluding to the Newtonian theory will be seen in the
preface.
It was quite natural that this spirit should call forth a somewhat vindictive
feeling, and with it not a little uncandid as well as unsparing criticism. "The
Doctrine of Colours" met with this reception in Germany long before it was
noticed in England, where a milder and fairer treatment could hardly be
expected, especially at a time when, owing perhaps to the limited intercourse
with the continent, German literature was far less popular than it is at present.
This last fact, it is true, can be of little importance in the present instance, for
although the change of opinion with regard to the genius of an enlightened nation
must be acknowledged to be beneficial, it is to be hoped there is no fashion in
science, and the translator begs to state once for all, that in advocating the
neglected merits of the "Doctrine of Colours," he is far from undertaking to
defend its imputed errors. Sufficient time has, however, now elapsed since the
publication of this work (in 1810) to allow a calmer and more candid
examination of its claims. In this more pleasing task Germany has again for
some time led the way, and many scientific investigators have followed up the
hints and observations of Goethe with a due acknowledgment of the acuteness of
his views.[2]
It may require more magnanimity in English scientific readers to do justice to the
merits of one who was so open and, in many respects, it is believed, so mistaken
an opponent of Newton; but it must be admitted that the statements of Goethe
contain more useful principles in all that relates to harmony of colour than any
that have been derived from the established doctrine. It is no derogation of the
more important truths of the Newtonian theory to say, that the views it contains
seldom appear in a form calculated for direct application to the arts. The
principle of contrast, so universally exhibited in nature, so apparent in the action
and re-action of the eye itself, is scarcely hinted at. The equal pretensions of

seven colours, as such, and the fanciful analogies which their assumed
proportions could suggest, have rarely found favour with the votaries of taste,—
indeed they have long been abandoned even by scientific authorities.[3] And here
the translator stops: he is quite aware that the defects which make the Newtonian
theory so little available for æsthetic application, are far from invalidating its
more important conclusions in the opinion of most scientific men. In carefully
abstaining therefore from any comparison between the two theories in these
latter respects, he may still be permitted to advocate the clearness and fulness of
Goethe's experiments. The German philosopher reduces the colours to their
origin and simplest elements; he sees and constantly bears in mind, and
sometimes ably elucidates, the phenomena of contrast and gradation, two
principles which may be said to make up the artist's world, and to constitute the
chief elements of beauty. These hints occur mostly in what may be called the
scientific part of the work. On the other hand, in the portion expressly devoted to
the æsthetic application of the doctrine, the author seems to have made but an
inadequate use of his own principles.
In that part of the chapter on chemical colours which relates to the colours of
plants and animals, the same genius and originality which are displayed in the
Essays on Morphology, and which have secured to Goethe undisputed rank
among the investigators of nature, are frequently apparent.
But one of the most interesting features of Goethe's theory, although it cannot be
a recommendation in a scientific point of view, is, that it contains, undoubtedly
with very great improvements, the general doctrine of the ancients and of the
Italians at the revival of letters. The translator has endeavoured, in some notes, to
point out the connexion between this theory and the practice of the Italian
painters.
The "Doctrine of Colours," as first published in 1810, consists of two volumes in
8vo., and sixteen plates, with descriptions, in 4to. It is divided into three parts, a
didactic, a controversial, and an historical part; the present translation is confined
to the first of these, with such extracts from the other two as seemed necessary,
in fairness to the author, to explain some of his statements. The polemical and
historical parts are frequently alluded to in the preface and elsewhere in the
present work, but it has not been thought advisable to omit these allusions. No
alterations whatever seem to have been made by Goethe in the didactic portion in
later editions, but he subsequently wrote an additional chapter on entoptic
colours, expressing his wish that it might be inserted in the theory itself at a
particular place which he points out. The form of this additional essay is,
however, very different from that of the rest of the work, and the translator has

therefore merely given some extracts from it in the appendix. The polemical
portion has been more than once omitted in later editions.
In the two first parts the author's statements are arranged numerically, in the style
of Bacon's Natural History. This, we are told, was for the convenience of
reference; but many passages are thus separately numbered which hardly seem to
have required it. The same arrangement is, however, strictly followed in the
translation to facilitate a comparison with the original where it may be desired;
and here the translator observes, that although he has sometimes permitted
himself to make slight alterations, in order to avoid unnecessary repetition, or to
make the author's meaning clearer, he feels that an apology may rather be
expected from him for having omitted so little. He was scrupulous on this point,
having once determined to translate the whole treatise, partly, as before stated,
from a wish to deal fairly with a controversial writer, and partly because many
passages, not directly bearing on the scientific views, are still characteristic of
Goethe. The observations which the translator has ventured to add are inserted in
the appendix: these observations are chiefly confined to such of the author's
opinions and conclusions as have direct reference to the arts; they seldom
interfere with the scientific propositions, even where these have been considered
most vulnerable.

[1] "Farbenlehre"—in the present translation generally rendered "Theory of Colours."
[2] Sixteen years after the appearance of the Farbenlehre, Dr. Johannes Müller devoted
a portion of his work, "Zur vergleichenden Physiologie des Gesichtssinnes des
Menschen und der Thiere," to the critical examination of Goethe's theory. In his
introductory remarks he expresses himself as follows—"For my own part I readily
acknowledge that I have been greatly indebted to Goethe's treatise, and can truly say
that without having studied it for some years in connexion with the actual phenomena,
the present work would hardly have been undertaken. I have no hesitation in
confessing more particularly that I have full faith in Goethe's statements, where they
are merely descriptive of the phenomena, and where the author does not enter into
explanations involving a decision on the great points of controversy." The names of
Hegel, Schelling, Seebeck, Steffens, may also be mentioned, and many others might
be added, as authorities more or less favourable to the Farbenlehre.
[3] "When Newton attempted to reckon up the rays of light decomposed by the
prism," says Sir John Leslie, "and ventured to assign the famous number seven, he was
apparently influenced by some lurking disposition towards mysticism. If any
unprejudiced person will fairly repeat the experiment, he must soon be convinced that
the various coloured spaces which paint the spectrum slide into each other by
indefinite shadings: he may name four or five principal colours, but the subordinate
spaces are evidently so multiplied as to be incapable of enumeration. The same
illustrious mathematician, we can hardly doubt, was betrayed by a passion for analogy,
when he imagined that the primary colours are distributed over the spectrum after the
proportions of the diatonic scale of music, since those intermediate spaces have really

no precise and defined limits."—Treatises on Various Subjects of Natural and
Chemical Philosophy, p. 59.

PREFACE TO THE FIRST EDITION OF 1810.

It may naturally be asked whether, in proposing to treat of colours, light itself
should not first engage our attention: to this we briefly and frankly answer that
since so much has already been said on the subject of light, it can hardly be
desirable to multiply repetitions by again going over the same ground.
Indeed, strictly speaking, it is useless to attempt to express the nature of a thing
abstractedly. Effects we can perceive, and a complete history of those effects
would, in fact, sufficiently define the nature of the thing itself. We should try in
vain to describe a man's character, but let his acts be collected and an idea of the
character will be presented to us.
The colours are acts of light; its active and passive modifications: thus
considered we may expect from them some explanation respecting light itself.
Colours and light, it is true, stand in the most intimate relation to each other, but
we should think of both as belonging to nature as a whole, for it is nature as a
whole which manifests itself by their means in an especial manner to the sense of
sight.
The completeness of nature displays itself to another sense in a similar way. Let
the eye be closed, let the sense of hearing be excited, and from the lightest breath
to the wildest din, from the simplest sound to the highest harmony, from the most
vehement and impassioned cry to the gentlest word of reason, still it is Nature
that speaks and manifests her presence, her power, her pervading life and the
vastness of her relations; so that a blind man to whom the infinite visible is
denied, can still comprehend an infinite vitality by means of another organ.
And thus as we descend the scale of being, Nature speaks to other senses—to
known, misunderstood, and unknown senses: so speaks she with herself and to
us in a thousand modes. To the attentive observer she is nowhere dead nor silent;
she has even a secret agent in inflexible matter, in a metal, the smallest portions
of which tell us what is passing in the entire mass. However manifold,
complicated, and unintelligible this language may often seem to us, yet its
elements remain ever the same. With light poise and counterpoise, Nature
oscillates within her prescribed limits, yet thus arise all the varieties and
conditions of the phenomena which are presented to us in space and time.

Infinitely various are the means by which we become acquainted with these
general movements and tendencies: now as a simple repulsion and attraction,
now as an upsparkling and vanishing light, as undulation in the air, as
commotion in matter, as oxydation and de-oxydation; but always, uniting or
separating, the great purpose is found to be to excite and promote existence in
some form or other.
The observers of nature finding, however, that this poise and counterpoise are
respectively unequal in effect, have endeavoured to represent such a relation in
terms. They have everywhere remarked and spoken of a greater and lesser
principle, an action and resistance, a doing and suffering, an advancing and
retiring, a violent and moderating power; and thus a symbolical language has
arisen, which, from its close analogy, may be employed as equivalent to a direct
and appropriate terminology.
To apply these designations, this language of Nature to the subject we have
undertaken: to enrich and amplify this language by means of the theory of
colours and the variety of their phenomena, and thus facilitate the
communication of higher theoretical views, was the principal aim of the present
treatise.
The work itself is divided into three parts. The first contains the outline of a
theory of colours. In this, the innumerable cases which present themselves to the
observer are collected under certain leading phenomena, according to an
arrangement which will be explained in the Introduction; and here it may be
remarked, that although we have adhered throughout to experiment, and
throughout considered it as our basis, yet the theoretical views which led to the
arrangement alluded to, could not but be stated. It is sometimes unreasonably
required by persons who do not even themselves attend to such a condition, that
experimental information should be submitted without any connecting theory to
the reader or scholar, who is himself to form his conclusions as he may list.
Surely the mere inspection of a subject can profit us but little. Every act of
seeing leads to consideration, consideration to reflection, reflection to
combination, and thus it may be said that in every attentive look on nature we
already theorise. But in order to guard against the possible abuse of this abstract
view, in order that the practical deductions we look to should be really useful, we
should theorise without forgetting that we are so doing, we should theorise with
mental self-possession, and, to use a bold word, with irony.
In the second part[1] we examine the Newtonian theory; a theory which by its
ascendancy and consideration has hitherto impeded a free inquiry into the
phenomena of colours. We combat that hypothesis, for although it is no longer

found available, it still retains a traditional authority in the world. Its real
relations to its subject will require to be plainly pointed out; the old errors must
be cleared away, if the theory of colours is not still to remain in the rear of so
many other better investigated departments of natural science. Since, however,
this second part of our work may appear somewhat dry as regards its matter, and
perhaps too vehement and excited in its manner, we may here be permitted to
introduce a sort of allegory in a lighter style, as a prelude to that graver portion,
and as some excuse for the earnestness alluded to.
We compare the Newtonian theory of colours to an old castle, which was at first
constructed by its architect with youthful precipitation; it was, however,
gradually enlarged and equipped by him according to the exigencies of time and
circumstances, and moreover was still further fortified and secured in
consequence of feuds and hostile demonstrations.
The same system was pursued by his successors and heirs: their increased wants
within, the harassing vigilance of their opponents without, and various accidents
compelled them in some places to build near, in others in connexion with the
fabric, and thus to extend the original plan.
It became necessary to connect all these incongruous parts and additions by the
strangest galleries, halls and passages. All damages, whether inflicted by the
hand of the enemy or the power of time, were quickly made good. As occasion
required, they deepened the moats, raised the walls, and took care there should
be no lack of towers, battlements, and embrasures. This care and these exertions
gave rise to a prejudice in favour of the great importance of the fortress, and still
upheld that prejudice, although the arts of building and fortification were by this
time very much advanced, and people had learnt to construct much better
dwellings and defences in other cases. But the old castle was chiefly held in
honour because it had never been taken, because it had repulsed so many
assaults, had baffled so many hostile operations, and had always preserved its
virgin renown. This renown, this influence lasts even now: it occurs to no one
that the old castle is become uninhabitable. Its great duration, its costly
construction, are still constantly spoken of. Pilgrims wend their way to it; hasty
sketches of it are shown in all schools, and it is thus recommended to the
reverence of susceptible youth. Meanwhile, the building itself is already
abandoned; its only inmates are a few invalids, who in simple seriousness
imagine that they are prepared for war.
Thus there is no question here respecting a tedious siege or a doubtful war; so far
from it we find this eighth wonder of the world already nodding to its fall as a
deserted piece of antiquity, and begin at once, without further ceremony, to

dismantle it from gable and roof downwards; that the sun may at last shine into
the old nest of rats and owls, and exhibit to the eye of the wondering traveller
that labyrinthine, incongruous style of building, with its scanty, make-shift
contrivances, the result of accident and emergency, its intentional artifice and
clumsy repairs. Such an inspection will, however, only be possible when wall
after wall, arch after arch, is demolished, the rubbish being at once cleared away
as well as it can be.
To effect this, and to level the site where it is possible to do so, to arrange the
materials thus acquired, so that they can be hereafter again employed for a new
building, is the arduous duty we have undertaken in this Second Part. Should we
succeed, by a cheerful application of all possible ability and dexterity, in razing
this Bastille, and in gaining a free space, it is thus by no means intended at once
to cover the site again and to encumber it with a new structure; we propose rather
to make use of this area for the purpose of passing in review a pleasing and
varied series of illustrative figures.
The third part is thus devoted to the historical account of early inquirers and
investigators. As we before expressed the opinion that the history of an
individual displays his character, so it may here be well affirmed that the history
of science is science itself. We cannot clearly be aware of what we possess till
we have the means of knowing what others possessed before us. We cannot
really and honestly rejoice in the advantages of our own time if we know not
how to appreciate the advantages of former periods. But it was impossible to
write, or even to prepare the way for a history of the theory of colours while the
Newtonian theory existed; for no aristocratic presumption has ever looked down
on those who were not of its order, with such intolerable arrogance as that
betrayed by the Newtonian school in deciding on all that had been done in earlier
times and all that was done around it. With disgust and indignation we find
Priestley, in his History of Optics, like many before and after him, dating the
success of all researches into the world of colours from the epoch of a
decomposed ray of light, or what pretended to be so; looking down with a
supercilious air on the ancient and less modern inquirers, who, after all, had
proceeded quietly in the right road, and who have transmitted to us observations
and thoughts in detail which we can neither arrange better nor conceive more
justly.
We have a right to expect from one who proposes to give the history of any
science, that he inform us how the phenomena of which it treats were gradually
known, and what was imagined, conjectured, assumed, or thought respecting
them. To state all this in due connexion is by no means an easy task; need we say

that to write a history at all is always a hazardous affair; with the most honest
intention there is always a danger of being dishonest; for in such an undertaking,
a writer tacitly announces at the outset that he means to place some things in
light, others in shade. The author has, nevertheless, long derived pleasure from
the prosecution of his task: but as it is the intention only that presents itself to the
mind as a whole, while the execution is generally accomplished portion by
portion, he is compelled to admit that instead of a history he furnishes only
materials for one. These materials consist in translations, extracts, original and
borrowed comments, hints, and notes; a collection, in short, which, if not
answering all that is required, has at least the merit of having been made with
earnestness and interest. Lastly, such materials,—not altogether untouched it is
true, but still not exhausted,—may be more satisfactory to the reflecting reader in
the state in which they are, as he can easily combine them according to his own
judgment.
This third part, containing the history of the science, does not, however, thus
conclude the subject: a fourth supplementary portion[2] is added. This contains a
recapitulation or revision; with a view to which, chiefly, the paragraphs are
headed numerically. In the execution of a work of this kind some things may be
forgotten, some are of necessity omitted, so as not to distract the attention, some
can only be arrived at as corollaries, and others may require to be exemplified
and verified: on all these accounts, postscripts, additions and corrections are
indispensable. This part contains, besides, some detached essays; for example,
that on the atmospheric colours; for as these are introduced in the theory itself
without any classification, they are here presented to the mind's eye at one view.
Again, if this essay invites the reader to consult Nature herself, another is
intended to recommend the artificial aids of science by circumstantially
describing the apparatus which will in future be necessary to assist researches
into the theory of colours.
In conclusion, it only remains to speak of the plates which are added at the end
of the work;[3] and here we confess we are reminded of that incompleteness and
imperfection which the present undertaking has, in common with all others of its
class; for as a good play can be in fact only half transmitted to writing, a great
part of its effect depending on the scene, the personal qualities of the actor, the
powers of his voice, the peculiarities of his gestures, and even the spirit and
favourable humour of the spectators; so it is, in a still greater degree, with a book
which treats of the appearances of nature. To be enjoyed, to be turned to account,
Nature herself must be present to the reader, either really, or by the help of a
lively imagination. Indeed, the author should in such cases communicate his

observations orally, exhibiting the phenomena he describes—as a text, in the first
instance,—partly as they appear to us unsought, partly as they may be presented
by contrivance to serve in particular illustration. Explanation and description
could not then fail to produce a lively impression.
The plates which generally accompany works like the present are thus a most
inadequate substitute for all this; a physical phenomenon exhibiting its effects on
all sides is not to be arrested in lines nor denoted by a section. No one ever
dreams of explaining chemical experiments with figures; yet it is customary in
physical researches nearly allied to these, because the object is thus found to be
in some degree answered. In many cases, however, such diagrams represent mere
notions; they are symbolical resources, hieroglyphic modes of communication,
which by degrees assume the place of the phenomena and of Nature herself, and
thus rather hinder than promote true knowledge. In the present instance we could
not dispense with plates, but we have endeavoured so to construct them that they
may be confidently referred to for the explanation of the didactic and polemical
portions. Some of these may even be considered as forming part of the apparatus
before mentioned.
We now therefore refer the reader to the work itself; first, only repeating a
request which many an author has already made in vain, and which the modern
German reader, especially, so seldom grants:—
Si quid novisti rectius istis
Candidus imperti; si non, his utere mecum.

[1] The Polemical part.
[2] This preface must have been written before the work was finished, for at the
conclusion of the historical part there is only an apology for the non-appearance of the
supplement here alluded to.
[3] In the present translation the necessary plates accompany the text.

CONTENTS

INTRODUCTION xxxvii
PART I.
PHYSIOLOGICAL COLOURS.
I. Effects of Light and Darkness on the Eye 2
II. Effects of Black and White Objects on the Eye 5

III.Grey Surfaces and Objects 14
IV.Dazzling Colourless Objects 16
V.Coloured Objects 20
VI.Coloured Shadows 29
VII.Faint Lights 38
VIII.Subjective Halos 40
Pathological Colours—Appendix 45
PART II.
PHYSICAL COLOURS.
IX. Dioptrical Colours 59
X. Dioptrical Colours of the First Class 60
XI.Dioptrical Colours of the Second Class
74
—Refraction
Subjective Experiments 80
XII. Refraction without the Appearance of Colour 80
XIII. Conditions of the Appearance of Colour 81
XIV. Conditions under which the Appearance of
Colour increases 86
XV. Explanation of the foregoing Phenomena 90
XVI. Decrease of the Appearance of Colour 100
XVII. Grey Objects displaced by Refraction 103
XVIII. Coloured Objects displaced by Refraction 106
XIX. Achromatism and Hyperchromatism 118
XX. Advantages of Subjective Experiments
—Transition to the Objective 123
Objective Experiments 125
XXI.Refraction without the Appearance of Colour 121
XXII.Conditions of the Appearance of Colour 128
XXIII.Conditions of the Increase of Colour 134
XXIV.Explanation of the foregoing Phenomena 139
XXV.Decrease of the Appearance of Colour 141
XXVI.Grey Objects 142
XXVII.Coloured Objects 143
XXVIII.Achromatism and Hyperchromatism 145
XXIX.Combination of Subjective and Objective
Experiments 147
XXX.Transition 150

XXXI.Catoptrical Colours 154
XXXII.Paroptical Colours 163
XXXIII.Epoptical Colours 177
PART III.
CHEMICAL COLOURS.
XXXIV.Chemical Contrast 202
XXXV.White 203
XXXVI.Black 205
XXXVII.First Excitation of Colour 206
XXXVIII.Augmentation of Colour 212
XXXIX.Culmination 214
XL.Fluctuation 217
XLI.Passage through the Whole Scale 218
XLII.Inversion 220
XLIII.Fixation 221
XLIV.Intermixture, Real 223
XLV.Intermixture, Apparent 226
XLVI.Communication, Actual 230
XLVII.Communication, Apparent 235
XLVIII.Extraction 237
XLIX.Nomenclature 242
L.Minerals 245
LI.Plants 247
LII.Worms, Insects, Fishes 252
LIII.Birds 259
LIV.Mammalia and Human Beings 262
LV.Physical and Chemical Effects of the Transmission
of Light through Coloured Mediums 266
LVI.Chemical Effect in Dioptrical Achromatism 270

PART IV.
GENERAL CHARACTERISTICS.

The Facility with which Colour appears 274
The Definite Nature of Colour 276
Combination of the Two Principles 277
Augmentation to Red 277
Junction of the Two Augmented Extremes 278
Completeness the Result of Variety in Colour 279

Harmony of the Complete State 280
Facility with which Colour may be made to tend either to
the Plus or Minus side 281
Evanescence of Colour 281
Permanence of Colour 282
PART V.
RELATION TO OTHER PURSUITS.

Relation to Philosophy 283
Relation to Mathematics 286
Relation to the Technical Operations of the Dyer 289
Relation to Physiology and Pathology 291
Relation to Natural History 292
Relation to General Physics 293
Relation to the Theory of Music 298
Concluding Observations on Terminology 300
PART VI.
EFFECT OF COLOUR WITH REFERENCE
TO MORAL ASSOCIATIONS.

Yellow 306
Red-Yellow 308
Yellow-Red 309
Blue 310
Red-Blue 312
Blue-Red 313
Red 313
Green 316
Completeness and Harmony 316
Characteristic Combinations 321
Yellow and Blue 322
Yellow and Red 322
Blue and Red 322
Yellow-Red and Blue-Red 323
Combinations Non-Characteristic 324
Relation of the Combinations to Light and Dark 325
Considerations derived from the Evidence of Experience
and History 326
Æsthetic Influence 330
Chiaro-Scuro 331
Tendency to Colour 334

Keeping 335
Colouring 337
Colour in General Nature 337
Colour of Particular Objects 338
Characteristic Colouring 339
Harmonious Colouring 341
Genuine Tone 342
False Tone 342
Weak Colouring 343
The Motley 344
Dread of Theory 344
Ultimate Aim 345
Grounds 345
Pigments 348
Allegorical, Symbolical, Mystical Application of Colour 350
Concluding Observations 352

OUTLINE OF A THEORY OF COLOURS.

"Si vera nostra sunt aut falsa, erunt talia, licet nostra per vitam defendimus.
Post fata nostra pueri qui nunc ludunt nostri judices erunt."

INTRODUCTION.

The desire of knowledge is first stimulated in us when remarkable phenomena
attract our attention. In order that this attention be continued, it is necessary that
we should feel some interest in exercising it, and thus by degrees we become
better acquainted with the object of our curiosity. During this process of
observation we remark at first only a vast variety which presses indiscriminately
on our view; we are forced to separate, to distinguish, and again to combine; by
which means at last a certain order arises which admits of being surveyed with
more or less satisfaction.

To accomplish this, only in a certain degree, in any department, requires an
unremitting and close application; and we find, for this reason, that men prefer
substituting a general theoretical view, or some system of explanation, for the
facts themselves, instead of taking the trouble to make themselves first
acquainted with cases in detail and then constructing a whole.
The attempt to describe and class the phenomena of colours has been only twice
made: first by Theophrastus,[1] and in modern times by Boyle. The pretensions of
the present essay to the third place will hardly be disputed.
Our historical survey enters into further details. Here we merely observe that in
the last century such a classification was not to be thought of, because Newton
had based his hypothesis on a phenomenon exhibited in a complicated and
secondary state; and to this the other cases that forced themselves on the
attention were contrived to be referred, when they could not be passed over in
silence; just as an astronomer would do, if from whim he were to place the moon
in the centre of our system; he would be compelled to make the earth, sun, and
planets revolve round the lesser body, and be forced to disguise and gloss over
the error of his first assumption by ingenious calculations and plausible
statements.
In our prefatory observations we assumed the reader to be acquainted with what
was known respecting light; here we assume the same with regard to the eye. We
observed that all nature manifests itself by means of colours to the sense of sight.
We now assert, extraordinary as it may in some degree appear, that the eye sees
no form, inasmuch as light, shade, and colour together constitute that which to
our vision distinguishes object from object, and the parts of an object from each
other. From these three, light, shade, and colour, we construct the visible world,
and thus, at the same time, make painting possible, an art which has the power of
producing on a flat surface a much more perfect visible world than the actual one
can be.
The eye may be said to owe its existence to light, which calls forth, as it were, a
sense that is akin to itself; the eye, in short, is formed with reference to light, to
be fit for the action of light; the light it contains corresponding with the light
without.
We are here reminded of a significant adage in constant use with the ancient
Ionian school—"Like is only known by Like;" and again, of the words of an old
mystic writer, which may be thus rendered, "If the eye were not sunny, how
could we perceive light? If God's own strength lived not in us, how could we
delight in Divine things?" This immediate affinity between light and the eye will

be denied by none; to consider them as identical in substance is less easy to
comprehend. It will be more intelligible to assert that a dormant light resides in
the eye, and that it may be excited by the slightest cause from within or from
without. In darkness we can, by an effort of imagination, call up the brightest
images; in dreams objects appear to us as in broad daylight; awake, the slightest
external action of light is perceptible, and if the organ suffers an actual shock,
light and colours spring forth. Here, however, those who are wont to proceed
according to a certain method, may perhaps observe that as yet we have not
decidedly explained what colour is. This question, like the definition of light and
the eye, we would for the present evade, and would appeal to our inquiry itself,
where we have circumstantially shown how colour is produced. We have only
therefore to repeat that colour is a law of nature in relation with the sense of
sight. We must assume, too, that every one has this sense, that every one knows
the operation of nature on it, for to a blind man it would be impossible to speak
of colours.
That we may not, however, appear too anxious to shun such an explanation, we
would restate what has been said as follows: colour is an elementary
phenomenon in nature adapted to the sense of vision; a phenomenon which, like
all others, exhibits itself by separation and contrast, by commixture and union,
by augmentation and neutralization, by communication and dissolution: under
these general terms its nature may be best comprehended.
We do not press this mode of stating the subject on any one. Those who, like
ourselves, find it convenient, will readily adopt it; but we have no desire to enter
the lists hereafter in its defence. From time immemorial it has been dangerous to
treat of colour; so much so, that one of our predecessors ventured on a certain
occasion to say, "The ox becomes furious if a red cloth is shown to him; but the
philosopher, who speaks of colour only in a general way, begins to rave."
Nevertheless, if we are to proceed to give some account of our work, to which
we have appealed, we must begin by explaining how we have classed the
different conditions under which colour is produced. We found three modes in
which it appears; three classes of colours, or rather three exhibitions of them all.
The distinctions of these classes are easily expressed.
Thus, in the first instance, we considered colours, as far as they may be said to
belong to the eye itself, and to depend on an action and re-action of the organ;
next, they attracted our attention as perceived in, or by means of, colourless
mediums; and lastly, where we could consider them as belonging to particular
substances. We have denominated the first, physiological, the second, physical,
the third, chemical colours. The first are fleeting and not to be arrested; the next

are passing, but still for a while enduring; the last may be made permanent for
any length of time.
Having separated these classes and kept them as distinct as possible, with a view
to a clear, didactic exposition, we have been enabled at the same time to exhibit
them in an unbroken series, to connect the fleeting with the somewhat more
enduring, and these again with the permanent hues; and thus, after having
carefully attended to a distinct classification in the first instance, to do away with
it again when a larger view was desirable.
In a fourth division of our work we have therefore treated generally what was
previously detailed under various particular conditions, and have thus, in fact,
given a sketch for a future theory of colours. We will here only anticipate our
statements so far as to observe, that light and darkness, brightness and obscurity,
or if a more general expression is preferred, light and its absence, are necessary
to the production of colour. Next to the light, a colour appears which we call
yellow; another appears next to the darkness, which we name blue. When these,
in their purest state, are so mixed that they are exactly equal, they produce a third
colour called green. Each of the two first-named colours can however of itself
produce a new tint by being condensed or darkened. They thus acquire a reddish
appearance which can be increased to so great a degree that the original blue or
yellow is hardly to be recognised in it: but the intensest and purest red, especially
in physical cases, is produced when the two extremes of the yellow-red and blue-
red are united. This is the actual state of the appearance and generation of
colours. But we can also assume an existing red in addition to the definite
existing blue and yellow, and we can produce contrariwise, by mixing, what we
directly produced by augmentation or deepening. With these three or six colours,
which may be conveniently included in a circle, the elementary doctrine of
colours is alone concerned. All other modifications, which may be extended to
infinity, have reference more to the application,—have reference to the technical
operations of the painter and dyer, and the various purposes of artificial life. To
point out another general quality, we may observe that colours throughout are to
be considered as half-lights, as half-shadows, on which account if they are so
mixed as reciprocally to destroy their specific hues, a shadowy tint, a grey, is
produced.
In the fifth division of our inquiry we had proposed to point out the relations in
which we should wish our doctrine of colours to stand to other pursuits.
Important as this part of our work is, it is perhaps on this very account not so
successful as we could wish. Yet when we reflect that strictly speaking these
relations cannot be described before they exist, we may console ourselves if we

have in some degree failed in endeavouring for the first time to define them. For
undoubtedly we should first wait to see how those whom we have endeavoured
to serve, to whom we have intended to make an agreeable and useful offering,
how such persons, we say, will accept the result of our utmost exertion: whether
they will adopt it, whether they will make use of it and follow it up, or whether
they will repel, reject, and suffer it to remain unassisted and neglected.
Meanwhile, we venture to express what we believe and hope. From the
philosopher we believe we merit thanks for having traced the phenomena of
colours to their first sources, to the circumstances under which they simply
appear and are, and beyond which no further explanation respecting them is
possible. It will, besides, be gratifying to him that we have arranged the
appearances described in a form that admits of being easily surveyed, even
should he not altogether approve of the arrangement itself.
The medical practitioner, especially him whose study it is to watch over the
organ of sight, to preserve it, to assist its defects and to cure its disorders, we
reckon to make especially our friend. In the chapter on the physiological colours,
in the Appendix relating to those that are more strictly pathological, he will find
himself quite in his own province. We are not without hopes of seeing the
physiological phenomena,—a hitherto neglected, and, we may add, most
important branch of the theory of colours,—completely investigated through the
exertions of those individuals who in our own times are treating this department
with success.
The investigator of nature should receive us cordially, since we enable him to
exhibit the doctrine of colours in the series of other elementary phenomena, and
at the same time enable him to make use of a corresponding nomenclature, nay,
almost the same words and designations as under the other rubrics. It is true we
give him rather more trouble as a teacher, for the chapter of colours is not now to
be dismissed as heretofore with a few paragraphs and experiments; nor will the
scholar submit to be so scantily entertained as he has hitherto been, without
murmuring. On the other hand, an advantage will afterwards arise out of this: for
if the Newtonian doctrine was easily learnt, insurmountable difficulties presented
themselves in its application. Our theory is perhaps more difficult to
comprehend, but once known, all is accomplished, for it carries its application
along with it.
The chemist who looks upon colours as indications by which he may detect the
more secret properties of material things, has hitherto found much inconvenience
in the denomination and description of colours; nay, some have been induced
after closer and nicer examination to look upon colour as an uncertain and

fallacious criterion in chemical operations. Yet we hope by means of our
arrangement and the nomenclature before alluded to, to bring colour again into
credit, and to awaken the conviction that a progressive, augmenting, mutable
quality, a quality which admits of alteration even to inversion, is not fallacious,
but rather calculated to bring to light the most delicate operations of nature.
In looking a little further round us, we are not without fears that we may fail to
satisfy another class of scientific men. By an extraordinary combination of
circumstances the theory of colours has been drawn into the province and before
the tribunal of the mathematician, a tribunal to which it cannot be said to be
amenable. This was owing to its affinity with the other laws of vision which the
mathematician was legitimately called upon to treat. It was owing, again, to
another circumstance: a great mathematician had investigated the theory of
colours, and having been mistaken in his observations as an experimentalist, he
employed the whole force of his talent to give consistency to this mistake. Were
both these circumstances considered, all misunderstanding would presently be
removed, and the mathematician would willingly co-operate with us, especially
in the physical department of the theory.
To the practical man, to the dyer, on the other hand, our labour must be
altogether acceptable; for it was precisely those who reflected on the facts
resulting from the operations of dyeing who were the least satisfied with the old
theory: they were the first who perceived the insufficiency of the Newtonian
doctrine. The conclusions of men are very different according to the mode in
which they approach a science or branch of knowledge; from which side,
through which door they enter. The literally practical man, the manufacturer,
whose attention is constantly and forcibly called to the facts which occur under
his eye, who experiences benefit or detriment from the application of his
convictions, to whom loss of time and money is not indifferent, who is desirous
of advancing, who aims at equalling or surpassing what others have
accomplished,—such a person feels the unsoundness and erroneousness of a
theory much sooner than the man of letters, in whose eyes words consecrated by
authority are at last equivalent to solid coin; than the mathematician, whose
formula always remains infallible, even although the foundation on which it is
constructed may not square with it. Again, to carry on the figure before
employed, in entering this theory from the side of painting, from the side of
æsthetic[2] colouring generally, we shall be found to have accomplished a most
thank-worthy office for the artist. In the sixth part we have endeavoured to define
the effects of colour as addressed at once to the eye and mind, with a view to
making them more available for the purposes of art. Although much in this
portion, and indeed throughout, has been suffered to remain as a sketch, it should

be remembered that all theory can in strictness only point out leading principles,
under the guidance of which, practice may proceed with vigour and be enabled
to attain legitimate results.

[1] The treatise to which the author alludes in more generally ascribed to Aristotle.—
T.
[2] Æsthetic—belonging to taste as mere internal sense, from αἰσθάνομαι, to feel; the
word was first used by Wolf.—T.

PART I.

PHYSIOLOGICAL COLOURS.

1.

We naturally place these colours first, because they belong altogether, or in a
great degree, to the subject[1]—to the eye itself. They are the foundation of the
whole doctrine, and open to our view the chromatic harmony on which so much
difference of opinion has existed. They have been hitherto looked upon as
extrinsic and casual, as illusion and infirmity: their appearances have been
known from ancient date; but, as they were too evanescent to be arrested, they
were banished into the region of phantoms, and under this idea have been very
variously described.
2.

Thus they are called colores adventicii by Boyle; imaginarii and phantastici by
Rizetti; by Buffon, couleurs accidentelles; by Scherfer, scheinfarben (apparent
colours); ocular illusions and deceptions of sight by many; by Hamberger, vitia
fugitiva; by Darwin, ocular spectra.

3.

We have called them physiological because they belong to the eye in a healthy
state; because we consider them as the necessary conditions of vision; the lively
alternating action of which, with reference to external objects and a principle
within it, is thus plainly indicated.

4.

To these we subjoin the pathological colours, which, like all deviations from a
constant law, afford a more complete insight into the nature of the physiological
colours.

I

EFFECTS OF LIGHT AND DARKNESS ON THE EYE.

5.

The retina, after being acted upon by light or darkness, is found to be in two
different states, which are entirely opposed to each other.

6.

If we keep the eyes open in a totally dark place, a certain sense of privation is
experienced. The organ is abandoned to itself; it retires into itself. That
stimulating and grateful contact is wanting by means of which it is connected
with the external world, and becomes part of a whole.

7.

If we look on a white, strongly illumined surface, the eye is dazzled, and for a
time is incapable of distinguishing objects moderately lighted.
8.

The whole of the retina is acted on in each of these extreme states, and thus we
can only experience one of these effects at a time. In the one case (6) we found
the organ in the utmost relaxation and susceptibility; in the other (7) in an
overstrained state, and scarcely susceptible at all.

9.

If we pass suddenly from the one state to the other, even without supposing these
to be the extremes, but only, perhaps, a change from bright to dusky, the
difference is remarkable, and we find that the effects last for some time.

10.

In passing from bright daylight to a dusky place we distinguish nothing at first:
by degrees the eye recovers its susceptibility; strong eyes sooner than weak ones;
the former in a minute, while the latter may require seven or eight minutes.

11.

The fact that the eye is not susceptible to faint impressions of light, if we pass
from light to comparative darkness, has led to curious mistakes in scientific
observations. Thus an observer, whose eyes required some time to recover their
tone, was long under the impression that rotten wood did not emit light at noon-
day, even in a dark room. The fact was, he did not see the faint light, because he
was in the habit of passing from bright sunshine to the dark room, and only
subsequently remained so long there that the eye had time to recover itself.
The same may have happened to Doctor Wall, who, in the daytime, even in a
dark room, could hardly perceive the electric light of amber.
Our not seeing the stars by day, as well as the improved appearance of pictures
seen through a double tube, is also to be attributed to the same cause.
12.

If we pass from a totally dark place to one illumined by the sun, we are dazzled.
In coming from a lesser degree of darkness to light that is not dazzling, we
perceive all objects clearer and better: hence eyes that have been in a state of
repose are in all cases better able to perceive moderately distinct appearances.
Prisoners who have been long confined in darkness acquire so great a
susceptibility of the retina, that even in the dark (probably a darkness very
slightly illumined) they can still distinguish objects.

13.

In the act which we call seeing, the retina is at one and the same time in different
and even opposite states. The greatest brightness, short of dazzling, acts near the
greatest darkness. In this state we at once perceive all the intermediate gradations
of chiaro-scuro, and all the varieties of hues.

14.

We will proceed in due order to consider and examine these elements of the
visible world, as well as the relation in which the organ itself stands to them, and
for this purpose we take the simplest objects.

[1] The German distinction between subject and object is so generally understood and
adopted, that it is hardly necessary to explain that the subject is the individual, in this
case the beholder; the object, all that is without him.—T.

II.

EFFECTS OF BLACK AND WHITE OBJECTS ON THE EYE.

15.

In the same manner as the retina generally is affected by brightness and darkness,
so it is affected by single bright or dark objects. If light and dark produce
different results on the whole retina, so black and white objects seen at the same
time produce the same states together which light and dark occasioned in
succession.

16.

A dark object appears smaller than a bright one of the same size. Let a white disk
be placed on a black ground, and a black disk on a white ground, both being
exactly similar in size; let them be seen together at some distance, and we shall
pronounce the last to be about a fifth part smaller than the other. If the black
circle be made larger by so much, they will appear equal.[1]

17.

Thus Tycho de Brahe remarked that the moon in conjunction (the darker state)
appears about a fifth part smaller than when in opposition (the bright full state).
The first crescent appears to belong to a larger disk than the remaining dark
portion, which can sometimes be distinguished at the period of the new moon.
Black dresses make people appear smaller than light ones. Lights seen behind an
edge make an apparent notch in it. A ruler, behind which the flame of a light just
appears, seems to us indented. The rising or setting sun appears to make a notch
in the horizon.

Plate 1.

18.

Black, as the equivalent of darkness, leaves the organ in a state of repose; white,
as the representative of light, excites it. We may, perhaps, conclude from the
above experiment (16) that the unexcited retina, if left to itself, is drawn together,
and occupies a less space than in its active state, produced by the excitement of
light.
Hence Kepler says very beautifully: "Certum est vel in retinâ caussâ picturæ, vel
in spiritibus caussâ impressionis, exsistere dilatationem lucidorum."—Paralip. in
Vitellionem, p. 220. Scherfer expresses a similar conjecture.—Note A.

19.

However this may be, both impressions derived from such objects remain in the
organ itself, and last for some time, even when the external cause is removed. In
ordinary experience we scarcely notice this, for objects are seldom presented to
us which are very strongly relieved from each other, and we avoid looking at
those appearances that dazzle the sight. In glancing from one object to another,
the succession of images appears to us distinct; we are not aware that some
portion of the impression derived from the object first contemplated passes to
that which is next looked at.

20.

If in the morning, on waking, when the eye is very susceptible, we look intently
at the bars of a window relieved against the dawning sky, and then shut our eyes
or look towards a totally dark place, we shall see a dark cross on a light ground
before us for some time.

21.

Every image occupies a certain space on the retina, and of course a greater or
less space in proportion as the object is seen near or at a distance. If we shut the
eyes immediately after looking at the sun we shall be surprised to find how small
the image it leaves appears.

22.

If, on the other hand, we turn the open eye towards the side of a room, and
consider the visionary image in relation to other objects, we shall always see it
larger in proportion to the distance of the surface on which it is thrown. This is

easily explained by the laws of perspective, according to which a small object
near covers a great one at a distance.

23.

The duration of these visionary impressions varies with the powers or structure
of the eye in different individuals, just as the time necessary for the recovery of
the tone of the retina varies in passing from brightness to darkness (10): it can be
measured by minutes and seconds, indeed much more exactly than it could
formerly have been by causing a lighted linstock to revolve rapidly, so as to
appear a circle.—Note B.
24.

But the force with which an impinging light impresses the eye is especially
worthy of attention. The image of the sun lasts longest; other objects, of various
degrees of brightness, leave the traces of their appearance on the eye for a
proportionate time.

25.

These images disappear by degrees, and diminish at once in distinctness and in
size.

26.

They are reduced from the contour inwards, and the impression on some persons
has been that in square images the angles become gradually blunted till at last a
diminished round image floats before the eye.

27.

Such an image, when its impression is no more observable, can, immediately
after, be again revived on the retina by opening and shutting the eye, thus
alternately exciting and resting it.

28.

Images may remain on the retina in morbid affections of the eye for fourteen,
seventeen minutes, or even longer. This indicates extreme weakness of the organ,
its inability to recover itself; while visions of persons or things which are the
objects of love or aversion indicate the connexion between sense and thought.

29.

If, while the image of the window-bars before mentioned lasts, we look upon a
light grey surface, the cross will then appear light and the panes dark. In the first
case (20) the image was like the original picture, so that the visionary impression
also could continue unchanged; but in the present instance our attention is
excited by a contrary effect being produced. Various examples have been given
by observers of nature.

30.

The scientific men who made observations in the Cordilleras saw a bright
appearance round the shadows of their heads on some clouds. This example is a
case in point; for, while they fixed their eyes on the dark shadow, and at the same
time moved from the spot, the compensatory light image appeared to float round
the real dark one. If we look at a black disk on a light grey surface, we shall
presently, by changing the direction of the eyes in the slightest degree, see a
bright halo floating round the dark circle.
A similar circumstance happened to myself: for while, as I sat in the open air, I
was talking to a man who stood at a little distance from me relieved on a grey
sky, it appeared to me, as I slightly altered the direction of my eyes, after having
for some time looked fixedly at him, that his head was encircled with a dazzling
light.
In the same way probably might be explained the circumstance that persons
crossing dewy meadows at sunrise see a brightness round each other's heads[2];
the brightness in this case may be also iridescent, as the phenomena of refraction
come into the account.
Thus again it has been asserted that the shadows of a balloon thrown on clouds
were bordered with bright and somewhat variegated circles.
Beccaria made use of a paper kite in some experiments on electricity. Round this
kite appeared a small shining cloud varying in size; the same brightness was
even observed round part of the string. Sometimes it disappeared, and if the kite
moved faster the light appeared to float to and fro for a few moments on the
place before occupied. This appearance, which could not be explained by those
who observed it at the time, was the image which the eye retained of the kite
relieved as a dark mass on a bright sky; that image being changed into a light
mass on a comparatively dark background.
In optical and especially in chromatic experiments, where the observer has to do
with bright lights whether colourless or coloured, great care should be taken that
the spectrum which the eye retains in consequence of a previous observation

does not mix with the succeeding one, and thus affect the distinctness and purity
of the impression.

31.

These appearances have been explained as follows: That portion of the retina on
which the dark cross (29) was impressed is to be considered in a state of repose
and susceptibility. On this portion therefore the moderately light surface acted in
a more lively manner than on the rest of the retina, which had just been
impressed with the light through the panes, and which, having thus been excited
by a much stronger brightness, could only view the grey surface as a dark.

32.

This mode of explanation appears sufficient for the cases in question, but, in the
consideration of phenomena hereafter to be adduced, we are forced to trace the
effects to higher sources.

33.

The eye after sleep exhibits its vital elasticity more especially by its tendency to
alternate its impressions, which in the simplest form change from dark to light,
and from light to dark. The eye cannot for a moment remain in a particular state
determined by the object it looks upon. On the contrary, it is forced to a sort of
opposition, which, in contrasting extreme with extreme, intermediate degree with
intermediate degree, at the same time combines these opposite impressions, and
thus ever tends to a whole, whether the impressions are successive, or
simultaneous and confined to one image.

34.

Perhaps the peculiarly grateful sensation which we experience in looking at the
skilfully treated chiaro-scuro of colourless pictures and similar works of art
arises chiefly from the simultaneous impression of a whole, which by the organ
itself is sought, rather than arrived at, in succession, and which, whatever may be
the result, can never be arrested.

[1] Plate 1. fig. 1.
[2] See the Life of Benvenuto Cellini, vol. i. p. 453. Milan edition, 1806.—T.

III.

GREY SURFACES AND OBJECTS.

35.

A moderate light is essential to many chromatic experiments. This can be
presently obtained by surfaces more or less grey, and thus we have at once to
make ourselves acquainted with this simplest kind of middle tint, with regard to
which it is hardly necessary to observe, that in many cases a white surface in
shadow, or in a low light, may be considered equivalent to a grey.

36.

Since a grey surface is intermediate between brightness and darkness, it admits
of our illustrating a phenomenon before described (29) by an easy experiment.

37.

Let a black object be held before a grey surface, and let the spectator, after
looking steadfastly at it, keep his eyes unmoved while it is taken away: the space
it occupied appears much lighter. Let a white object be held up in the same
manner: on taking it away the space it occupied will appear much darker than the
rest of the surface. Let the spectator in both cases turn his eyes this way and that
on the surface, the visionary images will move in like manner.
38.

A grey object on a black ground appears much brighter than the same object on a
white ground. If both comparisons are seen together the spectator can hardly
persuade himself that the two greys are identical. We believe this again to be a
proof of the great excitability of the retina, and of the silent resistance which
every vital principle is forced to exhibit when any definite or immutable state is
presented to it. Thus inspiration already presupposes expiration; thus every
systole its diastole. It is the universal formula of life which manifests itself in this
as in all other cases. When darkness is presented to the eye it demands
brightness, and vice versâ: it shows its vital energy, its fitness to receive the
impression of the object, precisely by spontaneously tending to an opposite state.

IV.

DAZZLING COLOURLESS OBJECTS.

39.

If we look at a dazzling, altogether colourless object, it makes a strong lasting
impression, and its after-vision is accompanied by an appearance of colour.

40.

Let a room be made as dark as possible; let there be a circular opening in the
window-shutter about three inches in diameter, which may be closed or not at
pleasure. The sun being suffered to shine through this on a white surface, let the
spectator from some little distance fix his eyes on the bright circle thus admitted.
The hole being then closed, let him look towards the darkest part of the room; a
circular image will now be seen to float before him. The middle of this circle will
appear bright, colourless, or somewhat yellow, but the border will at the same
moment appear red.
After a time this red, increasing towards the centre, covers the whole circle, and
at last the bright central point. No sooner, however, is the whole circle red than
the edge begins to be blue, and the blue gradually encroaches inwards on the red.
When the whole is blue the edge becomes dark and colourless. This darker edge
again slowly encroaches on the blue till the whole circle appears colourless. The
image then becomes gradually fainter, and at the same time diminishes in size.
Here again we see how the retina recovers itself by a succession of vibrations
after the powerful external impression it received. (25, 26.)

41.

By several repetitions similar in result, I found the comparative duration of these
appearances in my own case to be as follows:—
I looked on the bright circle five seconds, and then, having closed the aperture,
saw the coloured visionary circle floating before me. After thirteen seconds it
was altogether red; twenty-nine seconds next elapsed till the whole was blue, and
forty-eight seconds till it appeared colourless. By shutting and opening the eye I
constantly revived the image, so that it did not quite disappear till seven minutes
had elapsed.
Future observers may find these periods shorter or longer as their eyes may be
stronger or weaker (23), but it would be very remarkable if, notwithstanding
such variations, a corresponding proportion as to relative duration should be
found to exist.

42.

But this remarkable phenomenon no sooner excites our attention than we observe
a new modification of it.
If we receive the impression of the bright circle as before, and then look on a
light grey surface in a moderately lighted room, an image again floats before us;
but in this instance a dark one: by degrees it is encircled by a green border that
gradually spreads inwards over the whole circle, as the red did in the former
instance. As soon as this has taken place a dingy yellow appears, and, filling the
space as the blue did before, is finally lost in a negative shade.

43.

These two experiments may be combined by placing a black and a white plane
surface next each other in a moderately lighted room, and then looking
alternately on one and the other as long as the impression of the light circle lasts:
the spectator will then perceive at first a red and green image alternately, and
afterwards the other changes. After a little practice the two opposite colours may
be perceived at once, by causing the floating image to fall on the junction of the
two planes. This can be more conveniently done if the planes are at some
distance, for the spectrum then appears larger.

44.

I happened to be in a forge towards evening at the moment when a glowing mass
of iron was placed on the anvil; I had fixed my eyes steadfastly on it, and,
turning round, I looked accidentally into an open coal-shed: a large red image
now floated before my eyes, and, as I turned them from the dark opening to the
light boards of which the shed was constructed, the image appeared half green,
half red, according as it had a lighter or darker ground behind it. I did not at that
time take notice of the subsequent changes of this appearance.

45.

The after-vision occasioned by a total dazzling of the retina corresponds with
that of a circumscribed bright object. The red colour seen by persons who are
dazzled with snow belongs to this class of phenomena, as well as the singularly
beautiful green colour which dark objects seem to wear after looking long on
white paper in the sun. The details of such experiments may be investigated
hereafter by those whose young eyes are capable of enduring such trials further
for the sake of science.

46.

With these examples we may also class the black letters which in the evening
light appear red. Perhaps we might insert under the same category the story that
drops of blood appeared on the table at which Henry IV. of France had seated
himself with the Duc de Guise to play at dice.

V.

COLOURED OBJECTS.

47.

We have hitherto seen the physiological colours displayed in the after-vision of
colourless bright objects, and also in the after-vision of general colourless
brightness; we shall now find analogous appearances if a given colour be
presented to the eye: in considering this, all that has been hitherto detailed must
be present to our recollection.

48.

The impression of coloured objects remains in the eye like that of colourless
ones, but in this case the energy of the retina, stimulated as it is to produce the
opposite colour, will be more apparent.

49.

Let a small piece of bright-coloured paper or silk stuff be held before a
moderately lighted white surface; let the observer look steadfastly on the small
coloured object, and let it be taken away after a time while his eyes remain
unmoved; the spectrum of another colour will then be visible on the white plane.
The coloured paper may be also left in its place while the eye is directed to
another part of the white plane; the same spectrum will be visible there too, for it
arises from an image which now belongs to the eye.
50.

In order at once to see what colour will be evoked by this contrast, the chromatic
circle[1] may be referred to. The colours are here arranged in a general way
according to the natural order, and the arrangement will be found to be directly
applicable in the present case; for the colours diametrically opposed to each
other in this diagram are those which reciprocally evoke each other in the eye.

Thus, yellow demands purple; orange, blue; red, green; and vice versâ: thus
again all intermediate gradations reciprocally evoke each other; the simpler
colour demanding the compound, and vice versâ.—Note C.

51.

The cases here under consideration occur oftener than we are aware in ordinary
life; indeed, an attentive observer sees these appearances everywhere, while, on
the other hand, the uninstructed, like our predecessors, consider them as
temporary visual defects, sometimes even as symptoms of disorders in the eye,
thus exciting serious apprehensions. A few remarkable instances may here be
inserted.
52.

I had entered an inn towards evening, and, as a well-favoured girl, with a
brilliantly fair complexion, black hair, and a scarlet bodice, came into the room, I
looked attentively at her as she stood before me at some distance in half shadow.
As she presently afterwards turned away, I saw on the white wall, which was
now before me, a black face surrounded with a bright light, while the dress of the
perfectly distinct figure appeared of a beautiful sea-green.

53.

Among the materials for optical experiments, there are portraits with colours and
shadows exactly opposite to the appearance of nature. The spectator, after having
looked at one of these for a time, will see the visionary figure tolerably true to
nature. This is conformable to the same principles, and consistent with
experience, for, in the former instance, a negress with a white head-dress would
have given me a white face surrounded with black. In the case of the painted
figures, however, which are commonly small, the parts are not distinguishable by
every one in the after-image.

54.

A phenomenon which has before excited attention among the observers of nature
is to be attributed, I am persuaded, to the same cause.
It has been stated that certain flowers, towards evening in summer, coruscate,
become phosphorescent, or emit a momentary light. Some persons have
described their observation of this minutely. I had often endeavoured to witness
it myself, and had even resorted to artificial contrivances to produce it.

On the 19th of June, 1799, late in the evening, when the twilight was deepening
into a clear night, as I was walking up and down the garden with a friend, we
very distinctly observed a flame-like appearance near the oriental poppy, the
flowers of which are remarkable for their powerful red colour. We approached
the place and looked attentively at the flowers, but could perceive nothing
further, till at last, by passing and repassing repeatedly, while we looked
sideways on them, we succeeded in renewing the appearance as often as we
pleased. It proved to be a physiological phenomenon, such as others we have
described, and the apparent coruscation was nothing but the spectrum of the
flower in the compensatory blue-green colour.
In looking directly at a flower the image is not produced, but it appears
immediately as the direction of the eye is altered. Again, by looking sideways on
the object, a double image is seen for a moment, for the spectrum then appears
near and on the real object.
The twilight accounts for the eye being in a perfect state of repose, and thus very
susceptible, and the colour of the poppy is sufficiently powerful in the summer
twilight of the longest days to act with full effect and produce a compensatory
image. I have no doubt these appearances might be reduced to experiment, and
the same effect produced by pieces of coloured paper. Those who wish to take
the most effectual means for observing the appearance in nature—suppose in a
garden—should fix the eyes on the bright flowers selected for the purpose, and,
immediately after, look on the gravel path. This will be seen studded with spots
of the opposite colour. The experiment is practicable on a cloudy day, and even
in the brightest sunshine, for the sun-light, by enhancing the brilliancy of the
flower, renders it fit to produce the compensatory colour sufficiently distinct to
be perceptible even in a bright light. Thus, peonies produce beautiful green,
marigolds vivid blue spectra.

55.

As the opposite colour is produced by a constant law in experiments with
coloured objects on portions of the retina, so the same effect takes place when
the whole retina is impressed with a single colour. We may convince ourselves of
this by means of coloured glasses. If we look long through a blue pane of glass,
everything will afterwards appear in sunshine to the naked eye, even if the sky is
grey and the scene colourless. In like manner, in taking off green spectacles, we
see all objects in a red light. Every decided colour does a certain violence to the
eye, and forces the organ to opposition.

56.

We have hitherto seen the opposite colours producing each other successively on
the retina: it now remains to show by experiment that the same effects can exist
simultaneously. If a coloured object impinges on one part of the retina, the
remaining portion at the same moment has a tendency to produce the
compensatory colour. To pursue a former experiment, if we look on a yellow
piece of paper placed on a white surface, the remaining part of the organ has
already a tendency to produce a purple hue on the colourless surface: in this case
the small portion of yellow is not powerful enough to produce this appearance
distinctly, but, if a white paper is placed on a yellow wall, we shall see the white
tinged with a purple hue.

57.

Although this experiment may be made with any colours, yet red and green are
particularly recommended for it, because these colours seem powerfully to evoke
each other. Numerous instances occur in daily experience. If a green paper is
seen through striped or flowered muslin, the stripes or flowers will appear
reddish. A grey building seen through green pallisades appears in like manner
reddish. A modification of this tint in the agitated sea is also a compensatory
colour: the light side of the waves appears green in its own colour, and the
shadowed side is tinged with the opposite hue. The different direction of the
waves with reference to the eye produces the same effect. Objects seen through
an opening in a red or green curtain appear to wear the opposite hue. These
appearances will present themselves to the attentive observer on all occasions,
even to an unpleasant degree.

58.

Having made ourselves acquainted with the simultaneous exhibition of these
effects in direct cases, we shall find that we can also observe them by indirect
means. If we place a piece of paper of a bright orange colour on the white
surface, we shall, after looking intently at it, scarcely perceive the compensatory
colour on the rest of the surface: but when we take the orange paper away, and
when the blue spectrum appears in its place, immediately as this spectrum
becomes fully apparent, the rest of the surface will be overspread, as if by a
flash, with a reddish-yellow light, thus exhibiting to the spectator in a lively
manner the productive energy of the organ, in constant conformity with the same
law.

59.

As the compensatory colours easily appear, where they do not exist in nature,
near and after the original opposite ones, so they are rendered more intense
where they happen to mix with a similar real hue. In a court which was paved
with grey limestone flags, between which grass had grown, the grass appeared of
an extremely beautiful green when the evening clouds threw a scarcely
perceptible reddish light on the pavement. In an opposite case we find, in
walking through meadows, where we see scarcely anything but green, the stems
of trees and the roads often gleam with a reddish hue. This tone is not
uncommon in the works of landscape painters, especially those who practice in
water-colours: they probably see it in nature, and thus, unconsciously imitating
it, their colouring is criticised as unnatural.

60.

These phenomena are of the greatest importance, since they direct our attention
to the laws of vision, and are a necessary preparation for future observations on
colours. They show that the eye especially demands completeness, and seeks to
eke out the colorific circle in itself. The purple or violet colour suggested by
yellow contains red and blue; orange, which responds to blue, is composed of
yellow and red; green, uniting blue and yellow, demands red; and so through all
gradations of the most complicated combinations. That we are compelled in this
case to assume three leading colours has been already remarked by other
observers.

61.

When in this completeness the elements of which it is composed are still
appreciable by the eye, the result is justly called harmony. We shall subsequently
endeavour to show how the theory of the harmony of colours may be deduced
from these phenomena, and how, simply through these qualities, colours may be
capable of being applied to æsthetic purposes. This will be shown when we have
gone through the whole circle of our observations, returning to the point from
which we started.

[1] Plate 1, fig. 3.

VI.

COLOURED SHADOWS.

62.

Before, however, we proceed further, we have yet to observe some very
remarkable cases of the vivacity with which the suggested colours appear in the
neighbourhood of others: we allude to coloured shadows. To arrive at these we
first turn our attention to shadows that are colourless or negative.

63.

A shadow cast by the sun, in its full brightness, on a white surface, gives us no
impression of colour; it appears black, or, if a contrary light (here assumed to
differ only in degree) can act upon it, it is only weaker, half-lighted, grey.

64.

Two conditions are necessary for the existence of coloured shadows: first, that
the principal light tinge the white surface with some hue; secondly, that a
contrary light illumine to a certain extent the cast shadow.

65.

Let a short, lighted candle be placed at twilight on a sheet of white paper.
Between it and the declining daylight let a pencil be placed upright, so that its
shadow thrown by the candle may be lighted, but not overcome, by the weak
daylight: the shadow will appear of the most beautiful blue.

66.

That this shadow is blue is immediately evident; but we can only persuade
ourselves by some attention that the white paper acts as a reddish yellow, by
means of which the complemental blue is excited in the eye.—Note D.
67.

In all coloured shadows, therefore, we must presuppose a colour excited or
suggested by the hue of the surface on which the shadow is thrown. This may be
easily found to be the case by attentive consideration, but we may convince
ourselves at once by the following experiment.
68.

Place two candles at night opposite each other on a white surface; hold a thin rod
between them upright, so that two shadows be cast by it; take a coloured glass
and hold it before one of the lights, so that the white paper appear coloured; at

the same moment the shadow cast by the coloured light and slightly illumined by
the colourless one will exhibit the complemental hue.

69.

An important consideration suggests itself here, to which we shall frequently
have occasion to return. Colour itself is a degree of darkness σκιερόν; hence
Kircher is perfectly right in calling it lumen opacatum. As it is allied to shadow,
so it combines readily with it; it appears to us readily in and by means of shadow
the moment a suggesting cause presents itself. We could not refrain from
adverting at once to a fact which we propose to trace and develop hereafter.—
Note E.
70.

Select the moment in twilight when the light of the sky is still powerful enough
to cast a shadow which cannot be entirely effaced by the light of a candle. The
candle may be so placed that a double shadow shall be visible, one from the
candle towards the daylight, and another from the daylight towards the candle. If
the former is blue the latter will appear orange-yellow: this orange-yellow is in
fact, however, only the yellow-red light of the candle diffused over the whole
paper, and which becomes visible in shadow.

71.

This is best exemplified by the former experiment with two candles and coloured
glasses.
The surprising readiness with which shadow assumes a colour will again invite
our attention in the further consideration of reflections and elsewhere.

72.

Thus the phenomena of coloured shadows may be traced to their cause without
difficulty. Henceforth let any one who sees an instance of the kind observe only
with what hue the light surface on which they are thrown is tinged. Nay, the
colour of the shadow may be considered as a chromatoscope of the illumined
surface, for the spectator may always assume the colour of the light to be the
opposite of that of the shadow, and by an attentive examination may ascertain
this to be the fact in every instance.
73.

These appearances have been a source of great perplexity to former observers:
for, as they were remarked chiefly in the open air, where they commonly
appeared blue, they were attributed to a certain inherent blue or blue colouring
quality in the air. The inquirer can, however, convince himself, by the
experiment with the candle in a room, that no kind of blue light or reflection is
necessary to produce the effect in question. The experiment may be made on a
cloudy day with white curtains drawn before the light, and in a room where no
trace of blue exists, and the blue shadow will be only so much the more
beautiful.

74.

De Saussure, in the description of his ascent of Mont Blanc, says, "A second
remark, which may not be uninteresting, relates to the colour of the shadows.
These, notwithstanding the most attentive observation, we never found dark blue,
although this had been frequently the case in the plain. On the contrary, in fifty-
nine instances we saw them once yellowish, six times pale bluish, eighteen times
colourless or black, and thirty-four times pale violet. Some natural philosophers
suppose that these colours arise from accidental vapours diffused in the air,
which communicate their own hues to the shadows; not that the colours of the
shadows are occasioned by the reflection of any given sky colour or interposition
of any given air colour: the above observations seem to favour this opinion." The
instances given by De Saussure may be now explained and classed with
analogous examples without difficulty.
At a great elevation the sky was generally free from vapours, the sun shone in
full force on the snow, so that it appeared perfectly white to the eye: in this case
they saw the shadows quite colourless. If the air was charged with a certain
degree of vapour, in consequence of which the light snow would assume a
yellowish tone, the shadows were violet-coloured, and this effect, it appears,
occurred oftenest. They saw also bluish shadows, but this happened less
frequently; and that the blue and violet were pale was owing to the surrounding
brightness, by which the strength of the shadows was mitigated. Once only they
saw the shadow yellowish: in this case, as we have already seen (70), the shadow
is cast by a colourless light, and slightly illumined by a coloured one.

75.

In travelling over the Harz in winter, I happened to descend from the Brocken
towards evening; the wide slopes extending above and below me, the heath,
every insulated tree and projecting rock, and all masses of both, were covered
with snow or hoar-frost. The sun was sinking towards the Oder ponds[1]. During

the day, owing to the yellowish hue of the snow, shadows tending to violet had
already been observable; these might now be pronounced to be decidedly blue,
as the illumined parts exhibited a yellow deepening to orange.
But as the sun at last was about to set, and its rays, greatly mitigated by the
thicker vapours, began to diffuse a most beautiful red colour over the whole
scene around me, the shadow colour changed to a green, in lightness to be
compared to a sea-green, in beauty to the green of the emerald. The appearance
became more and more vivid: one might have imagined oneself in a fairy world,
for every object had clothed itself in the two vivid and so beautifully
harmonising colours, till at last, as the sun went down, the magnificent spectacle
was lost in a grey twilight, and by degrees in a clear moon-and-starlight night.

76.

One of the most beautiful instances of coloured shadows may be observed during
the full moon. The candle-light and moon-light may be contrived to be exactly
equal in force; both shadows may be exhibited with equal strength and clearness,
so that both colours balance each other perfectly. A white surface being placed
opposite the full moon, and the candle being placed a little on one side at a due
distance, an opaque body is held before the white plane, A double shadow will
then be seen: that cast by the moon and illumined by the candle-light will be a
powerful red-yellow; and contrariwise, that cast by the candle and illumined by
the moon will appear of the most beautiful blue. The shadow, composed of the
union of the two shadows, where they cross each other, is black. The yellow
shadow (74) cannot perhaps be exhibited in a more striking manner. The
immediate vicinity of the blue and the interposing black shadow make the
appearance the more agreeable. It will even be found, if the eye dwells long on
these colours, that they mutually evoke and enhance each other, the increasing
red in the one still producing its contrast, viz. a kind of sea-green.

77.

We are here led to remark that in this, and in all cases, a moment or two may
perhaps be necessary to produce the complemental colour. The retina must be
first thoroughly impressed with the demanding hue before the responding one
can be distinctly observable.

78.

When divers are under water, and the sunlight shines into the diving-bell,
everything is seen in a red light (the cause of which will be explained hereafter),
while the shadows appear green. The very same phenomenon which I observed

on a high mountain (75) is presented to others in the depths of the sea, and thus
Nature throughout is in harmony with herself.

79.

Some observations and experiments which equally illustrate what has been stated
with regard to coloured objects and coloured shadows may be here added. Let a
white paper blind be fastened inside the window on a winter evening; in this
blind let there be an opening, through which the snow of some neighbouring roof
can be seen. Towards dusk let a candle be brought into the room; the snow seen
through the opening will then appear perfectly blue, because the paper is tinged
with warm yellow by the candle-light. The snow seen through the aperture is
here equivalent to a shadow illumined by a contrary light (76), and may also
represent a grey disk on a coloured surface (56).
80.

Another very interesting experiment may conclude these examples. If we take a
piece of green glass of some thickness, and hold it so that the window bars be
reflected in it, they will appear double owing to the thickness of the glass. The
image which is reflected from the under surface of the glass will be green; the
image which is reflected from the upper surface, and which should be colourless,
will appear red.
The experiment may be very satisfactorily made by pouring water into a vessel,
the inner surface of which can act as a mirror; for both reflections may first be
seen colourless while the water is pure, and then by tinging it, they will exhibit
two opposite hues.

[1] Reservoirs in which water is collected from various small streams, to work the
mines.—T.

VII.

FAINT LIGHTS.

81.

Light, in its full force, appears purely white, and it gives this impression also in
its highest degree of dazzling splendour. Light, which is not so powerful, can
also, under various conditions, remain colourless. Several naturalists and

mathematicians have endeavoured to measure its degrees—Lambert, Bouguer,
Rumford.

82.

Yet an appearance of colour presently manifests itself in fainter lights, for in their
relation to absolute light they resemble the coloured spectra of dazzling objects
(39).

83.

A light of any kind becomes weaker, either when its own force, from whatever
cause, is diminished, or when the eye is so circumstanced or placed, that it
cannot be sufficiently impressed by the action of the light. Those appearances
which may be called objective, come under the head of physical colours. We will
only advert here to the transition from white to red heat in glowing iron. We may
also observe that the flames of lights at night appear redder in proportion to their
distance from the eye.—Note F.

84.

Candle-light at night acts as yellow when seen near; we can perceive this by the
effect it produces on other colours. At night a pale yellow is hardly to be
distinguished from white; blue approaches to green, and rose-colour to orange.

85.

Candle-light at twilight acts powerfully as a yellow light: this is best proved by
the purple blue shadows which, under these circumstances, are evoked by the
eye.
86.

The retina may be so excited by a strong light that it cannot perceive fainter
lights (11): if it perceive these they appear coloured: hence candle-light by day
appears reddish, thus resembling, in its relation to fuller light, the spectrum of a
dazzling object; nay, if at night we look long and intently on the flame of a light,
it appears to increase in redness.

87.

There are faint lights which, notwithstanding their moderate lustre, give an
impression of a white, or, at the most, of a light yellow appearance on the retina;
such as the moon in its full splendour. Rotten wood has even a kind of bluish
light. All this will hereafter be the subject of further remarks.

88.

If at night we place a light near a white or greyish wall so that the surface be
illumined from this central point to some extent, we find, on observing the
spreading light at some distance, that the boundary of the illumined surface
appears to be surrounded with a yellow circle, which on the outside tends to red-
yellow. We thus observe that when light direct or reflected does not act in its full
force, it gives an impression of yellow, of reddish, and lastly even of red. Here
we find the transition to halos which we are accustomed to see in some mode or
other round luminous points.

VIII.

SUBJECTIVE HALOS.

89.

Halos may be divided into subjective and objective. The latter will be considered
under the physical colours; the first only belong here. These are distinguished
from the objective halos by the circumstance of their vanishing when the point of
light which produces them on the retina is covered.

90.

We have before noticed the impression of a luminous object on the retina, and
seen that it appears larger: but the effect is not at an end here, it is not confined to
the impression of the image; an expansive action also takes place, spreading
from the centre.

91.

That a nimbus of this kind is produced round the luminous image in the eye may
be best seen in a dark room, if we look towards a moderately large opening in the
window-shutter. In this case the bright image is surrounded by a circular misty
light. I saw such a halo bounded by a yellow and yellow-red circle on opening
my eyes at dawn, on an occasion when I passed several nights in a bed-carriage.
92.

Halos appear most vivid when the eye is susceptible from having been in a state
of repose. A dark background also heightens their appearance. Both causes

account for our seeing them so strong if a light is presented to the eyes on
waking at night. These conditions were combined when Descartes after sleeping,
as he sat in a ship, remarked such a vividly-coloured halo round the light.

93.

A light must shine moderately, not dazzle, in order to produce the impression of
a halo in the eye; at all events the halos of dazzling lights cannot be observed.
We see a splendour of this kind round the image of the sun reflected from the
surface of water.
94.

A halo of this description, attentively observed, is found to be encircled towards
its edge with a yellow border: but even here the expansive action, before alluded
to, is not at an end, but appears still to extend in varied circles.

95.

Several cases seem to indicate a circular action of the retina, whether owing to
the round form of the eye itself and its different parts, or to some other cause.

96.

If the eye is pressed only in a slight degree from the inner corner, darker or
lighter circles appear. At night, even without pressure, we can sometimes
perceive a succession of such circles emerging from, or spreading over, each
other.

97.

We have already seen that a yellow border is apparent round the white space
illumined by a light placed near it. This may be a kind of objective halo. (88.)

98.

Subjective halos may be considered as the result of a conflict between the light
and a living surface. From the conflict between the exciting principle and the
excited, an undulating motion arises, which may be illustrated by a comparison
with the circles on water. The stone thrown in drives the water in all directions;
the effect attains a maximum, it reacts, and being opposed, continues under the
surface. The effect goes on, culminates again, and thus the circles are repeated. If
we have ever remarked the concentric rings which appear in a glass of water on
trying to produce a tone by rubbing the edge; if we call to mind the intermitting
pulsations in the reverberations of bells, we shall approach a conception of what

may take place on the retina when the image of a luminous object impinges on it,
not to mention that as a living and elastic structure, it has already a circular
principle in its organisation.—Note G.

99.

The bright circular space which appears round the shining object is yellow,
ending in red: then follows a greenish circle, which is terminated by a red border.
This appears to be the usual phenomenon where the luminous body is somewhat
considerable in size. These halos become greater the more distant we are from
the luminous object.

100.

Halos may, however, appear extremely small and numerous when the impinging
image is minute, yet powerful, in its effect. The experiment is best made with a
piece of gold-leaf placed on the ground and illumined by the sun. In these cases
the halos appear in variegated rays. The iridescent appearance produced in the
eye when the sun pierces through the leaves of trees seems also to belong to the
same class of phenomena.

PATHOLOGICAL COLOURS.

APPENDIX.

101.

We are now sufficiently acquainted with the physiological colours to distinguish
them from the pathological. We know what appearances belong to the eye in a
healthy state, and are necessary to enable the organ to exert its complete vitality
and activity.

102.

Morbid phenomena indicate in like manner the existence of organic and physical
laws: for if a living being deviates from those rules with reference to which it is
constructed, it still seeks to agree with the general vitality of nature in
conformity with general laws, and throughout its whole course still proves the
constancy of those principles on which the universe has existed, and by which it
is held together.

103.

We will here first advert to a very remarkable state in which the vision of many
persons is found to be. As it presents a deviation from the ordinary mode of
seeing colours, it might be fairly classed under morbid impressions; but as it is
consistent in itself, as it often occurs, may extend to several members of a family,
and probably does not admit of cure, we may consider it as bordering only on the
nosological cases, and therefore place it first.

104.

I was acquainted with two individuals not more than twenty years of age, who
were thus affected: both had bluish-grey eyes, an acute sight for near and distant
objects, by day-light and candle-light, and their mode of seeing colours was in
the main quite similar.
105.

They agreed with the rest of the world in denominating white, black, and grey in
the usual manner. Both saw white untinged with any hue. One saw a somewhat
brownish appearance in black, and in grey a somewhat reddish tinge. In general
they appeared to have a very delicate perception of the gradations of light and
dark.
106.

They appeared to see yellow, red-yellow, and yellow-red,[1] like others: in the
last case they said they saw the yellow passing as it were over the red as if
glazed: some thickly-ground carmine, which had dried in a saucer, they called
red.
107.

But now a striking difference presented itself. If the carmine was passed thinly
over the white saucer, they would compare the light colour thus produced to the
colour of the sky, and call it blue. If a rose was shown them beside it, they
would, in like manner, call it blue; and in all the trials which were made, it
appeared that they could not distinguish light blue from rose-colour. They
confounded rose-colour, blue, and violet on all occasions: these colours only
appeared to them to be distinguished from each other by delicate shades of
lighter, darker, intenser, or fainter appearance.
108.

Again they could not distinguish green from dark orange, nor, more especially,
from a red brown.

109.

If any one, accidentally conversing with these individuals, happened to question
them about surrounding objects, their answers occasioned the greatest perplexity,
and the interrogator began to fancy his own wits were out of order. With some
method we may, however, approach to a nearer knowledge of the law of this
deviation from the general law.

110.

These persons, as may be gathered from what has been stated, saw fewer colours
than other people: hence arose the confusion of different colours. They called the
sky rose-colour, and the rose blue, or vice versâ. The question now is: did they
see both blue or both rose-colour? did they see green orange, or orange green?

111.

This singular enigma appears to solve itself, if we assume that they saw no blue,
but, instead of it, a light pure red, a rose-colour. We can comprehend what would
be the result of this by means of the chromatic diagram.

112.

If we take away blue from the chromatic circle we shall miss violet and green as
well. Pure red occupies the place of blue and violet, and in again mixing with
yellow the red produces orange where green should be.
113.

Professing to be satisfied with this mode of explanation, we have named this
remarkable deviation from ordinary vision "Acyanoblepsia."[2] We have prepared
some coloured figures for its further elucidation, and in explaining these we shall
add some further details. Among the examples will be found a landscape,
coloured in the mode in which the individuals alluded to appeared to see nature:
the sky rose-colour, and all that should be green varying from yellow to brown
red, nearly as foliage appears to us in autumn[3].—Note H.

114.

We now proceed to speak of morbid and other extraordinary affections of the
retina, by which the eye may be susceptible of an appearance of light without

external light, reserving for a future occasion the consideration of galvanic light.

115.

If the eye receives a blow, sparks seem to spread from it. In some states of body,
again, when the blood is heated, and the system much excited, if the eye is
pressed first gently, and then more and more strongly, a dazzling and intolerable
light may be excited.
116.

If those who have been recently couched experience pain and heat in the eye,
they frequently see fiery flashes and sparks: these symptoms last sometimes for a
week or fortnight, or till the pain and heat diminish.

117.

A person suffering from ear-ache saw sparks and balls of light in the eye during
each attack, as long as the pain lasted.

118.

Persons suffering from worms often experience extraordinary appearances in the
eye, sometimes sparks of fire, sometimes spectres of light, sometimes frightful
figures, which they cannot by an effort of the will cease to see: sometimes these
appearances are double.

119.

Hypochondriacs frequently see dark objects, such as threads, hairs, spiders, flies,
wasps. These appearances also exhibit themselves in the incipient hard cataract.
Many see semi-transparent small tubes, forms like wings of insects, bubbles of
water of various sizes, which fall slowly down, if the eye is raised: sometimes
these congregate together so as to resemble the spawn of frogs; sometimes they
appear as complete spheres, sometimes in the form of lenses.

120.

As light appeared, in the former instances, without external light, so also these
images appear without corresponding external objects. The images are
sometimes transient, sometimes they last during the patient's life. Colour, again,
frequently accompanies these impressions: for hypochondriacs often see yellow-
red stripes in the eye: these are generally more vivid and numerous in the
morning, or when lasting.

121.

We have before seen that the impression of any object may remain for a time in
the eye: this we have found to be a physiological phenomenon (23): the
excessive duration of such an impression, on the other band, may be considered
as morbid.

122.

The weaker the organ the longer the impression of the image lasts. The retina
does not so soon recover itself; and the effect may be considered as a kind of
paralysis (28).

123.

This is not to be wondered at in the case of dazzling lights. If any one looks at
the sun, he may retain the image in his eyes for several days. Boyle relates an
instance of ten years.
124.

The same takes place, in a certain degree, with regard to objects that are not
dazzling. Büsch relates of himself that the image of an engraving, complete in all
its parts, was impressed on his eye for seventeen minutes.

125.

A person inclined to fulness of blood retained the image of a bright red calico,
with white spots, many minutes in the eye, and saw it float before everything like
a veil. It only disappeared by rubbing the eye for some time.

126.

Scherfer observes that the red colour, which is the consequence of a powerful
impression of light, may last for some hours.

127.

As we can produce an appearance of light on the retina by pressure on the
eyeball, so by a gentle pressure a red colour appears, thus corresponding with the
after-image of an impression of light.

128.

Many sick persons, on awaking, see everything in the colour of the morning sky,
as if through a red veil: so, if in the evening they doze and wake again, the same

appearance presents itself. It remains for some minutes, and always disappears if
the eye is rubbed a little. Red stars and balls sometimes accompany the
impression. This state may last for a considerable time.

129.

The aëronauts, particularly Zambeccari and his companions, relate that they saw
the moon blood-red at the highest elevation. As they had ascended above the
vapours of the earth, through which we see the moon and sun naturally of such a
colour, it may be suspected that this appearance may be classed with the
pathological colours. The senses, namely, may be so influenced by an unusual
state, that the whole nervous system, and particularly the retina, may sink into a
kind of inertness and inexcitability. Hence it is not impossible that the moon
might act as a very subdued light, and thus produce the impression of the red
colour. The sun even appeared blood-red to the aëronauts of Hamburgh.
If those who are at some elevation in a balloon scarcely hear each other speak,
may not this, too, be attributed to the inexcitable state of the nerves as well as to
the thinness of the air?

130.

Objects are often seen by sick persons in variegated colours. Boyle relates an
instance of a lady, who, after a fall by which an eye was bruised, saw all objects,
but especially white objects, glittering in colours, even to an intolerable degree.

131.

Physicians give the name of "Chrupsia" to an affection of the sight, occurring in
typhoid maladies. In these cases the patients state that they see the boundaries of
objects coloured where light and dark meet. A change probably takes place in the
humours of the eye, through which their achromatism is affected.

132.

In cases of milky cataract, a very turbid crystalline lens causes the patient to see
a red light. In a case of this kind, which was treated by the application of
electricity, the red light changed by degrees to yellow, and at last to white, when
the patient again began to distinguish objects. These changes of themselves
warranted the conclusion that the turbid state of the lens was gradually
approaching the transparent state. We shall be enabled easily to trace this effect
to its source as soon as we become better acquainted with the physical colours.
133.

If again it may be assumed that a jaundiced patient sees through an actually
yellow-coloured humour, we are at once referred to the department of chemical
colours, and it is thus evident that we can only thoroughly investigate the chapter
of pathological colours when we have made ourselves acquainted with the whole
range of the remaining phenomena. What has been adduced may therefore
suffice for the present, till we resume the further consideration of this portion of
our subject.

134.

In conclusion we may, however, at once advert to some peculiar states or
dispositions of the organ.
There are painters who, instead of rendering the colours of nature, diffuse a
general tone, a warm or cold hue, over the picture. In some, again, a predilection
for certain colours displays itself; in others a want of feeling for harmony.

135.

Lastly, it is also worthy of remark, that savage nations, uneducated people, and
children have a great predilection for vivid colours; that animals are excited to
rage by certain colours; that people of refinement avoid vivid colours in their
dress and the objects that are about them, and seem inclined to banish them
altogether from their presence.—Note I.

[1] It has been found necessary to follow the author's nomenclature throughout—T.
[2] Non-perception of blue.
[3] It has not been thought necessary to copy the plates here referred to.—T.

PART II.

PHYSICAL COLOURS.

136.

We give this designation to colours which are produced by certain material
mediums: these mediums, however, have no colour themselves, and may be
either transparent, semi-transparent yet transmitting light, or altogether opaque.
The colours in question are thus produced in the eye through such external given
causes, or are merely reflected to the eye when by whatever means they are
already produced without us. Although we thus ascribe to them a certain
objective character, their distinctive quality still consists in their being transient,
and not to be arrested.

137.

They are called by former investigators colores apparentes, fluxi, fugitivi,
phantastici, falsi, variantes. They are also called speciosi and emphatici, on
account of their striking splendour. They are immediately connected with the
physiological colours, and appear to have but little more reality: for, while in the
production of the physiological colours the eye itself was chiefly efficient, and
we could only perceive the phenomena thus evoked within ourselves, but not
without us, we have now to consider the fact that colours are produced in the eye
by means of colourless objects; that we thus too have a colourless surface before
us which is acted upon as the retina itself is, and that we can perceive the
appearance produced upon it without us. In such a process, however, every
observation will convince us that we have to do with colours in a progressive and
mutable, but not in a final or complete, state.

138.

Hence, in directing our attention to these physical colours, we find it quite
possible to place an objective phenomenon beside a subjective one, and often by
means of the union of the two successfully to penetrate farther into the nature of
the appearance.

139.

Thus, in the observations by which we become acquainted with the physical
colours, the eye is not to be considered as acting alone; nor is the light ever to be
considered in immediate relation with the eye: but we direct our attention
especially to the various effects produced by mediums, those mediums being
themselves colourless.

140.

Light under these circumstances may be affected by three conditions. First, when
it flashes back from the surface of a medium; in considering which catoptrical
experiments invite our attention. Secondly, when it passes by the edge of a
medium: the phenomena thus produced were formerly called perioptical; we
prefer the term paroptical. Thirdly, when it passes through either a merely light-
transmitting or an actually transparent body; thus constituting a class of
appearances on which dioptrical experiments are founded. We have called a
fourth class of physical colours epoptical, as the phenomena exhibit themselves
on the colourless surface of bodies under various conditions, without previous or
actual dye (βαφή).—Note K.
141.

In examining these categories with reference to our three leading divisions,
according to which we consider the phenomena of colours in a physiological,
physical, or chemical view, we find that the catoptrical colours are closely
connected with the physiological; the paroptical are already somewhat more
distinct and independent; the dioptrical exhibit themselves as entirely and strictly
physical, and as having a decidedly objective character; the epoptical, although
still only apparent, may be considered as the transition to the chemical colours.
142.

If we were desirous of prosecuting our investigation strictly in the order of
nature, we ought to proceed according to the classification which has just been
made; but in didactic treatises it is not of so much consequence to connect as to
duly distinguish the various divisions of a subject, in order that at last, when
every single class and case has been presented to the mind, the whole may be
embraced in one comprehensive view. We therefore turn our attention forthwith
to the dioptrical class, in order at once to give the reader the full impression of
the physical colours, and to exhibit their characteristics the more strikingly.

IX.

DIOPTRICAL COLOURS.

143.

Colours are called dioptrical when a colourless medium is necessary to produce
them; the medium must be such that light and darkness can act through it either
on the eye or on opposite surfaces. It is thus required that the medium should be
transparent, or at least capable, to a certain degree, of transmitting light.

144.

According to these conditions we divide the dioptrical phenomena into two
classes, placing in the first those which are produced by means of imperfectly
transparent, yet light-transmitting mediums; and in the second such as are
exhibited when the medium is in the highest degree transparent.

X.

DIOPTRICAL COLOURS OF THE FIRST CLASS.

145.

Space, if we assume it to be empty, would have the quality of absolute
transparency to our vision. If this space is filled so that the eye cannot perceive
that it is so, there exists a more or less material transparent medium, which may
be of the nature of air and gas, may be fluid or even solid.

146.

The pure and light-transmitting semi-transparent medium is only an accumulated
form of the transparent medium. It may therefore be presented to us in three
modes.
147.

The extreme degree of this accumulation is white; the simplest, brightest, first,
opaque occupation of space.
148.

Transparency itself, empirically considered, is already the first degree of the
opposite state. The intermediate degrees from this point to opaque white are
infinite.

149.

At whatever point short of opacity we arrest the thickening medium, it exhibits
simple and remarkable phenomena when placed in relation with light and
darkness.

150.

The highest degree of light, such as that of the sun, of phosphorus burning in
oxygen, is dazzling and colourless: so the light of the fixed stars is for the most
part colourless. This light, however, seen through a medium but very slightly
thickened, appears to us yellow. If the density of such a medium be increased, or
if its volume become greater, we shall see the light gradually assume a yellow-
red hue, which at last deepens to a ruby-colour.—Note L.

151.

If on the other hand darkness is seen through a semi-transparent medium, which
is itself illumined by a light striking on it, a blue colour appears: this becomes
lighter and paler as the density of the medium is increased, but on the contrary
appears darker and deeper the more transparent the medium becomes: in the least
degree of dimness short of absolute transparence, always supposing a perfectly
colourless medium, this deep blue approaches the most beautiful violet.

152.

If this effect takes place in the eye as here described, and may thus be
pronounced to be subjective, it remains further to convince ourselves of this by
objective phenomena. For a light thus mitigated and subdued illumines all
objects in like manner with a yellow, yellow-red, or red hue; and, although the
effect of darkness through the non-transparent medium does not exhibit itself so
powerfully, yet the blue sky displays itself in the camera obscura very distinctly
on white paper, as well as every other material colour.

153.

In examining the cases in which this important leading phenomenon appears, we
naturally mention the atmospheric colours first: most of these may be here
introduced in order.

154.

The sun seen through a certain degree of vapour appears with a yellow disk; the
centre is often dazzlingly yellow when the edges are already red. The orb seen
through a thick yellow mist appears ruby-red (as was the case in 1794, even in
the north); the same appearance is still more decided, owing to the state of the
atmosphere, when the scirocco prevails in southern climates: the clouds generally
surrounding the sun in the latter case are of the same colour, which is reflected
again on all objects.
The red hues of morning and evening are owing to the same cause. The sun is
announced by a red light, in shining through a greater mass of vapours. The
higher he rises, the yellower and brighter the light becomes.

155.

If the darkness of infinite space is seen through atmospheric vapours illumined
by the day-light, the blue colour appears. On high mountains the sky appears by
day intensely blue, owing to the few thin vapours that float before the endless
dark space: as soon as we descend in the valleys, the blue becomes lighter; till at
last, in certain regions, and in consequence of increasing vapours, it altogether
changes to a very pale blue.

156.

The mountains, in like manner, appear to us blue; for, as we see them at so great
a distance that we no longer distinguish the local tints, and as no light reflected
from their surface acts on our vision, they are equivalent to mere dark objects,
which, owing to the interposed vapours, appear blue.
157.

So we find the shadowed parts of nearer objects are blue when the air is charged
with thin vapours.

158.

The snow-mountains, on the other hand, at a great distance, still appear white, or
approaching to a yellowish hue, because they act on our eyes as brightness seen
through atmospheric vapour.

159.

The blue appearance at the lower part of the flame of a candle belongs to the
same class of phenomena. If the flame be held before a white ground, no blue

will be seen, but this colour will immediately appear if the flame is opposed to a
black ground. This phenomenon may be exhibited most strikingly with a
spoonful of lighted spirits of wine. We may thus consider the lower part of the
flame as equivalent to the vapour which, although infinitely thin, is still apparent
before the dark surface; it is so thin, that one may easily see to read through it: on
the other hand, the point of the flame which conceals objects from our sight is to
be considered as a self-illuminating body.

160.

Lastly, smoke is also to be considered as a semi-transparent medium, which
appears to us yellow or reddish before a light ground, but blue before a dark one.

161.

If we now turn our attention to fluid mediums, we find that water, deprived in a
very slight degree of its transparency, produces the same effects.

162.

The infusion of the lignum nephriticum (guilandina Linnæi), which formerly
excited so much attention, is only a semi-transparent liquor, which in dark
wooden cups must appear blue, but held towards the sun in a transparent glass
must exhibit a yellow appearance.

163.

A drop of scented water, of spirit varnish, of several metallic solutions, may be
employed to give various degrees of opacity to water for such experiments. Spirit
of soap perhaps answers best.
164.

The bottom of the sea appears to divers of a red colour in bright sunshine: in this
case the water, owing to its depth, acts as a semi-transparent medium. Under
these circumstances, they find the shadows green, which is the complemental
colour.

165.

Among solid mediums the opal attracts our attention first: its colours are, at least,
partly to be explained by the circumstance that it is, in fact, a semi-transparent
medium, through which sometimes light, sometimes dark, substrata are visible.

166.

For these experiments, however, the opal-glass (vitrum astroides, girasole) is the
most desirable material. It is prepared in various ways, and its semi-opacity is
produced by metallic oxydes. The same effect is produced also by melting
pulverised and calcined bones together with the glass, on which account it is also
known by the name of beinglas; but, prepared in this mode, it easily becomes too
opaque.

167.

This glass may be adapted for experiments in various ways: it may either be
made in a very slight degree non-transparent, in which case the light seen
through various layers placed one upon the other may be deepened from the
lightest yellow to the deepest red, or, if made originally more opaque, it may be
employed in thinner or thicker laminæ. The experiments may be successfully
made in both ways: in order, however, to see the bright blue colour, the glass
should neither be too opaque nor too thick. For, as it is quite natural that darkness
must act weakly through the semi-transparent medium, so this medium, if too
thick, soon approaches whiteness.
168.

Panes of glass throw a yellow light on objects through those parts where they
happen to be semi-opaque, and these same parts appear blue if we look at a dark
object through them.

169.

Smoked glass may be also mentioned here, and is, in like manner, to be
considered as a semi-opaque medium. It exhibits the sun more or less ruby-
coloured; and, although this appearance may be attributed to the black-brown
colour of the soot, we may still convince ourselves that a semi-transparent
medium here acts if we hold such a glass moderately smoked, and lit by the sun
on the unsmoked side, before a dark object, for we shall then perceive a bluish
appearance.

170.

A striking experiment may be made in a dark room with sheets of parchment. If
we fasten a piece of parchment before the opening in the window-shutter when
the sun shines, it will appear nearly white; by adding a second, a yellowish
colour appears, which still increases as more leaves are added, till at last it
changes to red.

171.

A similar effect, owing to the state of the crystalline lens in milky cataract, has
been already adverted to (131).

172.

Having now, in tracing these phenomena, arrived at the effect of a degree of
opacity scarcely capable of transmitting light, we may here mention a singular
appearance which was owing to a momentary state of this kind.
A portrait of a celebrated theologian had been painted some years before the
circumstance to which we allude, by an artist who was known to have
considerable skill in the management of his materials. The very reverend
individual was represented in a rich velvet dress, which was not a little admired,
and which attracted the eye of the spectator almost more than the face. The
picture, however, from the effect of the smoke of lamps and dust, had lost much
of its original vivacity. It was, therefore, placed in the hands of a painter, who
was to clean it, and give it a fresh coat of varnish. This person began his
operations by carefully washing the picture with a sponge: no sooner, however,
had he gone over the surface once or twice, and wiped away the first dirt, than to
his amazement the black velvet dress changed suddenly to a light blue plush,
which gave the ecclesiastic a very secular, though somewhat old-fashioned,
appearance. The painter did not venture to go on with his washing: he could not
comprehend how a light blue should be the ground of the deepest black, still less
how he could so suddenly have removed a glazing colour capable of converting
the one tint to the other.
At all events, he was not a little disconcerted at having spoilt the picture to such
an extent. Nothing to characterize the ecclesiastic remained but the richly-curled
round wig, which made the exchange of a faded plush for a handsome new velvet
dress far from desirable. Meanwhile, the mischief appeared irreparable, and the
good artist, having turned the picture to the wall, retired to rest with a mind ill at
ease. But what was his joy the next morning, when, on examining the picture, he
beheld the black velvet dress again in its full splendour. He could not refrain
from again wetting a corner, upon which the blue colour again appeared, and
after a time vanished. On hearing of this phenomenon, I went at once to see the
miraculous picture. A wet sponge was passed over it in my presence, and the
change quickly took place. I saw a somewhat faded, but decidedly light blue
plush dress, the folds under the arm being indicated by some brown strokes.
I explained this appearance to myself by the doctrine of the semi-opaque
medium. The painter, in order to give additional depth to his black, may have

passed some particular varnish over it: on being washed, this varnish imbibed
some moisture, and hence became semi-opaque, in consequence of which the
black underneath immediately appeared blue. Perhaps those who are practically
acquainted with the effect of varnishes may, through accident or contrivance,
arrive at some means of exhibiting this singular appearance, as an experiment, to
those who are fond of investigating natural phenomena. Notwithstanding many
attempts, I could not myself succeed in re-producing it.

173.

Having now traced the most splendid instances of atmospheric appearances, as
well as other less striking yet sufficiently remarkable cases, to the leading
examples of semi-transparent mediums, we have no doubt that attentive
observers of nature will carry such researches further, and accustom themselves
to trace and explain the various appearances which present themselves in every-
day experience on the same principle: we may also hope that such investigators
will provide themselves with an adequate apparatus in order to place remarkable
facts before the eyes of others who may be desirous of information.

174.

We venture, once for all, to call the leading appearance in question, as generally
described in the foregoing pages, a primordial and elementary phenomenon; and
we may here be permitted at once to state what we understand by the term.

175.

The circumstances which come under our notice in ordinary observation are, for
the most part, insulated cases, which, with some attention, admit of being classed
under general leading facts. These again range themselves under theoretical
rubrics which are more comprehensive, and through which we become better
acquainted with certain indispensable conditions of appearances in detail. From
henceforth everything is gradually arranged under higher rules and laws, which,
however, are not to be made intelligible by words and hypotheses to the
understanding merely, but, at the same time, by real phenomena to the senses. We
call these primordial phenomena, because nothing appreciable by the senses lies
beyond them, on the contrary, they are perfectly fit to be considered as a fixed
point to which we first ascended, step by step, and from which we may, in like
manner, descend to the commonest case of every-day experience. Such an
original phenomenon is that which has lately engaged our attention. We see on
the one side light, brightness; on the other darkness, obscurity: we bring the
semi-transparent medium between the two, and from these contrasts and this

medium the colours develop themselves, contrasted, in like manner, but soon,
through a reciprocal relation, directly tending again to a point of union.[1]

176.

With this conviction we look upon the mistake that has been committed in the
investigation of this subject to be a very serious one, inasmuch as a secondary
phenomenon has been thus placed higher in order—the primordial phenomenon
has been degraded to an inferior place; nay, the secondary phenomenon has been
placed at the head, a compound effect has been treated as simple, a simple
appearance as compound: owing to this contradiction, the most capricious
complication and perplexity have been introduced into physical inquiries, the
effects of which are still apparent.

177.

But when even such a primordial phenomenon is arrived at, the evil still is that
we refuse to recognise it as such, that we still aim at something beyond, although
it would become us to confess that we are arrived at the limits of experimental
knowledge. Let the observer of nature suffer the primordial phenomenon to
remain undisturbed in its beauty; let the philosopher admit it into his department,
and he will find that important elementary facts are a worthier basis for further
operations than insulated cases, opinions, and hypotheses.—Note M.

[1] That is (according to the author's statement 150. 151.) both tend to red; the yellow
deepening to orange as the comparatively dark medium is thickened before brightness;
the blue deepening to violet as the light medium is thinned before darkness.—T.

XI.

DIOPTRICAL COLOURS OF THE SECOND CLASS.—REFRACTION.

178.

Dioptrical colours of both classes are closely connected, as will presently appear
on a little examination. Those of the first class appeared through semi-
transparent mediums, those of the second class will now appear through
transparent mediums. But since every substance, however transparent, may be
already considered to partake of the opposite quality (as every accumulation of a

medium called transparent proves), so the near affinity of the two classes is
sufficiently manifest.

179.

We will, however, first consider transparent mediums abstractedly as such, as
entirely free from any degree of opacity, and direct our whole attention to a
phenomenon which here presents itself, and which is known by the name of
refraction.
180.

In treating of the physiological colours, we have already had occasion to
vindicate what were formerly called illusions of sight, as the active energies of
the healthy and duly efficient eye (2), and we are now again invited to consider
similar instances confirming the constancy of the laws of vision.

181.

Throughout nature, as presented to the senses, everything depends on the relation
which things bear to each other, but especially on the relation which man, the
most important of these, bears to the rest. Hence the world divides itself into two
parts, and the human being as subject, stands opposed to the object. Thus the
practical man exhausts himself in the accumulation of facts, the thinker in
speculation; each being called upon to sustain a conflict which admits of no
peace and no decision.

182.

But still the main point always is, whether the relations are truly seen. As our
senses, if healthy, are the surest witnesses of external relations, so we may be
convinced that, in all instances where they appear to contradict reality, they lay
the greater and surer stress on true relations. Thus a distant object appears to us
smaller; and precisely by this means we are aware of distance. We produced
coloured appearances on colourless objects, through colourless mediums, and at
the same moment our attention was called to the degree of opacity in the
medium.
183.

Thus the different degrees of opacity in so-called transparent mediums, nay, even
other physical and chemical properties belonging to them, are known to our
vision by means of refraction, and invite us to make further trials in order to

penetrate more completely by physical and chemical means into those secrets
which are already opened to our view on one side.

184.

Objects seen through mediums more or less transparent do not appear to us in the
place which they should occupy according to the laws of perspective. On this
fact the dioptrical colours of the second class depend.

185.

Those laws of vision which admit of being expressed in mathematical formulæ
are based on the principle that, as light proceeds in straight lines, it must be
possible to draw a straight line from the eye to any given object in order that it be
seen. If, therefore, a case arises in which the light arrives to us in a bent or
broken line, that we see the object by means of a bent or broken line, we are at
once informed that the medium between the eye and the object is denser, or that
it has assumed this or that foreign nature.

186.

This deviation from the law of right-lined vision is known by the general term of
refraction; and, although we may take it for granted that our readers are
sufficiently acquainted with its effects, yet we will here once more briefly exhibit
it in its objective and subjective point of view.

187.

Let the sun shine diagonally into an empty cubical vessel, so that the opposite
side be illumined, but not the bottom: let water be then poured into this vessel,
and the direction of the light will be immediately altered; for a part of the bottom
is shone upon. At the point where the light enters the thicker medium it deviates
from its rectilinear direction, and appears broken: hence the phenomenon is
called the breaking (brechung) or refraction. Thus much of the objective
experiment.
188.

We arrive at the subjective fact in the following mode:—Let the eye be
substituted for the sun: let the sight be directed in like manner diagonally over
one side, so that the opposite inner side be entirely seen, while no part of the
bottom is visible. On pouring in water the eye will perceive a part of the bottom;
and this takes place without our being aware that we do not see in a straight line;
for the bottom appears to us raised, and hence we give the term elevation

(hebung) to the subjective phenomenon. Some points, which are particularly
remarkable with reference to this, will be adverted to hereafter.

189.

Were we now to express this phenomenon generally, we might here repeat, in
conformity with the view lately taken, that the relation of the objects is changed
or deranged.
190.

But as it is our intention at present to separate the objective from the subjective
appearances, we first express the phenomenon in a subjective form, and say,—a
derangement or displacement of the object seen, or to be seen, takes place.

191.

But that which is seen without a limiting outline may be thus affected without
our perceiving the change. On the other hand, if what we look at has a visible
termination, we have an evident indication that a displacement occurs. If,
therefore, we wish to ascertain the relation or degree of such a displacement, we
must chiefly confine ourselves to the alteration of surfaces with visible
boundaries; in other words, to the displacement of circumscribed objects.

192.

The general effect may take place through parallel mediums, for every parallel
medium displaces the object by bringing it perpendicularly towards the eye. The
apparent change of position is, however, more observable through mediums that
are not parallel.

193.

These latter may be perfectly spherical, or may be employed in the form of
convex or concave lenses. We shall make use of all these as occasion may
require in our experiments. But as they not only displace the object from its
position, but alter it in various ways, we shall, in most cases, prefer employing
mediums with surfaces, not, indeed, parallel with reference to each other, but still
altogether plane, namely, prisms. These have a triangle for their base, and may, it
is true, be considered as portions of a lens, but they are particularly available for
our experiments, inasmuch as they very perceptibly displace the object from its
position, without producing a remarkable distortion.

194.

And now, in order to conduct our observations with as much exactness as
possible, and to avoid all confusion and ambiguity, we confine ourselves at first
to

SUBJECTIVE EXPERIMENTS,

in which, namely, the object is seen by the observer through a refracting medium.
As soon as we have treated these in due series, the objective experiments will
follow in similar order.

XII.

REFRACTION WITHOUT THE APPEARANCE OF COLOUR.

195.

Refraction can visibly take place without our perceiving an appearance of colour.
To whatever extent a colourless or uniformly coloured surface may be altered as
to its position by refraction, no colour consequent upon refraction appears within
it, provided it has no outline or boundary. We may convince ourselves of this in
various ways.

196.

Place a glass cube on any larger surface, and look through the glass
perpendicularly or obliquely, the unbroken surface opposite the eye appears
altogether raised, but no colour exhibits itself. If we look at a pure grey or blue
sky or a uniformly white or coloured wall through a prism, the portion of the
surface which the eye thus embraces will be altogether changed as to its position,
without our therefore observing the smallest appearance of colour.

XIII.

CONDITIONS OF THE APPEARANCE OF COLOUR.

197.

Although in the foregoing experiments we have found all unbroken surfaces,
large or small, colourless, yet at the outlines or boundaries, where the surface is
relieved upon a darker or lighter object, we observe a coloured appearance.

198.

Outline, as well as surface, is necessary to constitute a figure or circumscribed
object. We therefore express the leading fact thus: circumscribed objects must be
displaced by refraction in order to the exhibition of an appearance of colour.

199.

We place before us the simplest object, a light disk on a dark ground (A).[1] A
displacement occurs with regard to this object, if we apparently extend its outline
from the centre by magnifying it. This may be done with any convex glass, and
in this case we see a blue edge (B).
200.

We can, to appearance, contract the circumference of the same light disk towards
the centre by diminishing the object; the edge will then appear yellow (C). This
may be done with a concave glass, which, however, should not be ground thin
like common eye-glasses, but must have some substance. In order, however, to
make this experiment at once with the convex glass, let a smaller black disk be
inserted within the light disk on a black ground. If we magnify the black disk on
a white ground with a convex glass, the same result takes place as if we
diminished the white disk; for we extend the black outline upon the white, and
we thus perceive the yellow edge together with the blue edge (D).

201.

These two appearances, the blue and yellow, exhibit themselves in and upon the
white: they both assume a reddish hue, in proportion as they mingle with the
black.[2]

Plate 2.

202.

In this short statement we have described the primordial phenomena of all
appearance of colour occasioned by refraction. These undoubtedly may be
repeated, varied, and rendered more striking; may be combined, complicated,
confused; but, after all, may be still restored to their original simplicity.
203.

In examining the process of the experiment just given, we find that in the one
case we have, to appearance, extended the white edge upon the dark surface; in
the other we have extended the dark edge upon the white surface, supplanting
one by the other, pushing one over the other. We will now endeavour, step by
step, to analyse these and similar cases.

204.

If we cause the white disk to move, in appearance, entirely from its place, which
can be done effectually by prisms, it will be coloured according to the direction
in which it apparently moves, in conformity with the above laws. If we look at
the disk a[3] through a prism, so that it appear moved to b, the outer edge will
appear blue and blue-red, according to the law of the figure B (fig. 1), the other
edge being yellow, and yellow-red, according to the law of the figure C (fig. 1).
For in the first case the white figure is, as it were, extended over the dark
boundary, and in the other case the dark boundary is passed over the white
figure. The same happens if the disk is, to appearance, moved from a to c, from a
to d, and so throughout the circle.
205.

As it is with the simple effect, so it is with more complicated appearances. If we
look through a horizontal prism (a b[4]) at a white disk placed at some distance
behind it at e, the disk will be raised to f, and coloured according to the above
law. If we remove this prism, and look through a vertical one (c d) at the same
disk, it will appear at h, and coloured according to the same law. If we place the
two prisms one upon the other, the disk will appear displaced diagonally, in
conformity with a general law of nature, and will be coloured as before; that is,
according to its movement in the direction, e.g.:[5]

206.

If we attentively examine these opposite coloured edges, we find that they only
appear in the direction of the apparent change of place. A round figure leaves us
in some degree uncertain as to this: a quadrangular figure removes all doubt.

207.

The quadrangular figure a,[6] moved in the direction a b or a d exhibits no colour
on the sides which are parallel with the direction in which it moves: on the other
hand, if moved in the direction a c, parallel with its diagonal, all the edges of the
figure appear coloured.[7]

208.

Thus, a former position (203) is here confirmed; viz. to produce colour, an object
must be so displaced that the light edges be apparently carried over a dark
surface, the dark edges over a light surface, the figure over its boundary, the
boundary over the figure. But if the rectilinear boundaries of a figure could be
indefinitely extended by refraction, so that figure and background might only
pursue their course next, but not over each other, no colour would appear, not
even if they were prolonged to infinity.

[1] Plate 2, fig. 1.
[2] The author has omitted the orange and purple in the coloured diagrams which
illustrate these first experiments, from a wish probably to present the elementary
contrast, on which he lays a stress, in greater simplicity. The reddish tinge would be
apparent, as stated above, where the blue and yellow are in contact with the black.—T.
[3] Plate 2, fig. 2
[4] Plate 2, fig. 4
[5] In this case, according to the author, the refracting medium being increased in
mass, the appearance of colour is increased, and the displacement is greater.—T.
[6] Plate 2, fig. 3.
[7] Fig. 2, plate 1, contains a variety of forms, which, when viewed through a prism,
are intended to illustrate the statement in this and the following paragraph.

XIV.

CONDITIONS UNDER WHICH THE APPEARANCE OF COLOUR INCREASES.

209.

We have seen in the foregoing experiments that all appearance of colour
occasioned by refraction depends on the condition that the boundary or edge be
moved in upon the object itself, or the object itself over the ground, that the
figure should be, as it were, carried over itself, or over the ground. And we shall
now find that, by increased displacement of the object, the appearance of colour
exhibits itself in a greater degree. This takes place in subjective experiments, to
which, for the present, we confine ourselves, under the following conditions.

210.

First, if, in looking through parallel mediums, the eye is directed more obliquely.
Secondly, if the surfaces of the medium are no longer parallel, but form a more
or less acute angle.
Thirdly, owing to the increased proportion of the medium, whether parallel
mediums be increased in size, or whether the angle be increased, provided it does
not attain a right angle.
Fourthly, owing to the distance of the eye armed with a refracting medium from
the object to be displaced.
Fifthly, owing to a chemical property that may be communicated to the glass, and
which may be afterwards increased in effect.

211.

The greatest change of place, short of considerable distortion of the object, is
produced by means of prisms, and this is the reason why the appearance of
colour can be exhibited most powerfully through glasses of this form. Yet we will
not, in employing them, suffer ourselves to be dazzled by the splendid
appearances they exhibit, but keep the above well-established, simple principles
calmly in view.

212.

The colour which is outside, or foremost, in the apparent change of an object by
refraction, is always the broader, and we will henceforth call this a border: the
colour that remains next the outline is the narrower, and this we will call an edge.
213.

If we move a dark boundary towards a light surface, the yellow broader border is
foremost, and the narrower yellow-red edge follows close to the outline. If we

move a light boundary towards a dark surface, the broader violet border is
foremost, and the narrower blue edge follows.

214.

If the object is large, its centre remains uncoloured. Its inner surface is then to be
considered as unlimited (195): it is displaced, but not otherwise altered: but if the
object is so narrow, that under the above conditions the yellow border can reach
the blue edge, the space between the outlines will be entirely covered with
colour. If we make this experiment with a white stripe on a black ground,[1] the
two extremes will presently meet, and thus produce green. We shall then see the
following series of colours:—
Yellow-red.
Yellow.
Green.
Blue.
Blue-red.
215.

If we place a black band, or stripe, on white paper,[2] the violet border will spread
till it meets the yellow-red edge. In this case the intermediate black is effaced (as
the intermediate white was in the last experiment), and in its stead a splendid
pure red will appear.[3] The series of colours will now be as follows:—
Blue.
Blue-red.
Red.
Yellow-red.
Yellow.
216.

The yellow and blue, in the first case (214), can by degrees meet so fully, that the
two colours blend entirely in green, and the order will then be,
Yellow-red.
Green.
Blue-red.
In the second case (215), under similar circumstances, we see only
Blue.
Red.

Yellow.
This appearance is best exhibited by refracting the bars of a window when they
are relieved on a grey sky.[4]

217.

In all this we are never to forget that this appearance is not to be considered as a
complete or final state, but always as a progressive, increasing, and, in many
senses, controllable appearance. Thus we find that, by the negation of the above
five conditions, it gradually decreases, and at last disappears altogether.

[1] Plate 2, fig. 5, left.
[2] Plate 2, fig. 5, right.
[3] This pure red, the union of orange and violet, is considered by the author the
maximum of the coloured appearance: he has appropriated the term purpur to it. See
paragraph 703, and note.—T.
[4] The bands or stripes in fig. 4, plate 1, when viewed through a prism, exhibit the
colours represented in plate 2, fig. 5.

XV.

EXPLANATION OF THE FOREGOING PHENOMENA.

218.

Before we proceed further, it is incumbent on us to explain the first tolerably
simple phenomenon, and to show its connexion with the principles first laid
down, in order that the observer of nature may be enabled clearly to comprehend
the more complicated appearances that follow.
219.

In the first place, it is necessary to remember that we have to do with
circumscribed objects. In the act of seeing, generally, it is the circumscribed
visible which chiefly invites our observation; and in the present instance, in
speaking of the appearance of colour, as occasioned by refraction, the
circumscribed visible, the detached object solely occupies our attention.

220.

For our chromatic exhibitions we can, however, divide objects generally into
primary and secondary. The expressions of themselves denote what we
understand by them, but our meaning will be rendered still more plain by what
follows.

221.

Primary objects may be considered firstly as original, as images which are
impressed on the eye by things before it, and which assure us of their reality. To
these the secondary images may be opposed as derived images, which remain in
the organ when the object itself is taken away; those apparent after-images,
which have been circumstantially treated of in the doctrine of physiological
colours.
222.

The primary images, again, may be considered as direct images, which, like the
original impressions, are conveyed immediately from the object to the eye. In
contradistinction to these, the secondary images may be considered as indirect,
being only conveyed to us, as it were, at second-hand from a reflecting surface.
These are the mirrored, or catoptrical, images, which in certain cases can also
become double images:

223.

When, namely, the reflecting body is transparent, and has two parallel surfaces,
one behind the other: in such a case, an image may be reflected to the eye from
both surfaces, and thus arise double images, inasmuch as the upper image does
not quite cover the under one: this may take place in various ways.
Let a playing-card be held before a mirror. We shall at first see the distinct image
of the card, but the edge of the whole card, as well as that of every spot upon it,
will be bounded on one side with a border, which is the beginning of the second
reflection. This effect varies in different mirrors, according to the different
thickness of the glass, and the accidents of polishing. If a person wearing a white
waistcoat, with the remaining part of his dress dark, stands before certain
mirrors, the border appears very distinctly, and in like manner the metal buttons
on dark cloth exhibit the double reflection very evidently.

224.

The reader who has made himself acquainted with our former descriptions of
experiments (80) will the more readily follow the present statement. The
window-bars reflected by plates of glass appear double, and by increased

thickness of the glass, and a due adaptation of the angle of reflection, the two
reflections may be entirely separated from each other. So a vase full of water,
with a plane mirror-like bottom, reflects any object twice, the two reflections
being more or less separated under the same conditions. In these cases it is to be
observed that, where the two reflections cover each other, the perfect vivid image
is reflected, but where they are separated they exhibit only weak, transparent, and
shadowy images.

225.

If we wish to know which is the under and which the upper image, we have only
to take a coloured medium, for then a light object reflected from the under
surface is of the colour of the medium, while that reflected from the upper
surface presents the complemental colour. With dark objects it is the reverse;
hence black and white surfaces may be here also conveniently employed. How
easily the double images assume and evoke colours will here again be striking.
226.

Thirdly, the primary images may be considered as principal images, while the
secondary can be, as it were, annexed to these as accessory images. Such an
accessory image produces a sort of double form; except that it does not separate
itself from the principal object, although it may be said to be always
endeavouring to do so. It is with secondary images of this last description that we
have to do in prismatic appearances.

227.

A surface without a boundary exhibits no appearance of colour when refracted
(195). Whatever is seen must be circumscribed by an outline to produce this
effect. In other words a figure, an object, is required; this object undergoes an
apparent change of place by refraction: the change is however not complete, not
clean, not sharp; but incomplete, inasmuch as an accessory image only is
produced.

228.

In examining every appearance of nature, but especially in examining an
important and striking one, we should not remain in one spot, we should not
confine ourselves to the insulated fact, nor dwell on it exclusively, but look round
through all nature to see where something similar, something that has affinity to
it, appears: for it is only by combining analogies that we gradually arrive at a
whole which speaks for itself, and requires no further explanation.

229.

Thus we here call to mind that in certain cases refraction unquestionably
produces double images, as is the case in Iceland spar: similar double images are
also apparent in cases of refraction through large rock crystals, and in other
instances; phenomena which have not hitherto been sufficiently observed.[1]

230.

But since in the case under consideration (227) the question relates not to double
but to accessory images, we refer to a phenomenon already adverted to, but not
yet thoroughly investigated. We allude to an earlier experiment, in which it
appeared that a sort of conflict took place in regard to the retina between a light
object and its dark ground, and between a dark object and its light ground (16).
The light object in this case appeared larger, the dark one smaller.
231.

By a more exact observation of this phenomenon we may remark that the forms
are not sharply distinguished from the ground, but that they appear with a kind of
grey, in some degree, coloured edge; in short, with an accessory image. If, then,
objects seen only with the naked eye produce such effects, what may not take
place when a dense medium is interposed? It is not that alone which presents
itself to us in obvious operation which produces and suffers effects, but likewise
all principles that have a mutual relation only of some sort are efficient
accordingly, and indeed often in a very high degree.

232.

Thus when refraction produces its effect on an object there appears an accessory
image next the object itself: the real form thus refracted seems even to linger
behind, as if resisting the change of place; but the accessory image seems to
advance, and extends itself more or less in the mode already shown (212-216).
233.

We also remarked (224) that in double images the fainter appear only half
substantial, having a kind of transparent, evanescent character, just as the fainter
shades of double shadows must always appear as half-shadows. These latter
assume colours easily, and produce them readily (69), the former also (80); and
the same takes place in the instance of accessory images, which, it is true, do not
altogether quit the real object, but still advance or extend from it as half-
substantial images, and hence can appear coloured so quickly and so powerfully.

234.

That the prismatic appearance is in fact an accessory image we may convince
ourselves in more than one mode. It corresponds exactly with the form of the
object itself. Whether the object be bounded by a straight line or a curve,
indented or waving, the form of the accessory image corresponds throughout
exactly with the form of the object.[2]

235.

Again, not only the form but other qualities of the object are communicated to
the accessory image. If the object is sharply relieved from its ground, like white
on black, the coloured accessory image in like manner appears in its greatest
force. It is vivid, distinct, and powerful; but it is most especially powerful when a
luminous object is shown on a dark ground, which may be contrived in various
ways.
236.

But if the object is but faintly distinguished from the ground, like grey objects on
black or white, or even on each other, the accessory image is also faint, and,
when the original difference of tint or force is slight, becomes hardly discernible.

237.

The appearances which are observable when coloured objects are relieved on
light, dark, or coloured grounds are, moreover, well worthy of attention. In this
case a union takes place between the apparent colour of the accessory image and
the real colour of the object; a compound colour is the result, which is either
assisted and enhanced by the accordance, or neutralised by the opposition of its
ingredients.
238.

But the common and general characteristic both of the double and accessory
image is semi-transparence. The tendency of a transparent medium to become
only half transparent, or merely light-transmitting, has been before adverted to
(147, 148). Let the reader assume that he sees within or through such a medium a
visionary image, and he will at once pronounce this latter to be a semi-
transparent image.
239.

Thus the colours produced by refraction may be fitly explained by the doctrine of
the semi-transparent mediums. For where dark passes over light, as the border of
the semi-transparent accessory image advances, yellow appears; and, on the
other hand, where a light outline passes over the dark background, blue appears
(150, 151).

240.

The advancing foremost colour is always the broader. Thus the yellow spreads
over the light with a broad border, but the yellow-red appears as a narrower
stripe and is next the dark, according to the doctrine of augmentation, as an effect
of shade.[3]

241.

On the opposite side the condensed blue is next the edge, while the advancing
border, spreading as a thinner veil over the black, produces the violet colour,
precisely on the principles before explained in treating of semi-transparent
mediums, principles which will hereafter be found equally efficient in many
other cases.

242.

Since an analysis like the present requires to be confirmed by ocular
demonstration, we beg every reader to make himself acquainted with the
experiments hitherto adduced, not in a superficial manner, but fairly and
thoroughly. We have not placed arbitrary signs before him instead of the
appearances themselves; no modes of expression are here proposed for his
adoption which may be repeated for ever without the exercise of thought and
without leading any one to think; but we invite him to examine intelligible
appearances, which must be present to the eye and mind, in order to enable him
clearly to trace these appearances to their origin, and to explain them to himself
and to others.

[1] The date of the publication, 1810, is sometimes to be remembered.—T.
[2] The forms in fig. 2, plate 1, when seen through a prism, are again intended to
exemplify this. In the plates to the original work curvilinear figures are added, but the
circles, fig. 1, in the same plate, may answer the same end.—T.

[3] The author has before observed that colour is a degree of darkness, and he here
means that increase of darkness, produced by transparent mediums, is, to a certain
extent, increase of colour.—T.

XVI.

DECREASE OF THE APPEARANCE OF COLOUR.

243.

We need only take the five conditions (210) under which the appearance of
colour increases in the contrary order, to produce the contrary or decreasing
state; it may be as well, however, briefly to describe and review the
corresponding modifications which are presented to the eye.

244.

At the highest point of complete junction of the opposite edges, the colours
appear as follows (216):—
Yellow-red. Blue.
Green. Red.
Blue-red. Yellow.

245.

Where the junction is less complete, the appearance is as follows (214, 215):—
Yellow-red. Blue.
Yellow. Blue-red.
Green. Red.
Blue. Yellow-red.
Blue-red. Yellow.
Here, therefore, the surface still appears completely coloured, but neither series
is to be considered as an elementary series, always developing itself in the same
manner and in the same degrees; on the contrary, they can and should be resolved
into their elements; and, in doing this, we become better acquainted with their
nature and character.

246.

These elements then are (199, 200, 201)—

Yellow-red. Blue.
Yellow. Blue-red.
White. Black.
Blue. Yellow-red.
Blue-red. Yellow.
Here the surface itself, the original object, which has been hitherto completely
covered, and as it were lost, again appears in the centre of the colours, asserts its
right, and enables us fully to recognise the secondary nature of the accessory
images which exhibit themselves as "edges" and "borders."—Note N.

247.

We can make these edges and borders as narrow as we please; nay, we can still
have refraction in reserve after having done away with all appearance of colour
at the boundary of the object.
Having now sufficiently investigated the exhibition of colour in this
phenomenon, we repeat that we cannot admit it to be an elementary
phenomenon. On the contrary, we have traced it to an antecedent and a simpler
one; we have derived it, in connexion with the theory of secondary images, from
the primordial phenomenon of light and darkness, as affected or acted upon by
semi-transparent mediums. Thus prepared, we proceed to describe the
appearances which refraction produces on grey and coloured objects, and this
will complete the section of subjective phenomena.

XVII.

GREY OBJECTS DISPLACED BY REFRACTION.

248.

Hitherto we have confined our attention to black and white objects relieved on
respectively opposite grounds, as seen through the prism, because the coloured
edges and borders are most clearly displayed in such cases. We now repeat these
experiments with grey objects, and again find similar results.

249.

As we called black the equivalent of darkness, and white the representative of
light (18), so we now venture to say that grey represents half-shadow, which
partakes more or less of light and darkness, and thus stands between the two. We
invite the reader to call to mind the following facts as bearing on our present
view.

250.

Grey objects appear lighter on a black than on a white ground (33); they appear
as a light on a black ground, and larger; as a dark on the white ground, and
smaller. (16.)

251.

The darker the grey the more it appears as a faint light on black, as a strong dark
on white, and vice versâ; hence the accessory images of dark-grey on black are
faint, on white strong: so the accessory images of light-grey on white are faint,
on black strong.

252.

Grey on black, seen through the prism, will exhibit the same appearances as
white on black; the edges are coloured according to the same law, only the
borders appear fainter. If we relieve grey on white, we have the same edges and
borders which would be produced if we saw black on white through the prism.—
Note O.

253.

Various shades of grey placed next each other in gradation will exhibit at their
edges, either blue and violet only, or red and yellow only, according as the darker
grey is placed over or under.
254.

A series of such shades of grey placed horizontally next each other will be
coloured conformably to the same law according as the whole series is relieved,
on a black or white ground above or below.

255.

The observer may see the phenomena exhibited by the prism at one glance, by
enlarging the plate intended to illustrate this section.[1]

256.

It is of great importance duly to examine and consider another experiment in
which a grey object is placed partly on a black and partly on a white surface, so
that the line of division passes vertically through the object.

257.

The colours will appear on this grey object in conformity with the usual law, but
according to the opposite relation of the light to the dark, and will be contrasted
in a line. For as the grey is as a light to the black, so it exhibits the red and
yellow above the blue and violet below: again, as the grey is as a dark to the
white, the blue and violet appear above the red and yellow below. This
experiment will be found of great importance with reference to the next chapter.
[1] It has been thought unnecessary to give all the examples in the plate alluded to, but
the leading instance referred to in the next paragraph will be found in plate 3, fig. 1.
The grey square when seen through a prism will exhibit the effects described in par.
257.—T.

XVIII.

COLOURED OBJECTS DISPLACED BY REFRACTION.

258.

An unlimited coloured surface exhibits no prismatic colour in addition to its own
hue, thus not at all differing from a black, white, or grey surface. To produce the
appearance of colour, light and dark boundaries must act on it either accidentally
or by contrivance. Hence experiments and observations on coloured surfaces, as
seen through the prism, can only be made when such surfaces are separated by an
outline from another differently tinted surface, in short when circumscribed
objects are coloured.
259.

All colours, whatever they may be, correspond so far with grey, that they appear
darker than white and lighter than black. This shade-like quality of colour
(σκιέρον) has been already alluded to (69), and will become more and more
evident. If then we begin by placing coloured objects on black and white
surfaces, and examine them through the prism, we shall again have all that we
have seen exhibited with grey surfaces.

Plate 3.

260.

If we displace a coloured object by refraction, there appears, as in the case of
colourless objects and according to the same laws, an accessory image. This
accessory image retains, as far as colour is concerned, its usual nature, and acts
on one side as a blue and blue-red, on the opposite side as a yellow and yellow-
red. Hence the apparent colour of the edge and border will be either
homogeneous with the real colour of the object, or not so. In the first case the
apparent image identifies itself with the real one, and appears to increase it,
while, in the second case, the real image may be vitiated, rendered indistinct, and
reduced in size by the apparent image. We proceed to review the cases in which
these effects are most strikingly exhibited.

261.

If we take a coloured drawing enlarged from the plate, which illustrates this
experiment[1], and examine the red and blue squares placed next each other on a
black ground, through the prism as usual, we shall find that as both colours are
lighter than the ground, similarly coloured edges and borders will appear above
and below, at the outlines of both, only they will not appear equally distinct to
the eye.

262.

Red is proportionally much lighter on black than blue is. The colours of the
edges will therefore appear stronger on the red than on the blue, which here acts
as a dark-grey, but little different from black. (251.)

263.

The extreme red edge will identify itself with the vermilion colour of the square,
which will thus appear a little elongated in this direction; while the yellow border
immediately underneath it only gives the red surface a more brilliant appearance,
and is not distinguished without attentive observation.

264.

On the other hand the red edge and yellow border are heterogeneous with the
blue square; a dull red appears at the edge, and a dull green mingles with the
figure, and thus the blue square seems, at a hasty glance, to be comparatively
diminished on this side.

265.

At the lower outline of the two squares a blue edge and a violet border will
appear, and will produce the contrary effect; for the blue edge, which is

heterogeneous with the warm red surface, will vitiate it and produce a neutral
colour, so that the red on this side appears comparatively reduced and driven
upwards, and the violet border on the black is scarcely perceptible.

266.

On the other hand, the blue apparent edge will identify itself with the blue
square, and not only not reduce, but extend it. The blue edge and even the violet
border next it have the apparent effect of increasing the surface, and elongating it
in that direction.

267.

The effect of homogeneous and heterogeneous edges, as I have now minutely
described it, is so powerful and singular that the two squares at the first glance
seem pushed out of their relative horizontal position and moved in opposite
directions, the red upwards, the blue downwards. But no one who is accustomed
to observe experiments in a certain succession, and respectively to connect and
trace them, will suffer himself to be deceived by such an unreal effect.

268.

A just impression with regard to this important phenomenon will, however, much
depend on some nice and even troublesome conditions, which are necessary to
produce the illusion in question. Paper should be tinged with vermilion or the
best minium for the red square, and with deep indigo for the blue square. The
blue and red prismatic edges will then unite imperceptibly with the real surfaces
where they are respectively homogeneous; where they are not, they vitiate the
colours of the squares without producing a very distinct middle tint. The real red
should not incline too much to yellow, otherwise the apparent deep red edge
above will be too distinct; at the same time it should be somewhat yellow,
otherwise the transition to the yellow border will be too observable. The blue
must not be light, otherwise the red edge will be visible, and the yellow border
will produce a too decided green, while the violet border underneath would not
give us the impression of being part of an elongated light blue square.

269.

All this will be treated more circumstantially hereafter, when we speak of the
apparatus intended to facilitate the experiments connected with this part of our
subject.[2] Every inquirer should prepare the figures himself, in order fairly to
exhibit this specimen of ocular deception, and at the same time to convince

himself that the coloured edges, even in this case, cannot escape accurate
examination.

270.

Meanwhile various other combinations, as exhibited in the plate, are fully
calculated to remove all doubt on this point in the mind of every attentive
observer.
271.

If, for instance, we look at a white square, next the blue one, on a black ground,
the prismatic hues of the opposite edges of the white, which here occupies the
place of the red in the former experiment, will exhibit themselves in their utmost
force. The red edge extends itself above the level of the blue almost in a greater
degree than was the case with the red square itself in the former experiment. The
lower blue edge, again, is visible in its full force next the white, while, on the
other hand, it cannot be distinguished next the blue square. The violet border
underneath is also much more apparent on the white than on the blue.

272.

If the observer now compares these double squares, carefully prepared and
arranged one above the other, the red with the white, the two blue squares
together, the blue with the red, the blue with the white, he will clearly perceive
the relations of these surfaces to their coloured edges and borders.

273.

The edges and their relations to the coloured surfaces appear still more striking if
we look at the coloured squares and a black square on a white ground; for in this
case the illusion before mentioned ceases altogether, and the effect of the edges
is as visible as in any case that has come under our observation. Let the blue and
red squares be first examined through the prism. In both the blue edge now
appears above; this edge, homogeneous with the blue surface, unites with it, and
appears to extend it upwards, only the blue edge, owing to its lightness, is
somewhat too distinct in its upper portion; the violet border underneath it is also
sufficiently evident on the blue. The apparent blue edge is, on the other hand,
heterogeneous with the red square; it is neutralised by contrast, and is scarcely
visible; meanwhile the violet border, uniting with the real red, produces a hue
resembling that of the peach-blossom.
274.

If thus, owing to the above causes, the upper outlines of these squares do not
appear level with each other, the correspondence of the under outlines is the
more observable; for since both colours, the red and the blue, are darks compared
with the white (as in the former case they were light compared with the black),
the red edge with its yellow border appears very distinctly under both. It exhibits
itself under the warm red surface in its full force, and under the dark blue nearly
as it appears under the black: as may be seen if we compare the edges and
borders of the figures placed one above the other on the white ground.

275.

In order to present these experiments with the greatest variety and perspicuity,
squares of various colours are so arranged[3] that the boundary of the black and
white passes through them vertically. According to the laws now known to us,
especially in their application to coloured objects, we shall find the squares as
usual doubly coloured at each edge; each square will appear to be split in two,
and to be elongated upwards or downwards. We may here call to mind the
experiment with the grey figure seen in like manner on the line of division
between black and white (257).[4]

276.

A phenomenon was before exhibited, even to illusion, in the instance of a red and
blue square on a black ground; in the present experiment the elongation upwards
and downwards of two differently coloured figures is apparent in the two halves
of one and the same figure of one and the same colour. Thus we are still referred
to the coloured edges and borders, and to the effects of their homogeneous and
heterogeneous relations with respect to the real colours of the objects.
277.

I leave it to observers themselves to compare the various gradations of coloured
squares, placed half on black half on white, only inviting their attention to the
apparent alteration which takes place in contrary directions; for red and yellow
appear elongated upwards if on a black ground, downwards if on a white; blue,
downwards if on a black ground, upwards if on a white. All which, however, is
quite in accordance with the diffusely detailed examples above given.

278.

Let the observer now turn the figures so that the before-mentioned squares
placed on the line of division between black and white may be in a horizontal
series; the black above, the white underneath. On looking at these squares

through the prism, he will observe that the red square gains by the addition of
two red edges; on more accurate examination he will observe the yellow border
on the red figure, and the lower yellow border upon the white will be perfectly
apparent.

279.

The upper red edge on the blue square is on the other hand hardly visible; the
yellow border next it produces a dull green by mingling with the figure; the
lower red edge and the yellow border are displayed in lively colours.

280.

After observing that the red figure in these cases appears to gain by an addition
on both sides, while the dark blue, on one side at least, loses something; we shall
see the contrary effect produced by turning the same figures upside down, so that
the white ground be above, the black below.

281.

For as the homogeneous edges and borders now appear above and below the blue
square, this appears elongated, and a portion of the surface itself seems even
more brilliantly coloured: it is only by attentive observation that we can
distinguish the edges and borders from the colour of the figure itself.

282.

The yellow and red squares, on the other hand, are comparatively reduced by the
heterogeneous edges in this position of the figures, and their colours are, to a
certain extent, vitiated. The blue edge in both is almost invisible. The violet
border appears as a beautiful peach-blossom hue on the red, as a very pale colour
of the same kind on the yellow; both the lower edges are green; dull on the red,
vivid on the yellow; the violet border is but faintly perceptible under the red, but
is more apparent under the yellow.

283.

Every inquirer should make it a point to be thoroughly acquainted with all the
appearances here adduced, and not consider it irksome to follow out a single
phenomenon through so many modifying circumstances. These experiments, it is
true, may be multiplied to infinity by differently coloured figures, upon and

between differently coloured grounds. Under all such circumstances, however, it
will be evident to every attentive observer that coloured squares only appear
relatively altered, or elongated, or reduced by the prism, because an addition of
homogeneous or heterogeneous edges produces an illusion. The inquirer will
now be enabled to do away with this illusion if he has the patience to go through
the experiments one after the other, always comparing the effects together, and
satisfying himself of their correspondence.
Experiments with coloured objects might have been contrived in various ways:
why they have been exhibited precisely in the above mode, and with so much
minuteness, will be seen hereafter. The phenomena, although formerly not
unknown, were much misunderstood; and it was necessary to investigate them
thoroughly to render some portions of our intended historical view clearer.

284.

In conclusion, we will mention a contrivance by means of which our scientific
readers may be enabled to see these appearances distinctly at one view, and even
in their greatest splendour. Cut in a piece of pasteboard five perfectly similar
square openings of about an inch, next each other, exactly in a horizontal line:
behind these openings place five coloured glasses in the natural order, orange,
yellow, green, blue, violet. Let the series thus adjusted be fastened in an opening
of the camera obscura, so that the bright sky may be seen through the squares, or
that the sun may shine on them; they will thus appear very powerfully coloured.
Let the spectator now examine them through the prism, and observe the
appearances, already familiar by the foregoing experiments, with coloured
objects, namely, the partly assisting, partly neutralising effects of the edges and
borders, and the consequent apparent elongation or reduction of the coloured
squares with reference to the horizontal line. The results witnessed by the
observer in this case, entirely correspond with those in the cases before analysed;
we do not, therefore, go through them again in detail, especially as we shall find
frequent occasions hereafter to return to the subject.—Note P.

[1] Plate 3, fig. 1. The author always recommends making the experiments on an
increased scale, in order to see the prismatic effects distinctly.
[2] Neither the description of the apparatus nor the recapitulation of the whole theory,
so often alluded to by the author, were ever given.—T.
[3] Plate 3. fig. 1.
[4] The grey square is introduced in the same plate, fig. 1, above the coloured squares.

XIX.

ACHROMATISM AND HYPERCHROMATISM.

285.

Formerly when much that is regular and constant in nature was considered as
mere aberration and accident, the colours arising from refraction were but little
attended to, and were looked upon as an appearance attributable to particular
local circumstances.

286.

But after it had been assumed that this appearance of colour accompanies
refraction at all times, it was natural that it should be considered as intimately
and exclusively connected with that phenomenon; the belief obtaining that the
measure of the coloured appearance was in proportion to the measure of the
refraction, and that they must advance pari passu with each other.

287.

If, again, philosophers ascribed the phenomenon of a stronger or weaker
refraction, not indeed wholly, but in some degree, to the different density of the
medium, (as purer atmospheric air, air charged with vapours, water, glass,
according to their increasing density, increase the so-called refraction, or
displacement of the object;) so they could hardly doubt that the appearance of
colour must increase in the same proportion; and hence took it for granted, in
combining different mediums which were to counteract refraction, that as long as
refraction existed, the appearance of colour must take place, and that as soon as
the colour disappeared, the refraction also must cease.
288.

Afterwards it was, however, discovered that this relation which was assumed to
correspond, was, in fact, dissimilar; that two mediums can refract an object with
equal power, and yet produce very dissimilar coloured borders.

289.

It was found that, in addition to the physical principle to which refraction was
ascribed, a chemical one was also to be taken into the account. We propose to
pursue this subject hereafter, in the chemical division of our inquiry, and we shall
have to describe the particulars of this important discovery in our history of the
doctrine of colours. What follows may suffice for the present.

290.

In mediums of similar or nearly similar refracting power, we find the remarkable
circumstance that a greater and lesser appearance of colour can be produced by a
chemical treatment; the greater effect is owing, namely, to acids, the lesser to
alkalis. If metallic oxydes are introduced into a common mass of glass, the
coloured appearance through such glasses becomes greatly increased without any
perceptible change of refracting power. That the lesser effect, again, is produced
by alkalis, may be easily supposed.

291.

Those kinds of glass which were first employed after the discovery, are called
flint and crown glass; the first produces the stronger, the second the fainter
appearance of colour.

292.

We shall make use of both these denominations as technical terms in our present
statement, and assume that the refractive power of both is the same, but that flint-
glass produces the coloured appearance more strongly by one-third than the
crown-glass. The diagram (Plate 3, fig. 2,) may serve in illustration.
293.

A black surface is here divided into compartments for more convenient
demonstration: let the spectator imagine five white squares between the parallel
lines a, b, and c, d. The square No. 1, is presented to the naked eye unmoved
from its place.

294.

But let the square No. 2, seen through a crown-glass prism g, be supposed to be
displaced by refraction three compartments, exhibiting the coloured borders to a
certain extent; again, let the square No. 3, seen through a flint glass prism h, in
like manner be moved downwards three compartments, when it will exhibit the
coloured borders by about a third wider than No. 2.

295.

Again, let us suppose that the square No. 4, has, like No. 2, been moved
downwards three compartments by a prism of crown-glass, and that then by an
oppositely placed prism h, of flint-glass, it has been again raised to its former
situation, where it now stands.

296.

Here, it is true, the refraction is done away with by the opposition of the two; but
as the prism h, in displacing the square by refraction through three
compartments, produces coloured borders wider by a third than those produced
by the prism g, so, notwithstanding the refraction is neutralised, there must be an
excess of coloured border remaining. (The position of this colour, as usual,
depends on the direction of the apparent motion (204) communicated to the
square by the prism h, and, consequently, it is the reverse of the appearance in
the two squares 2 and 3, which have been moved in an opposite direction.) This
excess of colour we have called Hyperchromatism, and from this the achromatic
state may be immediately arrived at.

297.

For assuming that it was the square No. 5 which was removed three
compartments from its first supposed place, like No. 2, by a prism of crown-glass
g, it would only be necessary to reduce the angle of a prism of flint-glass h, and
to connect it, reversed, to the prism g, in order to raise the square No. 5 two
degrees or compartments; by which means the Hyperchromatism of the first case
would cease, the figure would not quite return to its first position, and yet be
already colourless. The prolonged lines of the united prisms, under No. 5, show
that a single complete prism remains: again, we have only to suppose the lines
curved, and an object-glass presents itself. Such is the principle of the achromatic
telescopes.
298.

For these experiments, a small prism composed of three different prisms, as
prepared in England, is extremely well adapted. It is to be hoped our own
opticians will in future enable every friend of science to provide himself with this
necessary instrument.

XX.

ADVANTAGES OF SUBJECTIVE EXPERIMENTS.—TRANSITION TO THE OBJECTIVE.

299.

We have presented the appearances of colour as exhibited by refraction, first, by
means of subjective experiments; and we have so far arrived at a definite result,

that we have been enabled to deduce the phenomena in question from the
doctrine of semi-transparent mediums and double images.

300.

In statements which have reference to nature, everything depends on ocular
inspection, and these experiments are the more satisfactory as they may be easily
and conveniently made. Every amateur can procure his apparatus without much
trouble or cost, and if he is a tolerable adept in pasteboard contrivances, he may
even prepare a great part of his machinery himself. A few plain surfaces, on
which black, white, grey, and coloured objects may be exhibited alternately on a
light and dark ground, are all that is necessary. The spectator fixes them before
him, examines the appearances at the edge of the figures conveniently, and as
long as he pleases; he retires to a greater distance, again approaches, and
accurately observes the progressive states of the phenomena.

301.

Besides this, the appearances may be observed with sufficient exactness through
small prisms, which need not be of the purest glass. The other desirable
requisites in these glass instruments will, however, be pointed out in the section
which treats of the apparatus.[1]

302.

A great advantage in these experiments, again, is, that they can be made at any
hour of the day in any room, whatever aspect it may have. We have no need to
wait for sunshine, which in general is not very propitious to northern observers.

[1] This description of the apparatus was never given.

OBJECTIVE EXPERIMENTS.

303.

The objective experiments, on the contrary, necessarily require the sun-light
which, even when it is to be had, may not always have the most desirable relation
with the apparatus placed opposite to it. Sometimes the sun is too high,
sometimes too low, and withal only a short time in the meridian of the best
situated room. It changes its direction during the observation, the observer is

forced to alter his own position and that of his apparatus, in consequence of
which the experiments in many cases become uncertain. If the sun shines through
the prism it exhibits all inequalities, lines, and bubbles in the glass, and thus the
appearance is rendered confused, dim, and discoloured.

304.

Yet both kinds of experiments must be investigated with equal accuracy. They
appear to be opposed to each other, and yet are always parallel. What one order
of experiments exhibits the other exhibits likewise, and yet each has its peculiar
capabilities, by means of which certain effects of nature are made known to us in
more than one way.

305.

In the next place there are important phenomena which may be exhibited by the
union of subjective and objective experiments. The latter experiments again have
this advantage, that we can in most cases represent them by diagrams, and
present to view the component relations of the phenomena. In proceeding,
therefore, to describe the objective experiments, we shall so arrange them that
they may always correspond with the analogous subjective examples; for this
reason, too, we annex to the number of each paragraph the number of the former
corresponding one. But we set out by observing generally that the reader must
consult the plates, that the scientific investigator must be familiar with the
apparatus in order that the twin-phenomena in one mode or the other may be
placed before them.

XXI.

REFRACTION WITHOUT THE APPEARANCE OF COLOUR.

306 (195, 196).

That refraction may exhibit its effects without producing an appearance of
colour, is not to be demonstrated so perfectly in objective as in subjective
experiments. We have, it is true, unlimited spaces which we can look at through
the prism, and thus convince ourselves that no colour appears where there is no
boundary; but we have no unlimited source of light which we can cause to act
through the prism. Our light comes to us from circumscribed bodies; and the sun,

which chiefly produces our prismatic appearances, is itself only a small,
circumscribed, luminous object.

307.

We may, however, consider every larger opening through which the sun shines,
every larger medium through which the sun-light is transmitted and made to
deviate from its course, as so far unlimited that we can confine our attention to
the centre of the surface without considering its boundaries.

308 (197).

If we place a large water-prism in the sun, a large bright space is refracted
upwards by it on the plane intended to receive the image, and the middle of this
illumined space will be colourless. The same effect may be produced if we make
the experiment with glass prisms having angles of few degrees: the appearance
may be produced even through glass prisms, whose refracting angle is sixty
degrees, provided we place the recipient surface near enough.

XXII.

CONDITIONS OF THE APPEARANCE OF COLOUR.

309 (198).

Although, then, the illumined space before mentioned appears indeed refracted
and moved from its place, but not coloured, yet on the horizontal edges of this
space we observe a coloured appearance. That here again the colour is solely
owing to the displacement of a circumscribed object may require to be more fully
proved.
The luminous body which here acts is circumscribed: the sun, while it shines and
diffuses light, is still an insulated object. However small the opening in the lid of
a camera obscura be made, still the whole image of the sun will penetrate it. The
light which streams from all parts of the sun's disk, will cross itself in the
smallest opening, and form the angle which corresponds with the sun's apparent
diameter. On the outside we have a cone narrowing to the orifice; within, this
apex spreads again, producing on an opposite surface a round image, which still
increases in size in proportion to the distance of the recipient surface from the

apex. This image, together with all other objects of the external landscape,
appears reversed on the white surface in question in a dark room.

310.

How little therefore we have here to do with single sun-rays, bundles or fasces of
rays, cylinders of rays, pencils, or whatever else of the kind may be imagined, is
strikingly evident. For the convenience of certain diagrams the sun-light may be
assumed to arrive in parallel lines, but it is known that this is only a fiction; a
fiction quite allowable where the difference between the assumption and the true
appearance is unimportant; but we should take care not to suffer such a postulate
to be equivalent to a fact, and proceed to further operations on such a fictitious
basis.

311.

Let the aperture in the window-shutter be now enlarged at pleasure, let it be
made round or square, nay, let the whole shutter be opened, and let the sun shine
into the room through the whole window; the space which the sun illumines will
always be larger according to the angle which its diameter makes; and thus even
the whole space illumined by the sun through the largest window is only the
image of the sun plus the size of the opening. We shall hereafter have occasion to
return to this.

312 (199).

If we transmit the image of the sun through convex glasses we contract it
towards the focus. In this case, according to the laws before explained, a yellow
border and a yellow-red edge must appear when the spectrum is thrown on white
paper. But as this experiment is dazzling and inconvenient, it may be made more
agreeably with the image of the full moon. On contracting this orb by means of a
convex glass, the coloured edge appears in the greatest splendour; for the moon
transmits a mitigated light in the first instance, and can thus the more readily
produce colour which to a certain extent accompanies the subduing of light: at
the same time the eye of the observer is only gently and agreeably excited.

313 (200).

If we transmit a luminous image through concave glasses, it is dilated. Here the
image appears edged with blue.
314.

The two opposite appearances may be produced by a convex glass,
simultaneously or in succession; simultaneously by fastening an opaque disk in
the centre of the convex glass, and then transmitting the sun's image. In this case
the luminous image and the black disk within it are both contracted, and,
consequently, the opposite colours must appear. Again, we can present this
contrast in succession by first contracting the luminous image towards the focus,
and then suffering it to expand again beyond the focus, when it will immediately
exhibit a blue edge.

315 (201).

Here too what was observed in the subjective experiments is again to be
remarked, namely, that blue and yellow appear in and upon the white, and that
both assume a reddish appearance in proportion as they mingle with the black.

316 (202, 203).

These elementary phenomena occur in all subsequent objective experiments, as
they constituted the groundwork of the subjective ones. The process too which
takes place is the same; a light boundary is carried over a dark surface, a dark
surface is carried over a light boundary. The edges must advance, and as it were
push over each other in these experiments as in the former ones.

317 (204).

If we admit the sun's image through a larger or smaller opening into the dark
room, if we transmit it through a prism so placed that its refracting angle, as
usual, is underneath; the luminous image, instead of proceeding in a straight line
to the floor, is refracted upwards on a vertical surface placed to receive it. This is
the moment to take notice of the opposite modes in which the subjective and
objective refractions of the object appear.

318.

If we look through a prism, held with its refracting angle underneath, at an object
above us, the object is moved downwards; whereas a luminous image refracted
through the same prism is moved upwards. This, which we here merely mention
as a matter of fact for the sake of brevity, is easily explained by the laws of
refraction and elevation.

319.

The luminous object being moved from its place in this manner, the coloured
borders appear in the order, and according to the laws before explained. The

violet border is always foremost, and thus in objective cases proceeds upwards,
in subjective cases downwards.

320 (205).

The observer may convince himself in like manner of the mode in which the
appearance of colour takes place in the diagonal direction when the displacement
is effected by means of two prisms, as has been plainly enough shown in the
subjective example; for this experiment, however, prisms should be procured of
few degrees, say about fifteen.
321 (206, 207).

That the colouring of the image takes place here too, according to the direction in
which it moves, will be apparent if we make a square opening of moderate size
in a shutter, and cause the luminous image to pass through a water-prism; the
spectrum being moved first in the horizontal and vertical directions, then
diagonally, the coloured edges will change their position accordingly.

322 (208).

Whence it is again evident that to produce colour the boundaries must be carried
over each other, not merely move side by side.

XXIII.

CONDITIONS OF THE INCREASE OF COLOUR.

323 (209).

Here too an increased displacement of the object produces a greater appearance
of colour.

324 (210).

This increased displacement occurs,
1. By a more oblique direction of the impinging luminous object through
mediums with parallel surfaces.
2. By changing the parallel form for one more or less acute angled.

3. By increased proportion of the medium, whether parallel or acute angled;
partly because the object is by this means more powerfully displaced, partly
because an effect depending on the mere mass co-operates.
4. By the distance of the recipient surface from the refracting medium so that the
coloured spectrum emerging from the prism may be said to have a longer way to
travel.
5. When a chemical property produces its effects under all these circumstances:
this we have already entered into more fully under the head of achromatism and
hyperchromatism.

325 (211).

The objective experiments have this advantage that the progressive states of the
phenomenon may be arrested and clearly represented by diagrams, which is not
the case with the subjective experiments.

326.

We can observe the luminous image after it has emerged from the prism, step by
step, and mark its increasing colour by receiving it on a plane at different
distances, thus exhibiting before our eyes various sections of this cone, with an
elliptical base: again, the phenomenon may at once be rendered beautifully
visible throughout its whole course in the following manner:—Let a cloud of fine
white dust be excited along the line in which the image passes through the dark
space; the cloud is best produced by fine, perfectly dry, hair-powder. The more or
less coloured appearance will now be painted on the white atoms, and presented
in its whole length and breadth to the eye of the spectator.

327.

By this means we have prepared some diagrams, which will be found among the
plates. In these the appearance is exhibited from its first origin, and by these the
spectator can clearly comprehend why the luminous image is so much more
powerfully coloured through prisms than through parallel mediums.

328 (212).

At the two opposite outlines of the image an opposite appearance presents itself,
beginning from an acute angle;[1] the appearance spreads as it proceeds further in
space, according to this angle. On one side, in the direction in which the
luminous image is moved, a violet border advances on the dark, a narrower blue
edge remains next the outline of the image. On the opposite side a yellow border

advances into the light of the image itself, and a yellow-red edge remains at the
outline.

329 (213).

Here, therefore, the movement of the dark against the light, of the light against
the dark, may be clearly observed.

Plate 4.

330 (214).

The centre of a large object remains long uncoloured, especially with mediums
of less density and smaller angles; but at last the opposite borders and edges
touch each other, upon which a green appears in the centre of the luminous
image.

331 (215).

Objective experiments have been usually made with the sun's image: an
objective experiment with a dark object has hitherto scarcely been thought of.
We have, however, prepared a convenient contrivance for this also. Let the large
water-prism before alluded to be placed in the sun, and let a round pasteboard
disk be fastened either inside or outside. The coloured appearance will again take
place at the outline, beginning according to the usual law; the edges will appear,
they will spread in the same proportion, and when they meet, red will appear in
the centre[2]. An intercepting square may be added near the round disk, and
placed in any direction ad libitum, and the spectator can again convince himself
of what has been before so often described.

332 (216).

If we take away these dark objects from the prism, in which case, however, the
glass is to be carefully cleaned, and hold a rod or a large pencil before the centre
of the horizontal prism, we shall then accomplish the complete immixture of the
violet border and the yellow-red edge, and see only the three colours, the external
blue, and yellow, and the central red.

333.

If again we cut a long horizontal opening in the middle of a piece of pasteboard,
fastened on the prism, and then cause the sun-light to pass through it, we shall
accomplish the complete union of the yellow border with the blue edge upon the
light, and only see yellow-red, green and violet. The details of this are further
entered into in the description of the plates.

334 (217).

The prismatic appearance is thus by no means complete and final when the
luminous image emerges from the prism. It is then only that we perceive its
elements in contrast; for as it increases these contrasting elements unite, and are

at last intimately joined. The section of this phenomenon arrested on a plane
surface is different at every degree of distance from the prism; so that the notion
of an immutable series of colours, or of a pervading similar proportion between
them, cannot be a question for a moment.

[1] Plate 4. fig. 1.
[2] Plate 4. fig. 2.

XXIV.

EXPLANATION OF THE FOREGOING PHENOMENA.

335 (218).

As we have already entered into this analysis circumstantially while treating of
the subjective experiments, as all that was of force there is equally valid here, it
will require no long details in addition to show that the phenomena, which are
entirely parallel in the two cases, may also be traced precisely to the same
sources.
336 (219).

That in objective experiments also we have to do with circumscribed images, has
been already demonstrated at large. The sun may shine through the smallest
opening, yet the image of the whole disk penetrates beyond. The largest prism
may be placed in the open sun-light, yet it is still the sun's image that is bounded
by the edges of the refracting surfaces, and produces the accessory images of this
boundary. We may fasten pasteboard, with many openings cut in it, before the
water-prism, yet we still merely see multiplied images which, after having been
moved from their place by refraction, exhibit coloured edges and borders, and in
these mere accessory images.
337 (235).

In subjective experiments we have seen that objects strongly relieved from each
other produce a very lively appearance of colour, and this will be the case in
objective experiments in a much more vivid and splendid degree. The sun's
image is the most powerful brightness we know; hence its accessory image will
be energetic in proportion, and notwithstanding its really secondary dimmed and
darkened character, must be still very brilliant. The colours thrown by the sun-

light through the prism on any object, carry a powerful light with them, for they
have the highest and most intense source of light, as it were, for their ground.

338.

That we are warranted in calling even these accessory images semi-transparent,
thus deducing the appearances from the doctrine of the semi-transparent
mediums, will be clear to every one who has followed us thus far, but
particularly to those who have supplied themselves with the necessary apparatus,
so as to be enabled at all times to witness the precision and vivacity with which
semi-transparent mediums act.

XXV.

DECREASE OF THE APPEARANCE OF COLOUR.

339 (243).

If we could afford to be concise in the description of the decreasing coloured
appearance in subjective cases, we may here be permitted to proceed with still
greater brevity while we refer to the former distinct statement. One circumstance,
only on account of its great importance, may be here recommended to the
reader's especial attention as a leading point of our whole thesis.
340 (244, 247).

The decline of the prismatic appearance must be preceded by its separation, by
its resolution into its elements. At a due distance from the prism, the image of the
sun being entirely coloured, the blue and yellow at length mix completely, and
we see only yellow-red, green, and blue-red. If we bring the recipient surface
nearer to the refracting medium, yellow and blue appear again, and we see the
five colours with their gradations. At a still shorter distance the yellow and blue
separate from each other entirely, the green vanishes, and the image itself
appears, colourless, between the coloured edges and borders. The nearer we
bring the recipient surface to the prism, the narrower the edges and borders
become, till at last, when in contact with the prism, they are reduced to nothing.

XXVI.

GREY OBJECTS.

341 (218).

We have exhibited grey objects as very important to our inquiry in the subjective
experiments. They show, by the faintness of the accessory images, that these
same images are in all cases derived from the principal object. If we wish here,
too, to carry on the objective experiments parallel with the others, we may
conveniently do this by placing a more or less dull ground glass before the
opening through which the sun's image enters. By this means a subdued image
would be produced, which on being refracted would exhibit much duller colours
on the recipient plane than those immediately derived from the sun's disk; and
thus, even from the intense sun-image, only a faint accessory image would
appear, proportioned to the mitigation of the light by the glass. This experiment,
it is true, will only again and again confirm what is already sufficiently familiar
to us.

XXVII.

COLOURED OBJECTS.

342 (260).

There are various modes of producing coloured images in objective experiments.
In the first place, we can fix coloured glass before the opening, by which means a
coloured image is at once produced; secondly, we can fill the water-prism with
coloured fluids; thirdly, we can cause the colours, already produced in their full
vivacity by the prism, to pass through proportionate small openings in a tin plate,
and thus prepare small circumscribed colours for a second operation. This last
mode is the most difficult; for owing to the continual progress of the sun, the
image cannot be arrested in any direction at will. The second method has also its
inconveniences, since not all coloured liquids can be prepared perfectly bright
and clear. On these accounts the first is to be preferred, and deserves the more to
be adopted because natural philosophers have hitherto chosen to consider the
colours produced from the sun-light through the prism, those produced through
liquids and glasses, and those which are already fixed on paper or cloth, as

exhibiting effects equally to be depended on, and equally available in
demonstration.

343.

As it is thus merely necessary that the image should be coloured, so the large
water-prism before alluded to affords us the best means of effecting this. A
pasteboard screen may be contrived to slide before the large surfaces of the
prism, through which, in the first instance, the light passes uncoloured. In this
screen openings of various forms may be cut, in order to produce different
images, and consequently different accessory images. This being done, we need
only fix coloured glasses before the openings, in order to observe what effect
refraction produces on coloured images in an objective sense.
344.

A series of glasses may be prepared in a mode similar to that before described
(284); these should be accurately contrived to slide in the grooves of the large
water-prism. Let the sun then shine through them, and the coloured images
refracted upwards will appear bordered and edged, and will vary accordingly: for
these borders and edges will be exhibited quite distinctly on some images, and on
others will be mixed with the specific colour of the glass, which they will either
enhance or neutralize. Every observer will be enabled to convince himself here
again that we have only to do with the same simple phenomenon so
circumstantially described subjectively and objectively.

XXVIII.

ACHROMATISM AND HYPERCHROMATISM.

345 (285, 290).

It is possible to make the hyperchromatic and achromatic experiments
objectively as well as subjectively. After what has been already stated, a short
description of the method will suffice, especially as we take it for granted that the
compound prism before mentioned is in the hands of the observer.

346.

Let the sun's image pass through an acute-angled prism of few degrees, prepared
from crown-glass, so that the spectrum be refracted upwards on an opposite

surface; the edges will appear coloured, according to the constant law, namely,
the violet and blue above and outside, the yellow and yellow-red below and
within the image. As the refracting angle of this prism is undermost, let another
proportionate prism of flint-glass be placed against it, with its refracting angle
uppermost. The sun's image will by this means be again moved to its place,
where, owing to the excess of the colouring power of the prism of flint-glass, it
will still appear a little coloured, and, in consequence of the direction in which it
has been moved, the blue and violet will now appear underneath and outside, the
yellow and yellow-red above and inside.

347.

If the whole image be now moved a little upwards by a proportionate prism of
crown-glass, the hyperchromatism will disappear, the sun's image will be moved
from its place, and yet will appear colourless.
348.

With an achromatic object-glass composed of three glasses, this experiment may
be made step by step, if we do not mind taking out the glasses from their setting.
The two convex glasses of crown-glass in contracting the sun's image towards
the focus, the concave glass of flint-glass in dilating the image beyond it, exhibit
at the edges the usual colours. A convex glass united with a concave one exhibits
the colours according to the law of the latter. If all three glasses are placed
together, whether we contract the sun's image towards the focus, or suffer it to
dilate beyond the focus, coloured edges never appear, and the achromatic effect
intended by the optician is, in this case, again attained.

349.

But as the crown-glass has always a greenish tint, and as a tendency to this hue
may be more decided in large and strong object-glasses, and under certain
circumstances produce the compensatory red, (which, however, in repeated
experiments with several instruments of this kind did not occur to us,)
philosophers have resorted to the most extraordinary modes of explaining such a
result; and having been compelled, in support of their system, theoretically to
prove the impossibility of achromatic telescopes, have felt a kind of satisfaction
in having some apparent ground for denying so great an improvement. Of this,
however, we can only treat circumstantially in our historical account of these
discoveries.

XXIX.

COMBINATION OF SUBJECTIVE AND OBJECTIVE EXPERIMENTS.

350.

Having shown above (318) that refraction, considered objectively and
subjectively, must act in opposite directions, it will follow that if we combine the
experiments, the effects will reciprocally destroy each other.

351.

Let the sun's image be thrown upwards on a vertical plane, through a
horizontally-placed prism. If the prism is long enough to admit of the spectator
also looking through it, he will see the image elevated by the objective refraction
again depressed, and in the same place in which it appeared without refraction.
352.

Here a remarkable case presents itself, but at the same time a natural result of a
general law. For since, as often before stated, the objective sun's image thrown
on the vertical plane is not an ultimate or unchangeable state of the phenomenon,
so in the above operation the image is not only depressed when seen through the
prism, but its edges and borders are entirely robbed of their hues, and the
spectrum is reduced to a colourless circular form.
353.

By employing two perfectly similar prisms placed next each other, for this
experiment, we can transmit the sun's image through one, and look through the
other.

354.

If the spectator advances nearer with the prism through which he looks, the
image is again elevated, and by degrees becomes coloured according to the law
of the first prism. If he again retires till he has brought the image to the
neutralized point, and then retires still farther away, the image, which had
become round and colourless, moves still more downwards and becomes
coloured in the opposite sense, so that if we look through the prism and upon the
refracted spectrum at the same time, we see the same image coloured according
to subjective and objective laws.

355.

The modes in which this experiment may be varied are obvious. If the refracting
angle of the prism, through which the sun's image was objectively elevated, is
greater than that of the prism through which the observer looks, he must retire to
a much greater distance, in order to depress the coloured image so low on the
vertical plane that it shall appear colourless, and vice versâ.

356.

It will be easily seen that we may exhibit achromatic and hyperchromatic effects
in a similar manner, and we leave it to the amateur to follow out such researches
more fully. Other complicated experiments in which prisms and lenses are
employed together, others again, in which objective and subjective experiments
are variously intermixed, we reserve for a future occasion, when it will be our
object to trace such effects to the simple phenomena with which we are now
sufficiently familiar.

XXX.

TRANSITION.

357.

In looking back on the description and analysis of dioptrical colours, we do not
repent either that we have treated them so circumstantially, or that we have taken
them into consideration before the other physical colours, out of the order we
ourselves laid down. Yet, before we quit this branch of our inquiry, it may be as
well to state the reasons that have weighed with us.

358.

If some apology is necessary for having treated the theory of the dioptrical
colours, particularly those of the second class, so diffusely, we should observe,
that the exposition of any branch of knowledge is to be considered partly with
reference to the intrinsic importance of the subject, and partly with reference to
the particular necessities of the time in which the inquiry is undertaken. In our
own case we were forced to keep both these considerations constantly in view. In
the first place we had to state a mass of experiments with our consequent
convictions; next, it was our especial aim to exhibit certain phenomena (known,
it is true, but misunderstood, and above all, exhibited in false connection,) in that
natural and progressive development which is strictly and truly conformable to

observation; in order that hereafter, in our polemical or historical investigations,
we might be enabled to bring a complete preparatory analysis to bear on, and
elucidate, our general view. The details we have entered into were on this
account unavoidable; they may be considered as a reluctant consequence of the
occasion. Hereafter, when philosophers will look upon a simple principle as
simple, a combined effect as combined; when they will acknowledge the first
elementary, and the second complicated states, for what they are; then, indeed,
all this statement may be abridged to a narrower form; a labour which, should we
ourselves not be able to accomplish it, we bequeath to the active interest of
contemporaries and posterity.

359.

With respect to the order of the chapters, it should be remembered that natural
phenomena, which are even allied to each other, are not connected in any
particular sequence or constant series; their efficient causes act in a narrow
circle, so that it is in some sort indifferent what phenomenon is first or last
considered; the main point is, that all should be as far as possible present to us, in
order that we may embrace them at last from one point of view, partly according
to their nature, partly according to generally received methods.

360.

Yet, in the present particular instance, it may be asserted that the dioptrical
colours are justly placed at the head of the physical colours; not only on account
of their striking splendour and their importance in other respects, but because, in
tracing these to their source, much was necessarily entered into which will assist
our subsequent enquiries.

361.

For, hitherto, light has been considered as a kind of abstract principle, existing
and acting independently; to a certain extent self-modified, and on the slightest
cause, producing colours out of itself. To divert the votaries of physical science
from this mode of viewing the subject; to make them attentive to the fact, that in
prismatic and other appearances we have not to do with light as an
uncircumscribed and modifying principle, but as circumscribed and modified;
that we have to do with a luminous image; with images or circumscribed objects
generally, whether light or dark: this was the purpose we had in view, and such is
the problem to be solved.

362.

All that takes place in dioptrical cases,—especially those of the second class
which are connected with the phenomena of refraction,—is now sufficiently
familiar to us, and will serve as an introduction to what follows.

363.

Catoptrical appearances remind us of the physiological phenomena, but as we
ascribe a more objective character to the former, we thought ourselves justified
in classing them with the physical examples. It is of importance, however, to
remember that here again it is not light, in an abstract sense, but a luminous
image that we have to consider.
364.

In proceeding onwards to the paroptrical class, the reader, if duly acquainted with
the foregoing facts, will be pleased to find himself once more in the region of
circumscribed forms. The shadows of bodies, especially, as secondary images, so
exactly accompanying the object, will serve greatly to elucidate analogous
appearances.

365.

We will not, however, anticipate these statements, but proceed as heretofore in
what we consider the regular course.

XXXI.

CATOPTRICAL COLOURS.

366.

Catoptrical colours are such as appear in consequence of a mirror-like reflection.
We assume, in the first place, that the light itself as well as the surface from
which it is reflected, is perfectly colourless. In this sense the appearances in
question come under the head of physical colours. They arise in consequence of
reflection, as we found the dioptrical colours of the second class appear by
means of refraction. Without further general definitions, we turn our attention at
once to particular cases, and to the conditions which are essential to the
exhibition of these phenomena.

367.

If we unroll a coil of bright steel-wire, and after suffering it to spring confusedly
together again, place it at a window in the light, we shall see the prominent parts
of the circles and convolutions illumined, but neither resplendent nor iridescent.
But if the sun shines on the wire, this light will be condensed into a point, and we
perceive a small resplendent image of the sun, which, when seen near, exhibits
no colour. On retiring a little, however, and fixing the eyes on this refulgent
appearance, we discern several small mirrored suns, coloured in the most varied
manner; and although the impression is that green and red predominate, yet, on a
more accurate inspection, we find that the other colours are also present.

368.

If we take an eye-glass, and examine the appearance through it, we find the
colours have vanished, as well as the radiating splendour in which they were
seen, and we perceive only the small luminous points, the repeated images of the
sun. We thus find that the impression is subjective in its nature, and that the
appearance is allied to those which we have adverted to under the name of
radiating halos (100).
369.

We can, however, exhibit this phenomenon objectively. Let a piece of white
paper be fastened beneath a small aperture in the lid of a camera-obscura, and
when the sun shines through this aperture, let the confusedly-rolled steel-wire be
held in the light, so that it be opposite to the paper. The sun-light will impinge on
and in the circles of the wire, and will not, as in the concentrating lens of the eye,
display itself in a point; but, as the paper can receive the reflection of the light in
every part of its surface will be seen in hair-like lines, which are also iridescent.

370.

This experiment is purely catoptrical; for as we cannot imagine that the light
penetrates the surface of the steel, and thus undergoes a change, we are soon
convinced that we have here a mere reflection which, in its subjective character,
is connected with the theory of faintly acting lights, and the after-image of
dazzling lights, and as far as it can be considered objective, announces even in
the minutest appearances, a real effect, independent of the action and reaction of
the eye.

371.

We have seen that to produce these effects not merely light but a powerful light
is necessary; that this powerful light again is not an abstract and general quality,

but a circumscribed light, a luminous image. We can convince ourselves still
further of this by analogous cases.

372.

A polished surface of silver placed in the sun reflects a dazzling light, but in this
case no colour is seen. If, however, we slightly scratch the surface, an iridescent
appearance, in which green and red are conspicuous, will be exhibited at a
certain angle. In chased and carved metals the effect is striking: yet it may be
remarked throughout that, in order to its appearance, some form, some
alternation of light and dark must co-operate with the reflection; thus a window-
bar, the stem of a tree, an accidentally or purposely interposed object produces a
perceptible effect. This appearance, too, may be exhibited objectively in the
camera-obscura.
373.

If we cause a polished plated surface to be so acted on by aqua fortis that the
copper within is touched, and the surface itself thus rendered rough, and if the
sun's image be then reflected from it, the splendour will be reverberated from
every minutest prominence, and the surface will appear iridescent. So, if we hold
a sheet of black unglazed paper in the sun, and look at it attentively, it will be
seen to glisten in its minutest points with the most vivid colours.

374.

All these examples are referable to the same conditions. In the first case the
luminous image is reflected from a thin line; in the second probably from sharp
edges; in the third from very small points. In all a very powerful and
circumscribed light is requisite. For all these appearances of colour again it is
necessary that the eye should be at a due distance from the reflecting points.

375.

If these observations are made with the microscope, the appearance will be
greatly increased in force and splendour, for we then see the smallest portion of
the surfaces, lit by the sun, glittering in these colours of reflection, which, allied
to the hues of refraction, now attain their highest degree of brilliancy. In such
cases we may observe a vermiform iridescence on the surface of organic bodies,
the further description of which will be given hereafter.
376.

Lastly, the colours which are chiefly exhibited in reflection are red and green,
whence we may infer that the linear appearance especially consists of a thin line
of red, bounded by blue on one side and yellow on the other. If these triple lines
approach very near together, the intermediate space must appear green; a
phenomenon which will often occur to us as we proceed.

377.

We frequently meet with these colours in nature. The colours of the spider's web
might be considered exactly of the same class with those reflected from the steel
wire, except that the non-translucent quality of the former is not so certain as in
the case of steel; on which account some have been inclined to class the colours
of the spider's web with the phenomena of refraction.

378.

In mother-of-pearl we perceive infinitely fine organic fibres and lamellæ in
juxta-position, from which, as from the scratched silver before alluded to, varied
colours, but especially red and green, may arise.

379.

The changing colours of the plumage of birds may also be mentioned here,
although in all organic instances a chemical principle and an adaptation of the
colour to the structure may be assumed; considerations to which we shall return
in treating of chemical colours.

380.

That the appearances of objective halos also approximate catoptrical phenomena
will be readily admitted, while we again do not deny that refraction as well may
here come into the account. For the present we restrict ourselves to one or two
observations; hereafter we may be enabled to make a fuller application of general
principles to particular examples.

381.

We first call to mind the yellow and red circles produced on a white or grey wall
by a light placed near it (88). Light when reflected appears subdued, and a
subdued light excites the impression of yellow, and subsequently of red.

382.

Let the wall be illumined by a candle placed quite close to it. The farther the light
is diffused the fainter it becomes; but it is still the effect of the flame, the

continuation of its action, the dilated effect of its image. We might, therefore,
very fairly call these circles reiterated images, because they constitute the
successive boundaries of the action of the light, and yet at the same time only
present an extended image of the flame.

383.

If the sky is white and luminous round the sun owing to the atmosphere being
filled with light vapours; if mists or clouds pass before the moon, the reflection
of the disk mirrors itself in them; the halos we then perceive are single or double,
smaller or greater, sometimes very large, often colourless, sometimes coloured.

384.

I witnessed a very beautiful halo round the moon the 15th of November, 1799,
when the barometer stood high; the sky was cloudy and vapoury. The halo was
completely coloured, and the circles were concentric round the light as in
subjective halos. That this halo was objective I was presently convinced by
covering the moon's disk, when the same circles were nevertheless perfectly
visible.

385.

The different extent of the halos appears to have a relation with the proximity or
distance of the vapour from the eye of the observer.

386.

As window-panes lightly breathed upon increase the brilliancy of subjective
halos, and in some degree give them an objective character, so, perhaps, with a
simple contrivance in winter, during a quickly freezing temperature, a more exact
definition of this might be arrived at.
387.

How much reason we have in considering these circles to insist on the image and
its effects, is apparent in the phenomenon of the so-called double suns. Similar
double images always occur in certain points of halos and circles, and only
present in a circumscribed form what takes place in a more general way in the
whole circle. All this will be more conveniently treated in connexion with the
appearance of the rainbow.—Note Q.

388.

In conclusion it is only necessary to point out the affinity between the catoptrical
and paroptical colours.
We call those paroptical colours which appear when the light passes by the edge
of an opaque colourless body. How nearly these are allied to the dioptrical
colours of the second class will be easily seen by those who are convinced with
us that the colours of refraction take place only at the edges of objects. The
affinity again between the catoptrical and paroptical colours will be evident in
the following chapter.

XXXII.

PAROPTICAL COLOURS.

389.

The paroptical colours have been hitherto called peri-optical, because a peculiar
effect of light was supposed to take place as it were round the object, and was
ascribed to a certain flexibility of the light to and from the object.

390.

These colours again may be divided into subjective and objective, because they
appear partly without us, as it were, painted on surfaces, and partly within us,
immediately on the retina. In this chapter we shall find it more to our purpose to
take the objective cases first, since the subjective are so closely connected with
other appearances already known to us, that it is hardly possible to separate them.

391.

The paroptical colours then are so called because the light must pass by an
outline or edge to produce them. They do not, however, always appear in this
case; to produce the effect very particular conditions are necessary besides.

392.

It is also to be observed that in this instance again light does not act as an abstract
diffusion (361), the sun shines towards an edge. The volume of light poured from
the sun-image passes by the edge of a substance, and occasions shadows. Within
these shadows we shall presently find colours appear.

393.

But, above all, we should make the experiments and observations that bear upon
our present inquiry in the fullest light. We, therefore, place the observer in the
open air before we conduct him to the limits of a dark room.

394.

A person walking in sun-shine in a garden, or on any level path, may observe that
his shadow only appears sharply defined next the foot on which he rests; farther
from this point, especially round the head, it melts away into the bright ground.
For as the sun-light proceeds not only from the middle of the sun, but also acts
cross-wise from the two extremes of every diameter, an objective parallax takes
place which produces a half-shadow on both sides of the object.

395.

If the person walking raises and spreads his hand, he distinctly sees in the
shadow of each finger the diverging separation of the two half-shadows
outwards, and the diminution of the principal shadow inwards, both being effects
of the cross action of the light.
396.

This experiment may be repeated and varied before a smooth wall, with rods of
different thicknesses, and again with balls; we shall always find that the farther
the object is removed from the surface of the wall, the more the weak double
shadow spreads, and the more the forcible main shadow diminishes, till at last
the main shadow appears quite effaced, and even the double shadows become so
faint, that they almost disappear; at a still greater distance they are, in fact,
imperceptible.

397.

That this is caused by the cross-action of the light we may easily convince
ourselves; for the shadow of a pointed object plainly exhibits two points. We
must thus never lose sight of the fact that in this case the whole sun-image acts,
produces shadows, changes them to double shadows, and finally obliterates
them.
398.

Instead of solid bodies let us now take openings cut of various given sizes next
each other, and let the sun shine through them on a plane surface at some little
distance; we shall find that the bright image produced by the sun on the surface,

is larger than the opening; this is because one edge of the sun shines towards the
opposite edge of the opening, while the other edge of the disk is excluded on that
side. Hence the bright image is more weakly lighted towards the edges.

399.

If we take square openings of any size we please, we shall find that the bright
image on a surface nine feet from the opening, is on every side about an inch
larger than the opening; thus nearly corresponding with the angle of the apparent
diameter of the sun.

400.

That the brightness should gradually diminish towards the edges of the image is
quite natural, for at last only a minimum of the light can act cross-wise from the
sun's circumference through the edge of the aperture.

401.

Thus we here again see how much reason we have in actual observation to guard
against the assumption of parallel rays, bundles and fasces of rays, and the like
hypothetical notions.

402.

We might rather consider the splendour of the sun, or of any light, as an infinite
specular multiplication of the circumscribed luminous image, whence it may be
explained that all square openings through which the sun shines, at certain
distances, according as the apertures are greater or smaller, must give a round
image of light.
403.

The above experiments may be repeated through openings of various shapes and
sizes, and the same effect will always take place at proportionate distances. In all
these cases, however, we may still observe that in a full light and while the sun
merely shines past an edge, no colour is apparent.

404.

We therefore proceed to experiments with a subdued light, which is essential to
the appearance of colour. Let a small opening be made in the window-shutter of a
dark room; let the crossing sun-light which enters, be received on a surface of
white paper, and we shall find that the smaller the opening is, the dimmer the

light image will be. This is quite obvious, because the paper does not receive
light from the whole sun, but partially from single points of its disk.

405.

If we look attentively at this dim image of the sun, we find it still dimmer
towards the outlines where a yellow border is perceptible. The colour is still
more apparent if a vapour or a transparent cloud passes before the sun, thus
subduing and dimming its brightness. The halo on the wall, the effect of the
decreasing brightness of a light placed near it, is here forced on our recollection.
(88.)
406.

If we examine the image more accurately, we perceive that this yellow border is
not the only appearance of colour; we can see, besides, a bluish circle, if not even
a halo-like repetition of the coloured border. If the room is quite dark, we discern
that the sky next the sun also has its effect: we see the blue sky, nay, even the
whole landscape, on the paper, and are thus again convinced that as far as regards
the sun, we have here only to do with a luminous image.

407.

If we take a somewhat larger square opening, so large that the image of the sun
shining through it does not immediately become round, we may distinctly
observe the half-shadows of every edge or side, the junction of these in the
corners, and their colours; just as in the above-mentioned appearance with the
round opening.

408.

We have now subdued a parallactic light by causing it to shine through small
apertures, but we have not taken from it its parallactic character; so that it can
produce double shadows of bodies, although with diminished power. These
double shadows which we have hitherto been describing, follow each other in
light and dark, coloured and colourless circles, and produce repeated, nay, almost
innumerable halos. These effects have been often represented in drawings and
engravings. By placing needles, hairs, and other small bodies, in the subdued
light, the numerous halo-like double shadows may be increased; thus observed,
they have been ascribed to an alternating flexile action of the light, and the same
assumption has been employed to explain the obliteration of the central shadow,
and the appearance of a light in the place of the dark.

409.

For ourselves, we maintain that these again are parallactic double shadows,
which appear edged with coloured borders and halos.

410.

After having seen and investigated the foregoing phenomena, we can proceed to
the experiments with knife-blades,[1] exhibiting effects which may be referred to
the contact and parallactic mutual intersection of the half-shadows and halos
already familiar to us.
411.

Lastly, the observer may follow out the experiments with hairs, needles, and
wires, in the half-light produced as before described by the sun, as well as in that
derived from the blue sky, and indicated on the white paper. He will thus make
himself still better acquainted with the true nature of this phenomenon.

412.

But since in these experiments everything depends on our being persuaded of the
parallactic action of the light, we can make this more evident by means of two
sources of light, the two shadows from which intersect each other, and may be
altogether separated. By day this may be contrived with two small openings in a
window-shutter; by night, with two candles. There are even accidental effects in
interiors, on opening and closing shutters, by means of which we can better
observe these appearances than with the most careful apparatus. But still, all and
each of these may be reduced to experiment by preparing a box which the
observer can look into from above, and gradually diminishing the openings after
having caused a double light to shine in. In this case, as might be expected, the
coloured shadow, considered under the physiological colours, appears very
easily.

413.

It is necessary to remember, generally, what has been before stated with regard to
the nature of double shadows, half-lights, and the like. Experiments also should
especially be made with different shades of grey placed next each other, where
every stripe will appear light by a darker, and dark by a lighter stripe next it. If at
night, with three or more lights, we produce shadows which cross each other
successively, we can observe this phenomenon very distinctly, and we shall be

convinced that the physiological case before more fully treated, here comes into
the account (38).

414.

To what extent the appearances that accompany the paroptical colours, may be
derived from the doctrine of subdued lights, from half-shadows, and from the
physiological disposition of the retina, or whether we shall be forced to take
refuge in certain intrinsic qualities of light, as has hitherto been done, time may
teach. Suffice it here to have pointed out the conditions under which the
paroptical colours appear, and we may hope that our allusion to their connexion
with the facts before adduced by us will not remain unnoticed by the observers of
nature.
415.

The affinity of the paroptical colours with the dioptrical of the second class will
also be readily seen and followed up by every reflecting investigator. Here, as in
those instances, we have to do with edges or boundaries; here, as in those
instances, with a light, which appears at the outline. How natural, therefore, it is
to conclude that the paroptical effects may be heightened, strengthened, and
enriched by the dioptrical. Since, however, the luminous image actually shines
through the medium, we can here only have to do with objective cases of
refraction: it is these which are strictly allied to the paroptical cases. The
subjective cases of refraction, where we see objects through the medium, are
quite distinct from the paroptical. We have already recommended them on
account of their clearness and simplicity.

416.

The connexion between the paroptical colours and the catoptrical may be already
inferred from what has been said: for as the catoptrical colours only appear on
scratches, points, steel-wire, and delicate threads, so it is nearly the same case as
if the light shone past an edge. The light must always be reflected from an edge
in order to produce colour. Here again, as before pointed out, the partial action of
the luminous image and the subduing of the light are both to be taken into the
account.

417.

We add but few observations on the subjective paroptical colours, because these
may be classed partly with the physiological colours, partly with the dioptrical of
the second order. The greater part hardly seem to belong here, but, when

attentively considered, they still diffuse a satisfactory light over the whole
doctrine, and establish its connexion.

418.

If we hold a ruler before the eyes so that the flame of a light just appears above
it, we see the ruler as it were indented and notched at the place where the light
appears. This seems deducible from the expansive power of light acting on the
retina (18).
419.

The same phenomenon on a large scale is exhibited at sun-rise; for when the orb
appears distinctly, but not too powerfully, so that we can still look at it, it always
makes a sharp indentation in the horizon.

420.

If, when the sky is grey, we approach a window, so that the dark cross of the
window-bars be relieved on the sky; if after fixing the eyes on the horizontal bar
we bend the head a little forward; on half closing the eyes as we look up, we
shall presently perceive a bright yellow-red border under the bar, and a bright
light-blue one above it. The duller and more monotonous the grey of the sky, the
more dusky the room, and, consequently, the more previously unexcited the eye,
the livelier the appearance will be; but it may be seen by an attentive observer
even in bright daylight.

421.

If we move the head backwards while half closing the eyes, so that the horizontal
bar be seen below, the phenomenon will appear reversed. The upper edge will
appear yellow, the under edge blue.

422.

Such observations are best made in a dark room. If white paper is spread before
the opening where the solar microscope is commonly fastened, the lower edge of
the circle will appear blue, the upper yellow, even while the eyes are quite open,
or only by half-closing them so far that a halo no longer appears round the white.
If the head is moved backwards the colours are reversed.

423.

These phenomena seem to prove that the humours of the eye are in fact only
really achromatic in the centre where vision takes place, but that towards the

circumference, and in unusual motions of the eyes, as in looking horizontally
when the head is bent backwards or forwards, a chromatic tendency remains,
especially when distinctly relieved objects are thus looked at. Hence such
phenomena may be considered as allied to the dioptrical colours of the second
class.

424.

Similar colours appear if we look on black and white objects, through a pin-hole
in a card. Instead of a white object we may take the minute light aperture in the
tin plate of a camera obscura, as prepared for paroptical experiments.
425.

If we look through a tube, the farther end of which is contracted or variously
indented, the same colours appear.

426.

The following phenomena appear to me to be more nearly allied to the paroptical
appearances. If we hold up a needle near the eye, the point appears double. A
particularly remarkable effect again is produced if we look towards a grey sky
through the blades of knives prepared for paroptical experiments. We seem to
look through a gauze; a multitude of threads appear to the eye; these are in fact
only the reiterated images of the sharp edges, each of which is successively
modified by the next, or perhaps modified in a parallactic sense by the oppositely
acting one, the whole mass being thus changed to a thread-like appearance.

427.

Lastly, it is to be remarked that if we look through the blades towards a minute
light in the window-shutter, coloured stripes and halos appear on the retina as on
the paper.

428.

The present chapter may be here terminated, the less reluctantly, as a friend has
undertaken to investigate this subject by further experiments. In our
recapitulation, in the description of the plates and apparatus, we hope hereafter to
give an account of his observations.[2]

[1] See Newton's Optics, book iii.
[2] The observations here alluded to never appeared.

XXXIII.

EPOPTICAL COLOURS.

429.

We have hitherto had to do with colours which appear with vivacity, but
which immediately vanish again when certain conditions cease. We have
now to become acquainted with others, which it is true are still to be
considered as transient, but which, under certain circumstances, become so
fixed that, even after the conditions which first occasioned their appearance
cease, they still remain, and thus constitute the link between the physical
and the chemical colours.

430.

They appear from various causes on the surface of a colourless body,
originally, without communication, die or immersion (βαφή); and we now
proceed to trace them, from their faintest indication to their most permanent
state, through the different conditions of their appearance, which for easier
survey we here at once summarily state.

431.

First condition.—The contact of two smooth surfaces of hard transparent
bodies.
First case: if masses or plates of glass, or if lenses are pressed against each
other.
Second case: if a crack takes place in a solid mass of glass, chrystal, or ice.
Third case: if lamellæ of transparent stones become separated.
Second condition.—If a surface of glass or a polished stone is breathed
upon.

Third condition.—The combination of the two last; first, breathing on the
glass, then placing another plate of glass upon it, thus exciting the colours
by pressure; then removing the upper glass, upon which the colours begin to
fade and vanish with the breath.
Fourth condition.—Bubbles of various liquids, soap, chocolate, beer, wine,
fine glass bubbles.
Fifth condition.—Very fine pellicles and lamellæ, produced by the
decomposition of minerals and metals. The pellicles of lime, the surface of
stagnant water, especially if impregnated with iron, and again pellicles of
oil on water, especially of varnish on aqua fortis.
Sixth condition.—If metals are heated; the operation of imparting tints to
steel and other metals.
Seventh condition.—If the surface of glass is beginning to decompose.
432.

First condition, first case. If two convex glasses, or a convex and plane
glass, or, best of all, a convex and concave glass come in contact, concentric
coloured circles appear. The phenomenon exhibits itself immediately on the
slightest pressure, and may then be gradually carried through various
successive states. We will describe the complete appearance at once, as we
shall then be better enabled to follow the different states through which it
passes.

433.

The centre is colourless; where the glasses are, so to speak, united in one by
the strongest pressure, a dark grey point appears with a silver white space
round it: then follow, in decreasing distances, various insulated rings, all
consisting of three colours, which are in immediate contact with each other.
Each of these rings, of which perhaps three or four might be counted, is
yellow on the inner side, blue on the outer, and red in the centre. Between
two rings there appears a silver white interval. The rings which are farthest
from the centre are always nearer together: they are composed of red and
green without a perceptible white space between them.

434.

We will now observe the appearances in their gradual formation, beginning
from the slightest pressure.

435.

On the slightest pressure the centre itself appears of a green colour. Then
follow as far as the concentric circles extend, red and green rings. They are
wide, accordingly, and no trace of a silver white space is to be seen between
them. The green is produced by the blue of an imperfectly developed circle,
mixing with the yellow of the first circle. All the remaining circles are, in
this slight contact, broad; their yellow and blue edges mix together, thus
producing a beautiful green. The red, however, of each circle, remains pure
and untouched; hence the whole series is composed of these two colours.

436.

A somewhat stronger pressure separates the first circle by a slight interval
from the imperfectly developed one: it is thus detached, and may be said to
appear in a complete state. The centre is now a blue point; for the yellow of
the first circle is now separated from this central point by a silver white
space. From the centre of the blue a red appears, which is thus, in all cases,
bounded on the outside by its blue edge. The second and third rings from
the centre are quite detached. Where deviations from this order present
themselves, the observer will be enabled to account for them, from what has
been or remains to be stated.

437.

On a stronger pressure the centre becomes yellow; this yellow is surrounded
by a red and blue edge: at last, the yellow also retires from the centre; the
innermost circle is formed and is bounded with yellow. The whole centre
itself now appears silver white, till at last, on the strongest pressure, the
dark point appears, and the phenomenon, as described at first, is complete.

438.

The relative size of the concentric circles and their intervals depends on the
form of the glasses which are pressed together.

439.

We remarked above, that the coloured centre is, in fact, an undeveloped
circle. It is, however, often found, on the slightest pressure, that several
undeveloped circles exist there, as it were, in the germ; these can be
successively developed before the eye of the observer.

440.

The regularity of these rings is owing to the form of the convex glasses, and
the diameter of the coloured appearance depends on the greater or lesser
section of a circle on which a lens is polished. We easily conclude from this,
that by pressing plane glasses together, irregular appearances only will be
produced; the colours, in fact, undulate like watered silks, and spread from
the point of pressure in all directions. Yet, the phenomenon as thus
exhibited is much more splendid than in the former instance, and cannot fail
to strike every spectator. If we make the experiment in this mode, we shall
distinctly see, as in the other case, that, on a slight pressure, the green and
red waves appear; on a stronger, stripes of blue, red, and yellow, become
detached. At first, the outer sides of these stripes touch; on increased
pressure they are separated by a silver white space.

441.

Before we proceed to a further description of this phenomenon, we may
point out the most convenient mode of exhibiting it. Place a large convex
glass on a table near the window; upon this glass lay a plate of well-
polished mirror-glass, about the size of a playing-card, and the mere weight
of the plate will press sufficiently to produce one or other of the phenomena
above described. So, also, by the different weight of plates of glass, by
other accidental circumstances, for instance, by slipping the plate on the
side of the convex glass where the pressure cannot be so strong as in the
centre, all the gradations above described can be produced in succession.

442.

In order to observe the phenomenon it is necessary to look obliquely on the
surface where it appears. But, above all, it is to be remarked that by
stooping still more, and looking at the appearance under a more acute angle,
the circles not only grow larger but other circles are developed from the
centre, of which no trace is to be discovered when we look perpendicularly,
even through the strongest magnifiers.

443.

In order to exhibit the phenomenon in its greatest beauty, the utmost
attention should be paid to the cleanness of the glasses. If the experiment is
made with plate-glass adapted for mirrors, the glass should be handled with
gloves. The inner surfaces, which must come in contact with the utmost
nicety, may be most conveniently cleaned before the experiment, and the
outer surfaces should be kept clean while the pressure is increased.

444.

From what has been said it will be seen that an exact contact of two smooth
surfaces is necessary. Polished glasses are best adapted for the purpose.
Plates of glass exhibit the most brilliant colours when they fit closely
together, and for this reason the phenomenon will increase in beauty if
exhibited under an air-pump, by exhausting the air.

445.

The appearance of the coloured rings may be produced in the greatest
perfection by placing a convex and concave glass together which have been
ground on similar segments of circles. I have never seen the effect more
brilliant than with the object-glass of an achromatic telescope, in which the
crown-glass and flint-glass were necessarily in the closest contact.

446.

A remarkable appearance takes place when dissimilar surfaces are pressed
together; for example, a polished crystal and a plate of glass. The
appearance does not at all exhibit itself in large flowing waves, as in the
combination of glass with glass, but it is small and angular, and, as it were,
disjointed: thus it appears that the surface of the polished crystal, which
consists of infinitely small sections of lamellæ, does not come so
uninterruptedly in contact with the glass as another glass-plate would.

447.

The appearance of colour vanishes on the strongest pressure, which so
intimately unites the two surfaces that they appear to make but one
substance. It is this which occasions the dark centre, because the pressed
lens no longer reflects any light from this point, for the very same point,

when seen against the light, is perfectly clear and transparent. On relaxing
the pressure, the colours, in like manner, gradually diminish, and disappear
entirely when the surfaces are separated.

448.

These same appearances occur in two similar cases. If entirely transparent
masses become partially separated, the surfaces of their parts being still
sufficiently in contact, we see the same circles and waves more or less.
They may be produced in great beauty by plunging a hot mass of glass in
water; the different fissures and cracks enabling us to observe the colours in
various forms. Nature often exhibits the same phenomena in split rock
crystals.

449.

This appearance, again, frequently displays itself in the mineral world in
those kinds of stone which by nature have a tendency to exfoliate. These
original lamellæ are, it is true, so intimately united, that stones of this kind
appear altogether transparent and colourless, yet, the internal layers become
separated, from various accidental causes, without altogether destroying the
contact: thus the appearance, which is now familiar to us by the foregoing
description, often occurs in nature, particularly in calcareous spars; the
specularis, adularia, and other minerals of similar structure. Hence it shows
an ignorance of the proximate causes of an appearance so often accidentally
produced, to consider it so important in mineralogy, and to attach especial
value to the specimens exhibiting it.

450.

We have yet to speak of the very remarkable inversion of this appearance,
as related by men of science. If, namely, instead of looking at the colours by
a reflected light, we examine them by a transmitted light, the opposite
colours are said to appear, and in a mode corresponding with that which we
have before described as physiological; the colours evoking each other.
Instead of blue, we should thus see red-yellow; instead of red, green, &c.,
and vice versâ. We reserve experiments in detail, the rather as we have
ourselves still some doubts on this point.

451.

If we were now called upon to give some general explanation of these
epoptical colours, as they appear under the first condition, and to show their
connexion with the previously detailed physical phenomena, we might
proceed to do so as follows:—

452.

The glasses employed for the experiments are to be regarded as the utmost
possible practical approach to transparence. By the intimate contact,
however, occasioned by the pressure applied to them, their surfaces, we are
persuaded, immediately become in a very slight degree dimmed. Within this
semi-transparence the colours immediately appear, and every circle
comprehends the whole scale; for when the two opposites, yellow and blue,
are united by their red extremities, pure red appears: the green, on the other
hand, as in prismatic experiments, when yellow and blue touch.

453.

We have already repeatedly found that where colour exists at all, the whole
scale is soon called into existence; a similar principle may be said to lurk in
the nature of every physical phenomenon; it already follows, from the idea
of polar opposition, from which an elementary unity or completeness
results.

454.

The fact that a colour exhibited by transmitted light is different from that
displayed by reflected light, reminds us of those dioptrical colours of the
first class which we found were produced precisely in the same way
through semi-opacity. That here, too, a diminution of transparency exists
there can scarcely be a doubt; for the adhesion of the perfectly smooth
plates of glass (an adhesion so strong that they remain hanging to each
other) produces a degree of union which deprives each of the two surfaces,
in some degree, of its smoothness and transparence. The fullest proof may,
however, be found in the fact that in the centre, where the lens is most
strongly pressed on the other glass, and where a perfect union is
accomplished, a complete transparence takes place, in which we no longer
perceive any colour. All this may be hereafter confirmed in a recapitulation
of the whole.

455.

Second condition.—If after breathing on a plate of glass, the breath is
merely wiped away with the finger, and if we then again immediately
breathe on the glass, we see very vivid colours gliding through each other;
these, as the moisture evaporates, change their place, and at last vanish
altogether. If this operation is repeated, the colours are more vivid and
beautiful, and remain longer than they did the first time.

456.

Quickly as this appearance passes, and confused as it appears to be, I have
yet remarked the following effects:—At first all the principal colours appear
with their combinations; on breathing more strongly, the appearance may be
perceived in some order. In this succession it may be remarked, that when
the breath in evaporating becomes contracted from all sides towards the
centre, the blue colour vanishes last.
457.

The phenomenon appears most readily between the minute lines, which the
action of passing the fingers leaves on the clear surface; a somewhat rough
state of the surface of the glass is otherwise requisite. On some glass the
appearance may be produced by merely breathing; in other cases the wiping
with the fingers is necessary: I have even met with polished mirror-glasses,
one side of which immediately showed the colours vividly; the other not. To
judge from some remaining pieces, the former was originally the front of
the glass, the latter the side which was covered with quicksilver.
458.

These experiments may be best made in cold weather, because the glass
may be more quickly and distinctly breathed upon, and the breath
evaporates more suddenly. In severe frost the phenomenon may be observed
on a large scale while travelling in a carriage; the glasses being well
cleaned, and all closed. The breath of the persons within is very gently
diffused over the glass, and immediately produces the most vivid play of
colours. How far they may present a regular succession I have not been able
to remark; but they appear particularly vivid when they have a dark object
as a background. This alternation of colours does not, however, last long;

for as soon as the breath gathers in drops, or freezes to points of ice, the
appearance is at once at an end.

459.

Third condition.—The two foregoing experiments of the pressure and
breathing may be united; namely, by breathing on a plate of glass, and
immediately after pressing the other upon it. The colours then appear as in
the case of two glasses unbreathed upon, with this difference, that the
moisture occasions here and there an interruption of the undulations. On
pushing one glass away from the other the moisture appears iridescent as it
evaporates.
460.

It might, however, be asserted that this combined experiment exhibits no
more than each single experiment; for it appears the colours excited by
pressure disappear in proportion as the glasses are less in contact, and the
moisture then evaporates with its own colours.

461.

Fourth condition.—Iridescent appearances are observable in almost all
bubbles; soap-bubbles are the most commonly known, and the effect in
question is thus exhibited in the easiest mode; but it may be observed in
wine, beer, in pure spirit, and again, especially, in the froth of chocolate.

462.

As in the above cases we required an infinitely narrow space between two
surfaces which are in contact, so we can consider the pellicle of the soap-
bubble as an infinitely thin lamina between two elastic bodies; for the
appearance in fact takes place between the air within, which distends the
bubble, and the atmospheric air.

463.

The bubble when first produced is colourless; then coloured stripes, like
those in marble paper, begin to appear: these at length spread over the
whole surface, or rather are driven round it as it is distended.

464.

In a single bubble, suffered to hang from the straw or tube, the appearance
of colour is difficult to observe, for the quick rotation prevents any accurate
observation, and all the colours seem to mix together; yet we can perceive
that the colours begin at the orifice of the tube. The solution itself may,
however, be blown into carefully, so that only one bubble shall appear. This
remains white (colourless) if not much agitated; but if the solution is not too
watery, circles appear round the perpendicular axis of the bubble; these
being near each other, are commonly composed alternately of green and
red. Lastly, several bubbles may be produced together by the same means;
in this case the colours appear on the sides where two bubbles have pressed
each other flat.

465.

The bubbles of chocolate-froth may perhaps be even more conveniently
observed than those of soap; though smaller, they remain longer. In these,
owing to the heat, an impulse, a movement, is produced and sustained,
which appears necessary to the development and succession of the
appearances.

466.

If the bubble is small, or shut in between others, coloured lines chase each
other over the surface, resembling marbled paper; all the colours of the
scale are seen to pass through each other; the pure, the augmented, the
combined, all distinctly clear and beautiful. In small bubbles the appearance
lasts for a considerable time.

467.

If the bubble is larger, or if it becomes by degrees detached, owing to the
bursting of others near, we perceive that this impulsion and attraction of the
colours has, as it were, an end in view; for on the highest point of the
bubble we see a small circle appear, which is yellow in the centre; the other
remaining coloured lines move constantly round this with a vermicular
action.

468.

In a short time the circle enlarges and sinks downwards on all sides; in the
centre the yellow remains; below and on the outside it becomes red, and
soon blue; below this again appears a new circle of the same series of
colours: if they approximate sufficiently, a green is produced by the union
of the border-colours.

469.

When I could count three such leading circles, the centre was colourless,
and this space became by degrees larger as the circles sank lower, till at last
the bubble burst.

470.

Fifth condition.—Very delicate pellicles may be formed in various ways: on
these films we discover a very lively play of colours, either in the usual
order, or more confusedly passing through each other. The water in which
lime has been slaked soon skims over with a coloured pellicle: the same
happens on the surface of stagnant water, especially if impregnated with
iron. The lamellæ of the fine tartar which adheres to bottles, especially in
red French wine, exhibit the most brilliant colours, on being exposed to the
light, if carefully detached. Drops of oil on water, brandy, and other fluids,
produce also similar circles and brilliant effects: but the most beautiful
experiment that can be made is the following:—Let aqua fortis, not too
strong, be poured into a flat saucer, and then with a brush drop on it some of
the varnish used by engravers to cover certain portions during the process
of biting their plates. After quick commotion there presently appears a film
which spreads itself out in circles, and immediately produces the most vivid
appearances of colour.

471.

Sixth condition.—When metals are heated, colours rapidly succeeding each
other appear on the surface: these colours can, however, be arrested at will.

472.

If a piece of polished steel is heated, it will, at a certain degree of warmth,
be overspread with yellow. If taken suddenly away from the fire, this
yellow remains.

473.

As the steel becomes hotter, the yellow appears darker, intenser, and
presently passes into red. This is difficult to arrest, for it hastens very
quickly to bright blue.

474.

This beautiful blue is to be arrested if the steel is suddenly taken out of the
heat and buried in ashes. The blue steel works are produced in this way. If,
again, the steel is held longer over the fire, it soon becomes a light blue, and
so it remains.
475.

These colours pass like a breath over the plate of steel; each seems to fly
before the other, but, in reality, each successive hue is constantly developed
from the preceding one.
476.

If we hold a penknife in the flame of a light, a coloured stripe will appear
across the blade. The portion of the stripe which was nearest to the flame is
light blue; this melts into blue-red; the red is in the centre; then follow
yellow-red and yellow.

477.

This phenomenon is deducible from the preceding ones; for the portion of
the blade next the handle is less heated than the end which is in the flame,
and thus all the colours which in other cases exhibited themselves in
succession, must here appear at once, and may thus be permanently
preserved.
478.

Robert Boyle gives this succession of colours as follows:—"A florido flavo
ad flavum saturum et rubescentem (quem artifices sanguineum vocant) inde
ad languidum, postea ad saturiorem cyaneum." This would be quite correct
if the words "languidus" and "saturior" were to change places. How far the
observation is correct, that the different colours have a relation to the degree
of temper which the metal afterwards acquires, we leave to others to decide.

The colours are here only indications of the different degrees of heat.—
Note R.

479.

When lead is calcined, the surface is first greyish. This greyish powder,
with greater heat, becomes yellow, and then orange. Silver, too, exhibits
colours when heated; the fracture of silver in the process of refining belongs
to the same class of examples. When metallic glasses melt, colours in like
manner appear on the surface.
480.

Seventh condition.—When the surface of glass becomes decomposed. The
accidental opacity (blindwerden) of glass has been already noticed: the term
(blindwerden) is employed to denote that the surface of the glass is so
affected as to appear dim to us.

481.

White glass becomes "blind" soonest; cast, and afterwards polished glass is
also liable to be so affected; the bluish less, the green least.

482.

Of the two sides of a plate of glass one is called the mirror side; it is that
which in the oven lies uppermost, on which one may observe roundish
elevations: it is smoother than the other, which is undermost in the oven,
and on which scratches may be sometimes observed. On this account the
mirror side is placed facing the interior of rooms, because it is less affected
by the moisture adhering to it from within, than the other would be, and the
glass is thus less liable to become "blind."
483.

This half-opacity or dimness of the glass assumes by degrees an appearance
of colour which may become very vivid, and in which perhaps a certain
succession, or otherwise regular order, might be discovered.

484.

Having thus traced the physical colours from their simplest effects to the
present instances, where these fleeting appearances are found to be fixed in
bodies, we are, in fact, arrived at the point where the chemical colours
begin; nay, we have in some sort already passed those limits; a
circumstance which may excite a favourable prejudice for the consistency
of our statement. By way of conclusion to this part of our inquiry, we
subjoin a general observation, which may not be without its bearing on the
common connecting principle of the phenomena that have been adduced.

485.

The colouring of steel and the appearances analogous to it, might perhaps
be easily deduced from the doctrine of the semi-opaque mediums. Polished
steel reflects light powerfully: we may consider the colour produced by the
heat as a slight degree of dimness: hence a bright yellow must immediately
appear; this, as the dimness increases, must still appear deeper, more
condensed, and redder, and at last pure and ruby-red. The colour has now
reached the extreme point of depth, and if we suppose the same degree of
semi-opacity still to continue, the dimness would now spread itself over a
dark ground, first producing a violet, then a dark-blue, and at last a light-
blue, and thus complete the series of the appearances.
We will not assert that this mode of explanation will suffice in all cases; our
object is rather to point out the road by which the all-comprehensive
formula, the very key of the enigma, may be at last discovered.—Note S.

PART III.

CHEMICAL COLOURS.

486.

We give this denomination to colours which we can produce, and more or
less fix, in certain bodies; which we can render more intense, which we can
again take away and communicate to other bodies, and to which, therefore,
we ascribe a certain permanency: duration is their prevailing characteristic.

487.

In this view the chemical colours were formerly distinguished with various
epithets; they were called colores proprii, corporei, materiales, veri,
permanentes, fixi.

488.

In the preceding chapter we observed how the fluctuating and transient
nature of the physical colours becomes gradually fixed, thus forming the
natural transition to our present subject.
489.

Colour becomes fixed in bodies more or less permanently; superficially, or
thoroughly.
490.

All bodies are susceptible of colour; it can either be excited, rendered
intense, and gradually fixed in them, or at least communicated to them.

XXXIV.

CHEMICAL CONTRAST.

491.

In the examination of coloured appearances we had occasion everywhere to
take notice of a principle of contrast: so again, in approaching the precincts
of chemistry, we find a chemical contrast of a remarkable nature. We speak
here, with reference to our present purpose, only of that which is
comprehended under the general names of acid and alkali.
492.

We characterised the chromatic contrast, in conformity with all other
physical contrasts as a more and less; ascribing the plus to the yellow side,
the minus to the blue; and we now find that these two divisions correspond
with the chemical contrasts. The yellow and yellow-red affect the acids, the

blue and blue-red the alkalis; thus the phenomena of chemical colours,
although still necessarily mixed up with other considerations, admit of
being traced with sufficient simplicity.

493.

The principal phenomena in chemical colours are produced by the
oxydation of metals, and it will be seen how important this consideration is
at the outset. Other facts which come into the account, and which are
worthy of attention, will be examined under separate heads; in doing this
we, however, expressly state that we only propose to offer some preparatory
suggestions to the chemist in a very general way, without entering into the
nicer chemical problems and questions, or presuming to decide on them.
Our object is only to give a sketch of the mode in which, according to our
conviction, the chemical theory of colours may be connected with general
physics.

XXXV.

WHITE.

494.

In treating of the dioptrical colours of the first class (155) we have already
in some degree anticipated this subject. Transparent substances may be said
to be in the highest class of inorganic matter. With these, colourless semi-
transparence is closely connected, and white may be considered the last
opaque degree of this.

495.

Pure water crystallised to snow appears white, for the transparence of the
separate parts makes no transparent whole. Various crystallised salts, when
deprived to a certain extent of moisture, appear as a white powder. The
accidentally opaque state of a pure transparent substance might be called
white; thus pounded glass appears as a white powder. The cessation of a

combining power, and the exhibition of the atomic quality of the substance
might at the same time be taken into the account.

496.

The known undecomposed earths are, in their pure state, all white. They
pass to a state of transparence by natural crystallization. Silex becomes
rock-crystal; argile, mica; magnesia, talc; calcareous earth and barytes
appear transparent in various spars.—Note T.
497.

As in the colouring of mineral bodies the metallic oxydes will often invite
our attention, we observe, in conclusion, that metals, when slightly
oxydated, at first appear white, as lead is converted to white lead by
vegetable acid.

XXXVI.

BLACK.

498.

Black is not exhibited in so elementary a state as white. We meet with it in
the vegetable kingdom in semi-combustion; and charcoal, a substance
especially worthy of attention on other accounts, exhibits a black colour.
Again, if woods—for example, boards, owing to the action of light, air, and
moisture, are deprived in part of their combustibility, there appears first the
grey then the black colour. So again, we can convert even portions of
animal substance to charcoal by semi-combustion.
499.

In the same manner we often find that a sub-oxydation takes place in metals
when the black colour is to be produced. Various metals, particularly iron,
become black by slight oxydation, by vinegar, by mild acid fermentations;
for example, a decoction of rice, &c.

500.

Again, it may be inferred that a de-oxydation may produce black. This
occurs in the preparation of ink, which becomes yellow by the solution of
iron in strong sulphuric acid, but when partly de-oxydised by the infusion of
gall-nuts, appears black.

XXXVII.

FIRST EXCITATION OF COLOUR.

501.

In the division of physical colours, where semi-transparent mediums were
considered, we saw colours antecedently to white and black. In the present
case we assume a white and black already produced and fixed; and the
question is, how colour can be excited in them?

502.

Here, too, we can say, white that becomes darkened or dimmed inclines to
yellow; black, as it becomes lighter, inclines to blue.—Note U.

503.

Yellow appears on the active (plus) side, immediately in the light, the
bright, the white. All white surfaces easily assume a yellow tinge; paper,
linen, wool, silk, wax: transparent fluids again, which have a tendency to
combustion, easily become yellow; in other words they easily pass into a
very slight state of semi-transparence.
504.

So again the excitement on the passive side, the tendency to obscure, dark,
black, is immediately accompanied with blue, or rather with a reddish-blue.
Iron dissolved in sulphuric acid, and much diluted with water, if held to the
light in a glass, exhibits a beautiful violet colour as soon as a few drops
only of the infusion of gall-nuts are added. This colour presents the peculiar

hues of the dark topaz, the orphninon of a burnt-red, as the ancients
expressed it.

505.

Whether any colour can be excited in the pure earths by the chemical
operations of nature and art, without the admixture of metallic oxydes, is an
important question, generally, indeed, answered in the negative. It is
perhaps connected with the question—to what extent changes may be
produced in the earths through oxydation?
506.

Undoubtedly the negation of the above question is confirmed by the
circumstance that wherever mineral colours are found, some trace of metal,
especially of iron, shows itself; we are thus naturally led to consider how
easily iron becomes oxydised, how easily the oxyde of iron assumes
different colours, how infinitely divisible it is, and how quickly it
communicates its colour. It were to be wished, notwithstanding, that new
experiments could be made in regard to the above point, so as either to
confirm or remove any doubt.

507.

However this may be, the susceptibility of the earths with regard to colours
already existing is very great; aluminous earth is thus particularly
distinguished.

508.

In proceeding to consider the metals, which in the inorganic world have the
almost exclusive prerogative of appearing coloured, we find that, in their
pure, independent, natural state, they are already distinguished from the
pure earths by a tendency to some one colour or other.
509.

While silver approximates most to pure white,—nay, really represents pure
white, heightened by metallic splendour,—steel, tin, lead, and so forth,
incline towards pale blue-grey; gold, on the other hand, deepens to pure
yellow, copper approaches a red hue, which, under certain circumstances,

increases almost to bright red, but which again returns to a yellow golden
colour when combined with zinc.

510.

But if metals in their pure state have so specific a determination towards
this or that exhibition of colour, they are, through the effect of oxydation, in
some degree reduced to a common character; for the elementary colours
now come forth in their purity, and although this or that metal appears to
have a particular tendency to this or that colour, we find some that can go
through the whole circle of hues, others, that are capable of exhibiting more
than one colour; tin, however, is distinguished by its comparative inaptitude
to become coloured. We propose to give a table hereafter, showing how far
the different metals can be more or less made to exhibit the different
colours.
511.

When the clean, smooth surface of a pure metal, on being heated, becomes
overspread with a mantling colour, which passes through a series of
appearances as the heat increases, this, we are persuaded, indicates the
aptitude of the metal to pass through the whole range of colours. We find
this phenomenon most beautifully exhibited in polished steel; but silver,
copper, brass, lead, and tin, easily present similar appearances. A superficial
oxydation is probably here taking place, as may be inferred from the effects
of the operation when continued, especially in the more easily oxydizable
metals.

512.

The same conclusion may be drawn from the fact that iron is more easily
oxydizable by acid liquids when it is red hot, for in this case the two effects
concur with each other. We observe, again, that steel, accordingly as it is
hardened in different stages of its colorification, may exhibit a difference of
elasticity: this is quite natural, for the various appearances of colour
indicate various degrees of heat.[1]

513.

If we look beyond this superficial mantling, this pellicle of colour, we
observe that as metals are oxydized throughout their masses, white or black
appears with the first degree of heat, as may be seen in white lead, iron, and
quicksilver.

514.

If we examine further, and look for the actual exhibition of colour, we find
it most frequently on the plus side. The mantling, so often mentioned, of
smooth metallic surfaces begins with yellow. Iron passes presently into
yellow ochre, lead from white lead to massicot, quicksilver from æthiops to
yellow turbith. The solutions of gold and platinum in acids are yellow.

515.

The exhibitions on the minus side are less frequent. Copper slightly
oxydized appears blue. In the preparation of Prussian-blue, alkalis are
employed.

516.

Generally, however, these appearances of colour are of so mutable a nature
that chemists look upon them as deceptive tests, at least in the nicer
gradations. For ourselves, as we can only treat of these matters in a general
way, we merely observe that the appearances of colour in metals may be
classed according to their origin, manifold appearance, and cessation, as
various results of oxydation, hyper-oxydation, ab-oxydation, and de-
oxydation.[2]

[1] See par. 478.
[2] As these terms are afterwards referred to (par. 525), it was necessary to
preserve them.

XXXVIII.

AUGMENTATION OF COLOUR.[1]

517.

The augmentation of colour exhibits itself as a condensation, a fulness, a
darkening of the hue. We have before seen, in treating of colourless
mediums, that by increasing the degree of opacity in the medium, we can
deepen a bright object from the lightest yellow to the intensest ruby-red.
Blue, on the other hand, increases to the most beautiful violet, if we rarefy
and diminish a semi-opaque medium, itself lighted, but through which we
see darkness (150, 151).

518.

If the colour is positive, a similar colour appears in the intenser state. Thus
if we fill a white porcelain cup with a pure yellow liquor, the fluid will
appear to become gradually redder towards the bottom, and at last appears
orange. If we pour a pure blue solution into another cup, the upper portion
will exhibit a sky-blue, that towards the bottom, a beautiful violet. If the
cup is placed in the sun, the shadowed side, even of the upper portion, is
already violet. If we throw a shadow with the hand, or any other substance,
over the illumined portion, the shadow in like manner appears reddish.
519.

This is one of the most important appearances connected with the doctrine
of colours, for we here manifestly find that a difference of quantity
produces a corresponding qualified impression on our senses. In speaking
of the last class of epoptical colours (452, 485), we stated our conjecture
that the colouring of steel might perhaps be traced to the doctrine of the
semi-transparent mediums, and we would here again recall this to the
reader's recollection.
520.

All chemical augmentation of colour, again, is the immediate consequence
of continued excitation. The augmentation advances constantly and
unremittingly, and it is to be observed that the increase of intenseness is
most common on the plus side. Yellow iron ochre increases, as well by fire
as by other operations, to a very strong red: massicot is increased to red
lead, turbith to vermilion, which last attains a very high degree of the

yellow-red. An intimate saturation of the metal by the acid, and its
separation to infinity, take place together with the above effects.

521.

The augmentation on the minus side is less frequent; but we observe that the
more pure and condensed the Prussian-blue or cobalt glass is prepared, the
more readily it assumes a reddish hue and inclines to the violet.

522.

The French have a happy expression for the less perceptible tendency of
yellow and blue towards red: they say the colour has "un œil de rouge,"
which we might perhaps express by a reddish glance (einen röthlichen
blick).

[1] Steigerung, literally gradual ascent. See the note to par. 523.

XXXIX.

CULMINATION[1]

523.

This is the consequence of still progressing augmentation. Red, in which
neither yellow nor blue is to be detected, here constitutes the acme.
524.

If we wish to select a striking example of a culmination on the plus side, we
again find it in the coloured steel, which attains the bright red acme, and
can be arrested at this point.

525.

Were we here to employ the terminology before proposed, we should say
that the first oxydation produces yellow, the hyper-oxydation yellow-red;

that here a kind of maximum exists, and that then an ab-oxydation, and
lastly a de-oxydation takes place.

526.

High degrees of oxydation produce a bright red. Gold in solution,
precipitated by a solution of tin, appears bright red: oxyde of arsenic, in
combination, with sulphur, produces a ruby colour.
527.

How far, however, a kind of sub-oxydation may co-operate in some
culminations, is matter for inquiry; for an influence of alkalis on yellow-red
also appears to produce the culmination; the colour reaching the acme by
being forced towards the minus side.

528.

The Dutch prepare a colour known by the name of vermilion, from the best
Hungarian cinnabar, which exhibits the brightest yellow-red. This vermilion
is still only a cinnabar, which, however, approximates the pure red, and it
may be conjectured that alkalis are used to bring it nearer to the culminating
point.

529.

Vegetable juices, treated in this way, offer very striking examples of the
above effects. The colouring-matter of turmeric, annotto, dyer's saffron,[2]
and other vegetables, being extracted with spirits of wine, exhibits tints of
yellow, yellow-red, and hyacinth-red; these, by the admixture of alkalis,
pass to the culminating point, and even beyond it to blue-red.

530.

No instance of a culmination on the minus side has come to my knowledge
in the mineral and vegetable kingdoms. In the animal kingdom the juice of
the murex is remarkable; of its augmentation and culmination on the minus
side, we shall hereafter have occasion to speak.

[1] Culmination, the original word. It might have been rendered maximum of
colour, but as the author supposes an ascent through yellow and blue to red, his

meaning is better expressed by his own term.
[2] Curcuma, Bixa Orellana, Carthamus Tinctorius.

XL.

FLUCTUATION.

531.

The mutability of colour is so great, that even those pigments, which may
have been considered to be defined and arrested, still admit of slight
variations on one side or the other. This mutability is most remarkable near
the culminating point, and is effected in a very striking manner by the
alternate employment of acids and alkalis.
532.

To express this appearance in dyeing, the French make use of the word
"virer," to turn from one side to the other; they thus very adroitly convey an
idea which others attempt to express by terms indicating the component
hues.
533.

The effect produced with litmus is one of the most known and striking of
this kind. This colouring substance is tendered red-blue by means of alkalis.
The red-blue is very readily changed to red-yellow by means of acids, and
again returns to its first state by again employing alkalis. The question
whether a culminating point is to be discovered and arrested by nice
experiments, is left to those who are practised in these operations. Dyeing,
especially scarlet-dyeing, might afford a variety of examples of this
fluctuation.

XLI.

PASSAGE THROUGH THE WHOLE SCALE.

534.

The first excitation and gradual increase of colour take place more on the
plus than on the minus side. So, also, in passing through the whole scale,
colour exhibits itself most on the plus side.

535.

A passage of this kind, regular and evident to the senses, from yellow
through red to blue, is apparent in the colouring of steel.

536.

The metals may be arrested at various points of the colorific circle by
various degrees and kinds of oxydation.

537.

As they also appear green, a question arises whether chemists know any
instance in the mineral kingdom of a constant transition from yellow,
through green, to blue, and vice versâ. Oxyde of iron, melted with glass,
produces first a green, and with a more powerful heat, a blue colour.

538.

We may here observe of green generally, that it appears, especially in an
atomic sense, and certainly in a pure state, when we mix blue and yellow:
but, again, an impure and dirty yellow soon gives us the impression of
green; yellow and black already produce green; this, however, is owing to
the affinity between black and blue. An imperfect yellow, such as that of
sulphur, gives us the impression of a greenish hue: thus, again, an imperfect
blue appears green. The green of wine bottles arises, it appears, from an
imperfect union of the oxyde of iron with the glass. If we produce a more
complete union by greater heat, a beautiful blue-glass is the result.
539.

From all this it appears that a certain chasm exists in nature between yellow
and blue, the opposite characters of which, it is true, may be done away
atomically by due immixture, and, thus combined, to green; but the true
reconciliation between yellow and blue, it seems, only takes place by means
of red.

540.

The process, however, which appears unattainable in inorganic substances,
we shall find to be possible when we turn our attention to organic
productions; for in these, the passage through the whole circle from yellow,
through green and blue, to red, really takes place.

XLII.

INVERSION.

541.

Again, an immediate inversion or change to the totally opposite hue, is a
very remarkable appearance which sometimes occurs; at present, we are
merely enabled to adduce what follows.

542.

The mineral chameleon, a name which has been given to an oxyde of
manganese, may be considered, in its perfectly dry state, as a green powder.
If we strew it in water, the green colour displays itself very beautifully in
the first moment of solution, but it changes presently to the bright red
opposite to green, without any apparent intermediate state.

543.

The same occurs with the sympathetic ink, which may be considered a
reddish liquid, but which, when dried by warmth, appears as a green colour
on paper.
544.

In fact, this phenomenon appears to be owing to the conflict between a dry
and moist state, as has been already observed, if we are not mistaken, by the
chemists. We may look to the improvements of time to point out what may
further be deduced from these phenomena, and to show what other facts
they may be connected with.

XLIII.

FIXATION.

545.

Mutable as we have hitherto found colour to be, even as a substance, yet
under certain circumstances it may at last be fixed.

546.

There are bodies capable of being entirely converted into colouring matter:
here it may be said that the colour fixes itself in its own substance, stops at
a certain point, and is there defined. Such colouring substances are found
throughout nature; the vegetable world affords a great quantity of examples,
among which some are particularly distinguished, and may be considered as
the representatives of the rest; such as, on the active side, madder, on the
passive side, indigo.
547.

In order to make these materials available in use, it is necessary that the
colouring quality in them should be intimately condensed, and the tinging
substance refined, practically speaking, to an infinite divisibility. This is
accomplished in various ways, and particularly by the well-known means of
fermentation and decomposition.

548.

These colouring substances now attach themselves again to other bodies.
Thus, in the mineral kingdom they adhere to earths and metallic oxydes;
they unite in melting with glasses; and in this case, as the light is
transmitted through them, they appear in the greatest beauty, while an
eternal duration may be ascribed to them.

549.

They fasten on vegetable and animal bodies with more or less power, and
remain more or less permanently; partly owing to their nature,—as yellow,
for instance, is more evanescent than blue,—or owing to the nature of the

substance on which they appear. They last less in vegetable than in animal
substances, and even within this latter kingdom there are again varieties.
Hemp or cotton threads, silk or wool, exhibit very different relations to
colouring substances.

550.

Here comes into the account the important operation of employing
mordants, which may be considered as the intermediate agents between the
colour and the recipient substance; various works on dyeing speak of this
circumstantially. Suffice it to have alluded to processes by means of which
the colour retains a permanency only to be destroyed with the substance,
and which may even increase in brightness and beauty by use.

XLIV.

INTERMIXTURE, REAL.

551.

Every intermixture pre-supposes a specific state of colour; and thus when
we speak of intermixture, we here understand it in an atomic sense. We
must first have before us certain bodies arrested at any given point of the
colorific circle, before we can produce gradations by their union.

552.

Yellow, blue, and red, may be assumed as pure elementary colours, already
existing; from these, violet, orange, and green, are the simplest combined
results.

553.

Some persons have taken much pains to define these intermixtures more
accurately, by relations of number, measure, and weight, but nothing very
profitable has been thus accomplished.

554.

Painting consists, strictly speaking, in the intermixture of such specific
colouring bodies and their infinite possible combinations—combinations
which can only be appreciated by the nicest, most practised eye, and only
accomplished under its influence.

555.

The intimate combination of these ingredients is effected, in the first
instance, through the most perfect comminution of the material by means of
grinding, washing, &c., as well as by vehicles or liquid mediums which
hold together the pulverized substance, and combine organically, as it were,
the unorganic; such are the oils, resins, &c.—Note V.
556.

If all the colours are mixed together they retain their general character as
σκιερόν, and as they are no longer seen next each other, no completeness,
no harmony, is experienced; the result is grey, which, like apparent colour,
always appears somewhat darker than white, and somewhat lighter than
black.

557.

This grey may be produced in various ways. By mixing yellow and blue to
an emerald green, and then adding pure red, till all three neutralize each
other; or, by placing the primitive and intermediate colours next each other
in a certain proportion, and afterwards mixing them.

558.

That all the colours mixed together produce white, is an absurdity which
people have credulously been accustomed to repeat for a century, in
opposition to the evidence of their senses.

559.

Colours when mixed together retain their original darkness. The darker the
colours, the darker will be the grey resulting from their union, till at last this
grey approaches black. The lighter the colours the lighter will be the grey,
which at last approaches white.

XLV.

INTERMIXTURE, APPARENT.

560.

The intermixture, which is only apparent, naturally invites our attention in
connexion with the foregoing; it is in many respects important, and, indeed,
the intermixture which we have distinguished as real, might be considered
as merely apparent. For the elements of which the combined colour consists
are only too small to be considered as distinct parts. Yellow and blue
powders mingled together appear green to the naked eye, but through a
magnifying glass we can still perceive yellow and blue distinct from each
other. Thus yellow and blue stripes seen at a distance, present a green mass;
the same observation is applicable with regard to the intermixture of other
specific colours.

561.

In the description of our apparatus we shall have occasion to mention the
wheel by means of which the apparent intermixture is produced by rapid
movement. Various colours are arranged near each other round the edge of a
disk, which is made to revolve with velocity, and thus by having several
such disks ready, every possible intermixture can be presented to the eye, as
well as the mixture of all colours to grey, darker or lighter, according to the
depth of the tints as above explained.

562.

Physiological colours admit, in like manner, of being mixed with others. If,
for example, we produce the blue shadow (65) on a light yellow paper, the
surface will appear green. The same happens with regard to the other
colours if the necessary preparations are attended to.
563.

If, when the eye is impressed with visionary images that last for a while, we
look on coloured surfaces, an intermixture also takes place; the spectrum is
determined to a new colour which is composed of the two.

564.

Physical colours also admit of combination. Here might be adduced the
experiments in which many-coloured images are seen through the prism, as
we have before shown in detail (258, 284).

565.

Those who have prosecuted these inquiries have, however, paid most
attention to the appearances which take place when the prismatic colours
are thrown on coloured surfaces.

566.

What is seen under these circumstances is quite simple. In the first place it
must be remembered that the prismatic colours are much more vivid than
the colours of the surface on which they are thrown. Secondly, we have to
consider that the prismatic colours may be either homogeneous or
heterogeneous, with the recipient surface. In the former case the surface
deepens and enhances them, and is itself enhanced in return, as a coloured
stone is displayed by a similarly coloured foil. In the opposite case each
vitiates, disturbs, and destroys the other.

567.

These experiments may be repeated with coloured glasses, by causing the
sun-light to shine through them on coloured surfaces. In every instance
similar results will appear.

568.

The same effect takes place when we look on coloured objects through
coloured glasses; the colours being thus according to the same conditions
enhanced, subdued, or neutralized.
569.

If the prismatic colours are suffered to pass through coloured glasses, the
appearances that take place are perfectly analogous; in these cases more or
less force, more or less light and dark, the clearness and cleanness of the
glass are all to be allowed for, as they produce many delicate varieties of

effect: these will not escape the notice of every accurate observer who takes
sufficient interest in the inquiry to go through the experiments.

570.

It is scarcely necessary to mention that several coloured glasses, as well as
oiled or transparent papers, placed over each other, may be made to produce
and exhibit every kind of intermixture at pleasure.

571.

Lastly, the operation of glazing in painting belongs to this kind of
intermixture; by this means a much more refined union may be produced
than that arising from the mechanical, atomic mixture which is commonly
employed.

XLVI.

COMMUNICATION, ACTUAL.

572.

Having now provided the colouring materials, as before shown, a further
question arises how to communicate these to colourless substances: the
answer is of the greatest importance from the connexion of the object with
the ordinary wants of men, with useful purposes, and with commercial and
technical interests.

573.

Here, again, the dark quality of every colour again comes into the account.
From a yellow, that is very near to white, through orange, and the hue of
minium to pure red and carmine, through all gradations of violet to the
deepest blue which is almost identified with black, colour still increases in
darkness. Blue once defined, admits of being diluted, made light, united
with yellow, and then, as green, it approaches the light side of the scale: but
this is by no means according to its own nature.

574.

In the physiological colours we have already seen that they are less than the
light, inasmuch as they are a repetition of an impression of light, nay, at last
they leave this impression quite as a dark. In physical experiments the
employment of semi-transparent mediums, the effect of semi-transparent
accessory images, taught us that in such cases we have to do with a subdued
light, with a transition to darkness.

575.

In treating of the chemical origin of pigments we found that the same effect
was produced on the very first excitement. The yellow tinge which mantles
over the steel, already darkens the shining surface. In changing white lead
to massicot it is evident that the yellow is darker than white.

576.

This process is in the highest degree delicate; the growing intenseness, as it
still increases, tinges the substance more and more intimately and
powerfully, and thus indicates the extreme fineness, and the infinite
divisibility of the coloured atoms.

577.

The colours which approach the dark side, and consequently, blue in
particular, can be made to approximate to black; in fact, a very perfect
Prussian blue, or an indigo acted on by vitriolic acid appears almost as a
black.

578.

A remarkable appearance may be here adverted to; pigments, in their
deepest and most condensed state, especially those produced from the
vegetable kingdom, such as the indigo just mentioned, or madder carried to
its intensest hue, no longer show their own colour; on the contrary, a
decided metallic shine is seen on their surface, in which the physiological
compensatory colour appears.

579.

All good indigo exhibits a copper-colour in its fracture, a circumstance
attended to, as a known characteristic, in trade. Again, the indigo which has
been acted on by sulphuric acid, if thickly laid on, or suffered to dry so that
neither white paper nor the porcelain can appear through, exhibits a colour
approaching to orange.

580.

The bright red Spanish rouge, probably prepared from madder, exhibits on
its surface a perfectly green, metallic shine. If this colour, or the blue before
mentioned, is washed with a pencil on porcelain or paper, it is seen in its
real state owing to the bright ground shining through.

581.

Coloured liquids appear black when no light is transmitted through them, as
we may easily see in cubic tin vessels with glass bottoms. In these every
transparent-coloured infusion will appear black and colourless if we place a
black surface under them.

582.

If we contrive that the image of a flame be reflected from the bottom, the
image will appear coloured. If we lift up the vessel and suffer the
transmitted light to fall on white paper under it, the colour of the liquid
appears on the paper. Every light ground seen through such a coloured
medium exhibits the colour of the medium.

583.

Thus every colour, in order to be seen, must have a light within or behind it.
Hence the lighter and brighter the grounds are, the more brilliant the colours
appear. If we pass lac-varnish over a shining white metal surface, as the so-
called foils are prepared, the splendour of the colour is displayed by this
internally reflected light as powerfully as in any prismatic experiment; nay,
the force of the physical colours is owing principally to the circumstance
that light is always acting with and behind them.

584.

Lichtenberg, who of necessity followed the received theory, owing to the
time and circumstances in which he lived, was yet too good an observer,
and too acute not to explain and classify, after his fashion, what was evident
to his senses. He says, in the preface to Delaval, "It appears to me also, on
other grounds, probable, that our organ, in order to be impressed by a
colour, must at the same time be impressed by all light (white)."

585.

To procure white as a ground is the chief business of the dyer. Every colour
may be easily communicated to colourless earths, especially to alum: but
the dyer has especially to do with animal and vegetable products as the
ground of his operations.
586.

Everything living tends to colour—to local, specific colour, to effect, to
opacity—pervading the minutest atoms. Everything in which life is extinct
approximates to white (494), to the abstract, the general state, to
clearness[1], to transparence.

587.

How this is put in practice in technical operations remains to be adverted to
in the chapter on the privation of colour. With regard to the communication
of colour, we have especially to bear in mind that animals and vegetables, in
a living state, produce colours, and hence their substances, if deprived of
colours, can the more readily re-assume them.

[1] Verklärung, literally clarification.

XLVII.

COMMUNICATION, APPARENT.

588.

The communication of colours, real as well as apparent, corresponds, as
may easily be seen, with their intermixture: we need not, therefore, repeat
what has been already sufficiently entered into.

589.

Yet we may here point out more circumstantially the importance of an
apparent communication which takes place by means of reflection. This
phenomenon is well known, but still it is pregnant with inferences, and is of
the greatest importance both to the investigator of nature and to the painter.
590.

Let a surface coloured with any one of the positive colours be placed in the
sun, and let its reflection be thrown on other colourless objects. This
reflection is a kind of subdued light, a half-light, a half-shadow, which, in a
subdued state, reflects the colours in question.

591.

If this reflection acts on light surfaces, it is so far overpowered that we can
scarcely perceive the colour which accompanies it; but if it acts on
shadowed portions, a sort of magical union takes place with the σκιερῷ.
Shadow is the proper element of colour, and in this case a subdued colour
approaches it, lighting up, tinging, and enlivening it. And thus arises an
appearance, as powerful as agreeable, which may render the most pleasing
service to the painter who knows how to make use of it. These are the types
of the so-called reflexes, which were only noticed late in the history of art,
and which have been too seldom employed in their full variety.

592.

The schoolmen called these colours colores notionales and intentionales,
and the history of the doctrine of colours will generally show that the old
inquirers already observed the phenomena well enough, and knew how to
distinguish them properly, although the whole method of treating such
subjects is very different from ours.

XLVIII.

EXTRACTION.

593.

Colour may be extracted from substances, whether they possess it naturally
or by communication, in various ways. We have thus the power to remove it
intentionally for a useful purpose, but, on the other hand, it often flies
contrary to our wish.

594.

Not only are the elementary earths in their natural state white, but vegetable
and animal substances can be reduced to a white state without disturbing
their texture. A pure white is very desirable for various uses, as in the
instance of our preferring to use linen and cotton stuffs uncoloured. In like
manner some silk stuffs, paper, and other substances, are the more
agreeable the whiter they can be. Again, the chief basis of all dyeing
consists in white grounds. For these reasons manufacturers, aided by
accident and contrivance, have devoted themselves assiduously to discover
means of extracting colour: infinite experiments have been made in
connexion with this object, and many important facts have been arrived at.

595.

It is in accomplishing this entire extraction of colour that the operation of
bleaching consists, which is very generally practised empirically or
methodically. We will here shortly state the leading principles.
596.

Light is considered as one of the first means of extracting colour from
substances, and not only the sun-light, but the mere powerless day-light: for
as both lights—the direct light of the sun, as well as the derived light of the
sky—kindle Bologna phosphorus, so both act on coloured surfaces.
Whether the light attacks the colour allied to it, and, as it were, kindles and
consumes it, thus reducing the definite quality to a general state, or whether
some other operation, unknown to us, takes place, it is clear that light
exercises a great power on coloured surfaces, and bleaches them more or
less. Here, however, the different colours exhibit a different degree of

durability; yellow, especially if prepared from certain materials, is, in this
case, the first to fly.

597.

Not only light, but air, and especially water, act strongly in destroying
colour. It has been even asserted that thread, well soaked and spread on the
grass at night, bleaches better than that which is exposed, after soaking, to
the sun-light. Thus, in this case, water proves to be a solving and
conducting agent, removing the accidental quality, and restoring the
substance to a general or colourless state.

598.

The extraction of colour is also effected by re-agents. Spirits of wine has a
peculiar tendency to attract the juice which tinges plants, and becomes
coloured with it often in a very permanent manner. Sulphuric acid is very
efficient in removing colour, especially from wool and silk, and every one
is acquainted with the use of sulphur vapours in bleaching.

599.

The strongest acids have been recommended more recently as more
expeditious agents in bleaching.

600.

The alkaline re-agents produce the same effects by contrary means—
lixiviums alone, oils and fat combined with lixiviums to soap, and so forth.

601.

Before we dismiss this subject, we observe [Pg 240] that it may be well
worth while to make certain delicate experiments as to how far light and air
exhibit their action in the removal of colour. It might be possible to expose
coloured substances to the light under glass bells, without air, or filled with
common or particular kinds of air. The colours might be those of known
fugacity, and it might be observed whether any of the volatilized colour
attached itself to the glass or was otherwise perceptible as a deposit or
precipitate; whether, again, in such a case, this appearance would be
perfectly like that which had gradually ceased to be visible, or whether it

had suffered any change. Skilful experimentalists might devise various
contrivances with a view to such researches.

602.

Having thus first considered the operations of nature as subservient to our
proposes, we add a few observations on the modes in which they act against
us.

603.

The art of painting is so circumstanced that the most beautiful results of
mind and labour are altered and destroyed in various ways by time. Hence
great pains have been always taken to find durable pigments, and so to unite
them with each other and with their ground, that their permanency might be
further insured. The technical history of the schools of painting affords
sufficient information on this point.

604.

We may here, too, mention a minor art, to which, in relation to dyeing, we
are much indebted, namely, the weaving of tapestry. As the manufacturers
were enabled to imitate the most delicate shades of pictures, and hence
often brought the most variously coloured materials together, it was soon
observed that the colours were not all equally durable, but that some faded
from the tapestry more quickly than others. Hence the most diligent efforts
were made to ensure an equal permanency to all the colours and their
gradations. This object was especially promoted in France, under Colbert,
whose regulations to this effect constitute an epoch in the history of dyeing.
The gay dye which only aimed at a transient beauty, was practised by a
particular guild. On the other hand, great pains were taken to define the
technical processes which promised durability.
And thus, after considering the artificial extraction, the evanescence, and
the perishable nature of brilliant appearances of colour, we are again
returned to the desideratum of permanency.

XLIX.

NOMENCLATURE.

605.

After what has been adduced respecting the origin, the increase, and the
affinity of colours, we may be better enabled to judge what nomenclature
would be desirable in future, and what might be retained of that hitherto in
use.

606.

The nomenclature of colours, like all other modes of designation, but
especially those employed to distinguish the objects of sense, proceeded in
the first instance from particular to general, and from general back again to
particular terms. The name of the species became a generic name to which
the individual was again referred.

607.

This method might have been followed in consequence of the mutability
and uncertainty of ancient modes of expression, especially since, in the
early ages, more reliance may be supposed to have been placed on the vivid
impressions of sense. The qualities of objects were described indistinctly,
because they were impressed clearly on every imagination.

608.

The pure chromatic circle was limited, it is true; but, specific as it was, it
appears to have been applied to innumerable objects, while it was
circumscribed by qualifying characteristics. If we take a glance at the
copiousness of the Greek and Roman terms, we shall perceive how mutable
the words were, and how easily each was adapted to almost every point in
the colorific circle.—Note W.

609.

In modern ages terms for many new gradations were introduced in
consequence of the various operations of dyeing. Even the colours of
fashion and their designations, represented an endless series of specific
hues. We shall, on occasion, employ the chromatic terminology of modern
languages, whence it will appear that the aim has gradually been to

introduce more exact definitions, and to individualise and arrest a fixed and
specific state by language equally distinct.

610.

With regard to the German terminology, it has the advantage of possessing
four monosyllabic names no longer to be traced to their origin, viz., yellow
(Gelb), blue, red, green. They represent the most general idea of colour to
the imagination, without reference to any very specific modification.
611.

If we were to add two other qualifying terms to each of these four, as thus—
red-yellow, and yellow-red, red-blue and blue-red, yellow-green and green-
yellow, blue-green and green-blue,[1] we should express the gradations of
the chromatic circle with sufficient distinctness; and if we were to add the
designations of light and dark, and again define, in some measure, the
degree of purity or its opposite by the monosyllables black, white, grey,
brown, we should have a tolerably sufficient range of expressions to
describe the ordinary appearances presented to us, without troubling
ourselves whether they were produced dynamically or atomically.

612.

The specific and proper terms in use might, however, still be conveniently
employed, and we have thus made use of the words orange and violet. We
have in like manner employed the word "purpur" to designate a pure central
red, because the secretion of the murex or "purpura" is to be carried to the
highest point of culmination by the action of the sun-light on fine linen
saturated with the juice.

[1] This description is suffered to remain because it accounts for the terminology
employed throughout.—T.

L.

MINERALS.

613.

The colours of minerals are all of a chemical nature, and thus the modes in
which they are produced may be explained in a general way by what has
been said on the subject of chemical colours.

614.

Among the external characteristics of minerals, the description of their
colours occupies the first place; and great pains have been taken, in the
spirit of modern times, to define and arrest every such appearance exactly:
by this means, however, new difficulties, it appears to us, have been
created, which occasion no little inconvenience in practice.

615.

It is true, this precision, when we reflect how it arose, carries with it its own
excuse. The painter has at all times been privileged in the use of colours.
The few specific hues, in themselves, admitted of no change; but from
these, innumerable gradations were artificially produced which imitated the
surface of natural objects. It was, therefore, not to be wondered at that these
gradations should also be adopted as criterions, and that the artist should be
invited to produce tinted patterns with which the objects of nature might be
compared, and according to which they were to receive their designations.

616.

But, after all, the terminology of colours which has been introduced in
mineralogy, is open to many objections. The terms, for instance, have not
been borrowed from the mineral kingdom, as was possible enough in most
cases, but from all kinds of visible objects. Too many specific terms have
been adopted; and in seeking to establish new definitions by combining
these, the nomenclators have not reflected that they thus altogether efface
the image from the imagination, and the idea from the understanding.
Lastly, these individual designations of colours, employed to a certain
extent as elementary definitions, are not arranged in the best manner as
regards their respective derivation from each other: hence, the scholar must
learn every single designation, and impress an almost lifeless but positive
language on his memory. The further consideration of this would be too
foreign to our present subject.[1]

[1] These remarks have reference to the German mineralogical terminology.—T.

LI.

PLANTS.

617.

The colours of organic bodies in general may be considered as a higher kind
of chemical operation, for which reason the ancients employed the word
concoction, πέψις, to designate the process. All the elementary colours, as
well as the combined and secondary hues, appear on the surface of organic
productions, while on the other hand, the interior, if not colourless, appears,
strictly speaking, negative when brought to the light. As we propose to
communicate our views respecting organic nature, to a certain extent, in
another place, we only insert here what has been before connected with the
doctrine of colours, while it may serve as an introduction to the further
consideration of the views alluded to: and first, of plants.

618.

Seeds, bulbs, roots, and what is generally shut out from the light, or
immediately surrounded by the earth, appear, for the most part, white.

619.

Plants reared from seed, in darkness, are white, or approaching to yellow.
Light, on the other hand, in acting on their colours, acts at the same time on
their form.

620.

Plants which grow in darkness make, it is true, long shoots from joint to
joint: but the stems between two joints are thus longer than they should be;
no side stems are produced, and the metamorphosis of the plant does not
take place.

621.

Light, on the other hand, places it at once in an active state; the plant
appears green, and the course of the metamorphosis proceeds
uninterruptedly to the period of reproduction.

622.

We know that the leaves of the stem are only preparations and pre-
significations of the instruments of florification and fructification, and
accordingly we can already see colours in the leaves of the stem which, as it
were, announce the flower from afar, as is the case in the amaranthus.

623.

There are white flowers whose petals have wrought or refined themselves to
the greatest purity; there are coloured ones, in which the elementary hues
may be said to fluctuate to and fro. There are some which, in tending to the
higher state, have only partially emancipated themselves from the green of
the plant.

624.

Flowers of the same genus, and even of the same kind, are found of all
colours. Roses, and particularly mallows, for example, vary through a great
portion of the colorific circle from white to yellow, then through red-yellow
to bright red, and from thence to the darkest hue it can exhibit as it
approaches blue.

625.

Others already begin from a higher degree in the scale, as, for example, the
poppy, which is yellow-red in the first instance, and which afterwards
approaches a violet hue.

626.

Yet the same colours in species, varieties, and even in families and classes,
if not constant, are still predominant, especially the yellow colour: blue is
throughout rarer.

627.

A process somewhat similar takes place in the juicy capsule of the fruit, for
it increases in colour from the green, through the yellowish and yellow, up
to the highest red, the colour of the rind thus indicating the degree of
ripeness. Some are coloured all round, some only on the sunny side, in
which last case the augmentation of the yellow into red,—the gradations
crowding in and upon each other,—may be very well observed.

628.

Many fruits, too, are coloured internally; pure red juices, especially, are
common.

629.

The colour which is found superficially in the flower and penetratingly in
the fruit, spreads itself through all the remaining parts, colouring the roots
and the juices of the stem, and this with a very rich and powerful hue.

630.

So, again, the colour of the wood passes from yellow through the different
degrees of red up to pure red and on to brown. Blue woods are unknown to
me; and thus in this degree of organisation the active side exhibits itself
powerfully, although both principles appear balanced in the general green of
the plant.

631.

We have seen above that the germ pushing from the earth is generally white
and yellowish, but that by means of the action of light and air it acquires a
green colour. The same happens with young leaves of trees, as may be seen,
for example, in the birch, the young leaves of which are yellowish, and if
boiled, yield a beautiful yellow juice: afterwards they become greener,
while the leaves of other trees become gradually blue-green.

632.

Thus a yellow ingredient appears to belong more essentially to leaves than a
blue one; for this last vanishes in the autumn, and the yellow of the leaf
appears changed to a brown colour. Still more remarkable, however, are the

particular cases where leaves in autumn again become pure yellow, and
others increase to the brightest red.

633.

Other plants, again, may, by artificial treatment be entirely converted to a
colouring matter, which is as fine, active, and infinitely divisible as any
other. Indigo and madder, with which so much is effected, are examples:
lichens are also used for dyes.
634.

To this fact another stands immediately opposed; we can, namely, extract
the colouring part of plants, and, as it were, exhibit it apart, while the
organisation does not on this account appear to suffer at all. The colours of
flowers may be extracted by spirits of wine, and tinge it; the petals
meanwhile becoming white.

635.

There are various modes of acting on flowers and their juices by re-agents.
This has been done by Boyle in many experiments. Roses are bleached by
sulphur, and may be restored to their first state by other acids; roses are
turned green by the smoke of tobacco.

LII.

WORMS, INSECTS, FISHES.

636.

With regard to creatures belonging to the lower degrees of organisation, we
may first observe that worms, which live in the earth and remain in
darkness and cold moisture, are imperfectly negatively coloured; worms
bred in warm moisture and darkness are colourless; light seems expressly
necessary to the definite exhibition of colour.

637.

Creatures which live in water, which, although a very dense medium,
suffers sufficient light to pass through it, appear more or less coloured.
Zoophytes, which appear to animate the purest calcareous earth, are mostly
white; yet we find corals deepened into the most beautiful yellow-red: in
other cells of worms this colour increases nearly to bright red.

638.

The shells of the crustaceous tribe are beautifully designed and coloured,
yet it is to be remarked that neither land-snails nor the shells of crustacea of
fresh water, are adorned with such bright colours as those of the sea.

639.

In examining shells, particularly such as are spiral, we find that a series of
animal organs, similar to each other, must have moved increasingly
forward, and in turning on an axis produced the shell in a series of
chambers, divisions, tubes, and prominences, according to a plan for ever
growing larger. We remark, however, that a tinging juice must have
accompanied the development of these organs, a juice which marked the
surface of the shell, probably through the immediate co-operation of the
sea-water, with coloured lines, points, spots, and shadings: this must have
taken place at regular intervals, and thus left the indications of increasing
growth lastingly on the exterior; meanwhile the interior is generally found
white or only faintly coloured.

640.

That such a juice is to be found in shell-fish is, besides, sufficiently proved
by experience; for the creatures furnish it in its liquid and colouring state:
the juice of the ink-fish is an example. But a much stronger is exhibited in
the red juice found in many shell-fish, which was so famous in ancient
times, and has been employed with advantage by the moderns. There is, it
appears, in the entrails of many of the crustaceous tribe a certain vessel
which is filled with a red juice; this contains a very strong and durable
colouring substance, so much so that the entire creature may be crushed and
boiled, and yet out of this broth a sufficiently strong tinging liquid may be
extracted. But the little vessel filled with colour may be separated from the
animal, by which means of course a concentrated juice is gained.

641.

This juice has the property that when exposed to light and air it appears first
yellowish, then greenish; it then passes to blue, then to a violet, gradually
growing redder; and lastly, by the action of the sun, and especially if
transferred to cambric, it assumes a pure bright red colour.

642.

Thus we should here have an augmentation, even to culmination, on the
minus side, which we cannot easily meet with in inorganic cases; indeed,
we might almost call this example a passage through the whole scale, and
we are persuaded that by due experiments the entire revolution of the circle
might really be effected, for there is no doubt that by acids duly employed,
the pure red may be pushed beyond the culminating point towards scarlet.
643.

This juice appears on the one hand to be connected with the phenomena of
reproduction, eggs being found, the embryos of future shell-fish, which
contain a similar colouring principle. On the other hand, in animals ranking
higher in the scale of being, the secretion appears to bear some relation to
the development of the blood. The blood exhibits similar properties in
regard to colour; in its thinnest state it appears yellow; thickened, as it is
found in the veins, it appears red; while the arterial blood exhibits a brighter
red, probably owing to the oxydation which takes place by means of
breathing. The venous blood approaches more to violet, and by this
mutability denotes the tendency to that augmentation and progression which
are now familiar to us.

644.

Before we quit the element whence we derived the foregoing examples, we
may add a few observations on fishes, whose scaly surface is coloured
either altogether in stripes, or in spots, and still oftener exhibits a certain
iridescent appearance, indicating the affinity of the scales with the coats of
shell-fish, mother-of-pearl, and even the pearl itself. At the same time it
should not be forgotten that warmer climates, the influence of which
extends to the watery regions, produce, embellish, and enhance these
colours in fishes in a still greater degree.

645.

In Otaheite, Forster observed fishes with beautifully iridescent surfaces, and
this effect was especially apparent at the moment when the fish died. We
may here call to mind the hues of the chameleon, and other similar
appearances; for when similar facts are presented together, we are better
enabled to trace them.

646.

Lastly, although not strictly in the same class, the iridescent appearance of
certain molluscæ may be mentioned, as well as the phosphorescence which,
in some marine creatures, it is said becomes iridescent just before it
vanishes.
647.

We now turn our attention to those creatures which belong to light, air and
dry warmth, and it is here that we first find ourselves in the living region of
colours. Here, in exquisitely organised parts, the elementary colours present
themselves in their greatest purity and beauty. They indicate, however, that
the creatures they adorn, are still low in the scale of organisation, precisely
because these colours can thus appear, as it were, unwrought. Here, too,
heat seems to contribute much to their development.

648.

We find insects which may be considered altogether as concentrated
colouring matter; among these, the cochineals especially are celebrated;
with regard to these we observe that their mode of settling on vegetables,
and even nestling in them, at the same time produces those excrescences
which are so useful as mordants in fixing colours.

649.

But the power of colour, accompanied by regular organisation, exhibits
itself in the most striking manner in those insects which require a perfect
metamorphosis for their development—in scarabæ, and especially in
butterflies.

650.

These last, which might be called true productions of light and air, often
exhibit the most beautiful colours, even in their chrysalis state, indicating
the future colours of the butterfly; a consideration which, if pursued further
hereafter, must undoubtedly afford a satisfactory insight into many a secret
of organised being.

651.

If, again, we examine the wings of the butterfly more accurately, and in its
net-like web discover the rudiments of an arm, and observe further the
mode in which this, as it were, flattened arm is covered with tender
plumage and constituted an organ of flying; we believe we recognise a law
according to which the great variety of tints is regulated. This will be a
subject for further investigation hereafter.

652.

That, again, heat generally has an influence on the size of the creature, on
the accomplishment of the form, and on the greater beauty of the colours,
hardly needs to be remarked.

LIII.

BIRDS.

653.

The more we approach the higher organisations, the more it becomes
necessary to limit ourselves to a few passing observations; for all the
natural conditions of such organised beings are the result of so many
premises, that, without having at least hinted at these, our remarks would
only appear daring, and at the same time insufficient.
654.

We find in plants, that the consummate flower and fruit are, as it were,
rooted in the stem, and that they are nourished by more perfect juices than
the original roots first afforded; we remark, too, that parasitical plants

which derive their support from organised structures, exhibit themselves
especially endowed as to their energies and qualities. We might in some
sense compare the feathers of birds with plants of this description; the
feathers spring up as a last structural result from the surface of a body
which has yet much in reserve for the completion of the external economy,
and thus are very richly endowed organs.

655.

The quills not only grow proportionally to a considerable size, but are
throughout branched, by which means they properly become feathers, and
many of these feathered branches are again subdivided; thus, again,
recalling the structure of plants.

656.

The feathers are very different in shape and size, but each still remains the
same organ, forming and transforming itself according to the constitution of
the part of the body from which it springs.

657.

With the form, the colour also becomes changed, and a certain law regulates
the general order of hues as well as that particular distribution by which a
single feather becomes party coloured, It is from this that all combination of
variegated plumage arises, and whence, at last, the eyes in the peacock's tail
are produced. It is a result similar to that which we have already unfolded in
treating of the metamorphosis of plants, and which we shall take an early
opportunity to prove.

658.

Although time and circumstances compel us here to pass by this organic
law, yet we are bound to refer to the chemical operations which commonly
exhibit themselves in the tinting of feathers in a mode now sufficiently
known to us.
659.

Plumage is of all colours, yet, on the whole, yellow deepening to red is
commoner than blue.

660.

The operation of light on the feathers and their colours, is to be remarked in
all cases. Thus, for example, the feathers on the breast of certain parrots, are
strictly yellow; the scale-like anterior portion, which is acted on by the
light, is deepened from yellow to red. The breast of such a bird appears
bright-red, but if we blow into the feathers the yellow appears.

661.

The exposed portion of the feathers is in all cases very different from that
which, in a quiet state, is covered; it is only the exposed portion, for
instance, in ravens, which exhibits the iridescent appearance; the covered
portion does not: from which indication, the feathers of the tail when ruffled
together, may be at once placed in the natural order again.

LIV.

MAMMALIA AND HUMAN BEINGS.

662.

Here the elementary colours begin to leave us altogether. We are arrived at
the highest degree of the scale, and shall not dwell on its characteristics
long.

663.

An animal of this class is distinguished among the examples of organised
being. Every thing that exhibits itself about him is living. Of the internal
structure we do not speak, but confine ourselves briefly to the surface. The
hairs are already distinguished from feathers, inasmuch as they belong more
to the skin, inasmuch as they are simple, thread-like, not branched. They are
however, like feathers, shorter, longer, softer, and firmer, colourless or
coloured, and all this in conformity to laws which might be defined.
664.

White and black, yellow, yellow-red and brown, alternate in various
modifications, but they never appear in such a state as to remind us of the
elementary hues. On the contrary, they are all broken colours subdued by
organic concoction, and thus denote, more or less, the perfection of life in
the being they belong to.

665.

One of the most important considerations connected with morphology, so
far as it relates to surfaces, is this, that even in quadrupeds the spots of the
skin have a relation with the parts underneath them. Capriciously as nature
here appears, on a hasty examination, to operate, she nevertheless
consistently observes a secret law. The development and application of this,
it is true, are reserved only for accurate and careful investigation and
sincere co-operation.

666.

If in some animals portions appear variegated with positive colours, this of
itself shows how far such creatures are removed from a perfect
organisation; for, it may be said, the nobler a creature is, the more all the
mere material of which he is composed, is disguised by being wrought
together; the more essentially his surface corresponds with the internal
organisation, the less can it exhibit the elementary colours. Where all tends
to make up a perfect whole, any detached specific developments cannot
take place.

667.

Of man we have little to say, for he is entirely distinct from the general
physiological results of which we now treat. So much in this case is in
affinity with the internal structure, that the surface can only be sparingly
endowed.

668.

When we consider that brutes are rather encumbered than advantageously
provided with intercutaneous muscles; when we see that much that is
superfluous tends to the surface, as, for instance, large ears and tails, as well

as hair, manes, tufts; we see that nature, in such cases, had much to give
away and to lavish.

669.

On the contrary, the general surface of the human form is smooth and clean,
and thus in the most perfect examples, the beautiful forms are apparent; for
it may be remarked in passing, that a superfluity of hair on the chest, arms,
and lower limbs, rather indicates weakness than strength. Poets only have
sometimes been induced, probably by the example of the ferine nature, so
strong in other respects, to extol similar attributes in their rough heroes.
670.

But we have here chiefly to speak of colour, and observe that the colour of
the human skin, in all its varieties, is never an elementary colour, but
presents, by means of organic concoction, a highly complicated result.—
Note X.

671.

That the colour of the skin and hair has relation with the differences of
character, is beyond question; and we are led to conjecture that the
circumstance of one or other organic system predominating, produces the
varieties we see. A similar hypothesis may be applied to nations, in which
case it might perhaps be observed, that certain colours correspond with
certain confirmations, which has always been observed of the negro
physiognomy.

672.

Lastly, we might here consider the problematical question, whether all
human forms and hues are not equally beautiful, and whether custom and
self-conceit are not the causes why one is preferred to another? We venture,
however, after what has been adduced, to assert that the white man, that is,
he whose surface varies from white to reddish, yellowish, brownish, in
short, whose surface appears most neutral in hue and least inclines to any
particular or positive colour, is the most beautiful. On the same principle a
similar point of perfection in human conformation may be defined hereafter,
when the question relates to form. We do not imagine that this long-

disputed question is to be thus, once for all, settled, for there are persons
enough who have reason to leave this significancy of the exterior in doubt;
but we thus express a conclusion, derived from observation and reflection,
such as might suggest itself to a mind aiming at a satisfactory decision. We
subjoin a few observations connected with the elementary chemical
doctrine of colours.—Note Y.

LV.

PHYSICAL AND CHEMICAL EFFECTS OF THE TRANSMISSION OF LIGHT THROUGH
COLOURED MEDIUMS.

673.

The physical and chemical effects of colourless light are known, so that it is
unnecessary here to describe them at length. Colourless light exhibits itself
under various conditions as exciting warmth, as imparting a luminous
quality to certain bodies, as promoting oxydation and de-oxydation. In the
modes and degrees of these effects many varieties take place, but no
difference is found indicating a principle of contrast such as we find in the
transmission of coloured light. We proceed briefly to advert to this.

674.

Let the temperature of a dark room be observed by means of a very sensible
air-thermometer; if the bulb is then brought to the direct sun light as it
shines into the room, nothing is more natural than that the fluid should
indicate a much higher degree of warmth. If upon this we interpose
coloured glasses, it follows again quite naturally that the degree of warmth
must be lowered; first, because the operation of the direct light is already
somewhat impeded by the glass, and again, more especially, because a
coloured glass, as a dark medium, admits less light through it.
675.

But here a difference in the excitation of warmth exhibits itself to the
attentive observer, according to the colour of the glass. The yellow and the

yellow-red glasses produce a higher temperature than the blue and blue-red,
the difference being considerable.

676.

This experiment may be made with the prismatic spectrum. The
temperature of the room being first remarked on the thermometer, the blue
coloured light is made to fall on the bulb, when a somewhat higher degree
of warmth is exhibited, which still increases as the other colours are
gradually brought to act on the mercury. If the experiment is made with the
water-prism, so that the white light can be retained in the centre, this,
refracted indeed, but not yet coloured light, is the warmest; the other
colours, stand in relation to each other as before.
677.

As we here merely describe, without undertaking to deduce or explain this
phenomenon, we only remark in passing, that the pure light is by no means
abruptly and entirely at an end with the red division in the spectrum, but
that a refracted light is still to be observed deviating from its course and, as
it were, insinuating itself beyond the prismatic image, so that on closer
examination it will hardly be found necessary to take refuge in invisible
rays and their refraction.

678.

The communication of light by means of coloured mediums exhibits the
same difference. The light communicates itself to Bologna phosphorus
through blue and violet glasses, but by no means through yellow and
yellow-red glasses. It has been even remarked that the phosphori which
have been rendered luminous under violet and blue glasses, become sooner
extinguished when afterwards placed under yellow and yellow-red glasses
than those which have been suffered to remain in a dark room without any
further influence.

679.

These experiments, like the foregoing, may also be made by means of the
prismatic spectrum, when the same results take place.

680.

To ascertain the effect of coloured light on oxydation and de-oxydation, the
following means may be employed:—Let moist, perfectly white muriate of
silver[1] be spread on a strip of paper; place it in the light, so that it may
become to a certain degree grey, and then cut it in three portions. Of these,
one may be preserved in a book, as a specimen of this state; let another be
placed under a yellow-red, and the third under a blue-red glass. The last will
become a darker grey, and exhibit a de-oxydation; the other, under the
yellow-red glass, will, on the contrary, become a lighter grey, and thus
approach nearer to the original state of more perfect oxydation. The change
in both may be ascertained by a comparison with the unaltered specimen.

681.

An excellent apparatus has been contrived to perform these experiments
with the prismatic image. The results are analogous to those already
mentioned, and we shall hereafter give the particulars, making use of the
labours of an accurate observer, who has been for some time carefully
prosecuting these experiments.[2]

[1] Now generally called chloride of silver: the term in the original is Hornsilber.
—T.
[2] The individual alluded to was Seebeck: the result of his experiments was
published in the second volume.—T.

LVI.

CHEMICAL EFFECT IN DIOPTRICAL ACHROMATISM.

682.

We first invite our readers to turn to what has been before observed on this
subject (285, 298), to avoid unnecessary repetition here.
683.

We can thus give a glass the property of producing much wider coloured
edges without refracting more strongly than before, that is, without

displacing the object much more perceptibly.

684.

This property is communicated to the glass by means of metallic oxydes.
Minium, melted and thoroughly united with a pure glass, produces this
effect, and thus flint-glass (291) is prepared with oxyde of lead.
Experiments of this kind have been carried farther, and the so-called butter
of antimony, which, according to a new preparation, may be exhibited as a
pure fluid, has been made use of in hollow lenses and prisms, producing a
very strong appearance of colour with a very moderate refraction, and
presenting the effect which we have called hyperchromatism in a very vivid
manner.
685.

In common glass, the alkaline nature obviously preponderates, since it is
chiefly composed of sand and alkaline salts; hence a series of experiments,
exhibiting the relation of perfectly alkaline fluids to perfect acids, might
lead to useful results.

686.

For, could the maximum and minimum be found, it would be a question
whether a refracting medium could not be discovered, in which the
increasing and diminishing appearance of colour, (an effect almost
independent of refraction,) could not be done away with altogether, while
the displacement of the object would be unaltered.

687.

How desirable, therefore, it would be with regard to this last point, as well
as for the elucidation of the whole of this third division of our work, and,
indeed, for the elucidation of the doctrine of colours generally, that those
who are occupied in chemical researches, with new views ever opening to
them, should take this subject in hand, pursuing into more delicate
combinations what we have only roughly hinted at, and prosecuting their
inquiries with reference to science as a whole.

PART IV.

GENERAL CHARACTERISTICS.

688.

We have hitherto, in a manner forcibly, kept phenomena asunder, which,
partly from their nature, partly in accordance with our mental habits, have,
as it were, constantly sought to be reunited. We have exhibited them in three
divisions. We have considered colours, first, as transient, the result of an
action and re-action in the eye itself; next, as passing effects of colourless,
light-transmitting, transparent, or opaque mediums on light; especially on
the luminous image; lastly, we arrived at the point where we could securely
pronounce them as permanent, and actually inherent in bodies.

689.

In following this order we have as far as possible endeavoured to define, to
separate, and to class the appearances. But now that we need no longer be
apprehensive of mixing or confounding them, we may proceed, first, to
state the general nature of these appearances considered abstractedly, as an
independent circle of facts, and, in the next place, to show how this
particular circle is connected with other classes of analogous phenomena in
nature.

THE FACILITY WITH WHICH COLOUR APPEARS.

690.

We have observed that colour under many conditions appears very easily.
The susceptibility of the eye with regard to light, the constant re-action of
the retina against it, produce instantaneously a slight iridescence. Every
subdued light may be considered as coloured, nay, we ought to call any
light coloured, inasmuch as it is seen. Colourless light, colourless surfaces,
are, in some sort, abstract ideas; in actual experience we can hardly be said
to be aware of them.—Note Z.

691.

If light impinges on a colourless body, is reflected from it or passes through
it, colour immediately appears; but it is necessary here to remember what
has been so often urged by us, namely, that the leading conditions of
refraction, reflection, &c., are not of themselves sufficient to produce the
appearance. Sometimes, it is true, light acts with these merely as light, but
oftener as a defined, circumscribed appearance, as a luminous image. The
semi-opacity of the medium is often a necessary condition; while half, and
double shadows, are required for many coloured appearances. In all cases,
however, colour appears instantaneously. We find, again, that by means of
pressure, breathing heat (432, 471), by various kinds of motion and
alteration on smooth clean surfaces (461), as well as on colourless fluids
(470), colour is immediately produced.

692.

The slightest change has only to take place in the component parts of
bodies, whether by immixture with other particles or other such effects, and
colour either makes its appearance or becomes changed.

THE FORCE OF COLOUR.

693.

The physical colours, and especially those of the prism, were formerly
called "colores emphatici," on account of their extraordinary beauty and
force. Strictly speaking, however, a high degree of effect may be ascribed to
all appearances of colour, assuming that they are exhibited under the purest
and most perfect conditions.

694.

The dark nature of colour, its full rich quality, is what produces the grave,
and at the same time fascinating impression we sometimes experience, and
as colour is to be considered a condition of light, so it cannot dispense with
light as the co-operating cause of its appearance, as its basis or ground; as a
power thus displaying and manifesting colour.

THE DEFINITE NATURE OF COLOUR.

695.

The existence and the relatively definite character of colour are one and the
same thing. Light displays itself and the face of nature, as it were, with a
general indifference, informing us as to surrounding objects perhaps devoid
of interest or importance; but colour is at all times specific, characteristic,
significant.

696.

Considered in a general point of view, colour is determined towards one of
two sides. It thus presents a contrast which we call a polarity, and which we
may fitly designate by the expressions plus and minus.
Plus. Minus.
Yellow. Blue.
Action. Negation.[1]
Light. Shadow.
Brightness. Darkness.
Force. Weakness.
Warmth. Coldness.
Proximity. Distance.
Repulsion Attraction.
Affinity with acids. Affinity with alkalis.

COMBINATION OF THE TWO PRINCIPLES.

697.

If these specific, contrasted principles are combined, the respective qualities
do not therefore destroy each other: for if in this intermixture the
ingredients are so perfectly balanced that neither is to be distinctly
recognised, the union again acquires a specific character; it appears as a

quality by itself in which we no longer think of combination. This union we
call green.

698.

Thus, if two opposite phenomena springing from the same source do not
destroy each other when combined, but in their union present a third
appreciable and pleasing appearance, this result at once indicates their
harmonious relation. The more perfect result yet remains to be adverted to.

AUGMENTATION TO RED.

699.

Blue and yellow do not admit of increased intensity without presently
exhibiting a new appearance in addition to their own. Each colour, in its
lightest state, is a dark; if condensed it must become darker, but this effect
no sooner takes place than the hue assumes an appearance which we
designate by the word reddish.
700.

This appearance still increases, so that when the highest degree of intensity
is attained it predominates over the original hue. A powerful impression of
light leaves the sensation of red on the retina. In the prismatic yellow-red
which springs directly from the yellow, we hardly recognise the yellow.
701.

This deepening takes place again by means of colourless semi-transparent
mediums, and here we see the effect in its utmost purity and extent.
Transparent fluids, coloured with any given hues, in a series of glass-
vessels, exhibit it very strikingly. The augmentation is unremittingly rapid
and constant; it is universal, and obtains in physiological as well as in
physical and chemical colours.

JUNCTION OF THE TWO AUGMENTED EXTREMES.

702.

As the extremes of the simple contrast produce a beautiful and agreeable
appearance by their union, so the deepened extremes on being united, will
present a still more fascinating colour; indeed, it might naturally be
expected that we should here find the acme of the whole phenomenon.

COMPLETENESS THE RESULT OF VARIETY.

703.

And such is the fact, for pure red appears; a colour to which, from its
excellence, we have appropriated the term "purpur."[2]

704.

There are various modes in which pure red may appear. By bringing
together the violet edge and yellow-red border in prismatic experiments, by
continued augmentation in chemical operations, and by the organic contrast
in physiological effects.
705.

As a pigment it cannot be produced by intermixture or union, but only by
arresting the hue in substances chemically acted on, at the high culminating
point. Hence the painter is justified in assuming that there are three
primitive colours from which he combines all the others. The natural
philosopher, on the other hand, assumes only two elementary colours, from
which he, in like manner, developes and combines the rest.

COMPLETENESS THE RESULT OF VARIETY IN COLOUR.

706.

The various appearances of colour arrested in their different degrees, and
seen in juxtaposition, produce a whole. This totality is harmony to the eye.

707.

The chromatic circle has been gradually presented to us; the various
relations of its progression are apparent to us. Two pure original principles
in contrast, are the foundation of the whole; an augmentation manifests
itself by means of which both approach a third state; hence there exists on
both sides a lowest and highest, a simplest and most qualified state. Again,
two combinations present themselves; first that of the simple primitive
contrasts, then that of the deepened contrasts.

HARMONY OF THE COMPLETE STATE.

708.

The whole ingredients of the chromatic scale, seen in juxtaposition, produce
an harmonious impression on the eye. The difference between the physical
contrast and harmonious opposition in all its extent should not be
overlooked. The first resides in the pure restricted original dualism,
considered in its antagonizing elements; the other results from the fully
developed effects of the complete state.
709.

Every single opposition in order to be harmonious must comprehend the
whole. The physiological experiments are sufficiently convincing on this
point. A development of all the possible contrasts of the chromatic scale
will be shortly given.[3]

FACILITY WITH WHICH COLOUR MAY BE MADE TO TEND EITHER TO THE PLUS
OR MINUS SIDE.

710.

We have already had occasion to take notice of the mutability of colour in
considering its so-called augmentation and progressive variations round the
whole circle; but the hues even pass and repass from one side to the other,
rapidly and of necessity.

711.

Physiological colours are different in appearance as they happen to fall on a
dark or on a light ground. In physical colours the combination of the
objective and subjective experiments is very remarkable. The epoptical
colours, it appears, are contrasted according as the light shines through or
upon them. To what extent the chemical colours may be changed by fire and
alkalis, has been sufficiently shown in its proper place.

EVANESCENCE OF COLOUR.

712.

All that has been adverted to as subsequent to the rapid excitation and
definition of colour, immixture, augmentation, combination, separation, not
forgetting the law of compensatory harmony, all takes place with the
greatest rapidity and facility; but with equal quickness colour again
altogether disappears.
713.

The physiological appearances are in no wise to be arrested; the physical
last only as long as the external condition lasts; even the chemical colours
have great mutability, they may be made to pass and repass from one side to
the other by means of opposite re-agents, and may even be annihilated
altogether.

PERMANENCE OF COLOUR.

714.

The chemical colours afford evidence of very great duration. Colours fixed
in glass by fusion, and by nature in gems, defy all time and re-action.

715.

The art of dyeing again fixes colour very powerfully. The hues of pigments
which might otherwise be easily rendered mutable by re-agents, may be
communicated to substances in the greatest permanency by means of
mordants.

[1] Wirkung, Beraubung; the last would be more literally rendered privation. The
author has already frequently made use of the terms active and passive as
equivalent to plus and minus.—T.
[2] Wherever this word occurs incidentally it is translated pure red, the English
word purple being generally employed to denote a colour similar to violet.—T.
[3] No diagram or table of this kind was ever given by the author.—T.

PART V.

RELATION TO OTHER PURSUITS—RELATION TO PHILOSOPHY.

716.

The investigator of nature cannot be required to be a philosopher, but it is
expected that he should so far have attained the habit of philosophizing, as
to distinguish himself essentially from the world, in order to associate
himself with it again in a higher sense. He should form to himself a method
in accordance with observation, but he should take heed not to reduce
observation to mere notion, to substitute words for this notion, and to use
and deal with these words as if they were things. He should be acquainted
with the labours of philosophers, in order to follow up the phenomena
which have been the subject of his observation, into the philosophic region.

717.

It cannot be required that the philosopher should be a naturalist, and yet his
co-operation in physical researches is as necessary as it is desirable. He
needs not an acquaintance with details for this, but only a clear view of
those conclusions where insulated facts meet.

718.

We have before (175) alluded to this important consideration, and repeat it
here where it is in its place. The worst that can happen to physical science
as well as to many other kinds of knowledge is, that men should treat a
secondary phenomenon as a primordial one, and (since it is impossible to
derive the original fact from the secondary state), seek to explain what is in
reality the cause by an effect made to usurp its place. Hence arises an
endless confusion, a mere verbiage, a constant endeavour to seek and to

find subterfuges whenever truth presents itself and threatens to be
overpowering.

719.

While the observer, the investigator of nature, is thus dissatisfied in finding
that the appearances he sees still contradict a received theory, the
philosopher can calmly continue to operate in his abstract department on a
false result, for no result is so false but that it can be made to appear valid,
as form without substance, by some means or other.

720.

If, on the other hand, the investigator of nature can attain to the knowledge
of that which we have called a primordial phenomenon, he is safe; and the
philosopher with him. The investigator of nature is safe, since he is
persuaded that he has here arrived at the limits of his science, that he finds
himself at the height of experimental research; a height whence he can look
back upon the details of observation in all its steps, and forwards into, if he
cannot enter, the regions of theory. The philosopher is safe, for he receives
from the experimentalist an ultimate fact, which, in his hands, now becomes
an elementary one. He now justly pays little attention to appearances which
are understood to be secondary, whether he already finds them scientifically
arranged, or whether they present themselves to his casual observation
scattered and confused. Should he even be inclined to go over this
experimental ground himself, and not be averse to examination in detail, he
does this conveniently, instead of lingering too long in the consideration of
secondary and intermediate circumstances, or hastily passing them over
without becoming accurately acquainted with them.

721.

To place the doctrine of colours nearer, in this sense, within the
philosopher's reach, was the author's wish; and although the execution of
his purpose, from various causes, does not correspond with his intention, he
will still keep this object in view in an intended recapitulation, as well as in
the polemical and historical portions of his work; for he will have to return
to the consideration of this point hereafter, on an occasion where it will be
necessary to speak with less reserve.

RELATION TO MATHEMATICS.

722.

It may be expected that the investigator of nature, who proposes to treat the
science of natural philosophy in its entire range, should be a mathematician.
In the middle ages, mathematics was the chief organ by means of which
men hoped to master the secrets of nature, and even now, geometry in
certain departments of physics, is justly considered of first importance.

723.

The author can boast of no attainments of this kind, and on this account
confines himself to departments of science which are independent of
geometry; departments which in modern times have been opened up far and
wide.

724.

It will be universally allowed that mathematics, one of the noblest
auxiliaries which can be employed by man, has, in one point of view, been
of the greatest use to the physical sciences; but that, by a false application
of its methods, it has, in many respects, been prejudicial to them, is also not
to be denied; we find it here and there reluctantly admitted.

725.

The theory of colours, in particular, has suffered much, and its progress has
been incalculably retarded by having been mixed up with optics generally, a
science which cannot dispense with mathematics; whereas the theory of
colours, in strictness, may be investigated quite independently of optics.

726.

But besides this there was an additional evil. A great mathematician was
possessed with an entirely false notion on the physical origin of colours;
yet, owing to his great authority as a geometer, the mistakes which he
committed as an experimentalist long became sanctioned in the eyes of a
world ever fettered in prejudices.

727.

The author of the present inquiry has endeavoured throughout to keep the
theory of colours distinct from the mathematics, although there are
evidently certain points where the assistance of geometry would be
desirable. Had not the unprejudiced mathematicians, with whom he has had,
or still has, the good fortune to be acquainted, been prevented by other
occupations from making common cause with him, his work would not
have wanted some merit in this respect. But this very want may be in the
end advantageous, since it may now become the object of the enlightened
mathematician to ascertain where the doctrine of colours is in need of his
aid, and how he can contribute the means at his command with a view to
the complete elucidation of this branch of physics.

728.

In general it were to be wished that the Germans, who render such good
service to science, while they adopt all that is good from other nations,
could by degrees accustom themselves to work in concert. We live, it must
be confessed, in an age, the habits of which are directly opposed to such a
wish. Every one seeks, not only to be original in his views, but to be
independent of the labours of others, or at least to persuade himself that he
is so, even in the course of his life and occupation. It is very often remarked
that men who undoubtedly have accomplished much, quote themselves
only, their own writings, journals, and compendiums; whereas it would be
far more advantageous for the individual, and for the world, if many were
devoted to a common pursuit. The conduct of our neighbours the French is,
in this respect, worthy of imitation; we have a pleasing instance in Cuvier's
preface to his "Tableau Élémentaire de l'Histoire Naturelle des Animaux."

729.

He who has observed science and its progress with an unprejudiced eye,
might even ask whether it is desirable that so many occupations and aims,
though allied to each other, should be united in one person, and whether it
would not be more suitable for the limited powers of the human mind to
distinguish, for example, the investigator and inventor, from him who
employs and applies the result of experiment? Astronomers, who devote
themselves to the observation of the heavens and the discovery or
enumeration of stars, have in modern times formed, to a certain extent, a

distinct class from those who calculate the orbits, consider the universe in
its connexion, and more accurately define its laws. The history of the
doctrine of colours will often lead us back to these considerations.

RELATION TO THE TECHNICAL OPERATIONS OF THE DYER.

730.

If in our labours we have gone out of the province of the mathematician, we
have, on the other hand, endeavoured to meet the practical views of the
dyer; and although the chapter which treats of colour in a chemical point of
view is not the most complete and circumstantial, yet in that portion, as well
as in our general observations respecting colour, the dyer will find his views
assisted far more than by the theory hitherto in vogue, which failed to afford
him any assistance.

731.

It is curious, in this view, to take a glance at the works containing directions
on the art of dyeing. As the Catholic, on entering his temple, sprinkles
himself with holy water, and after bending the knee, proceeds perhaps to
converse with his friends on his affairs, without any especial devotion; so
all the treatises on dyeing begin with a respectful allusion to the accredited
theory, without afterwards exhibiting a single trace of any principle deduced
from this theory, or showing that it has thrown light on any part of the art,
or that it offers any useful hints in furtherance of practical methods.

732.

On the other hand, there are men who, after having become thoroughly and
experimentally acquainted with the nature of dyes, have not been able to
reconcile their observations with the received theory; who have, in short,
discovered its weak points, and sought for a general view more consonant
to nature and experience. When we come to the names of Castel and
Gülich, in our historical review, we shall have occasion to enter into this
more fully, and an opportunity will then present itself to show that an
assiduous experience in taking advantage of every accident may, in fact, be

said almost to exhaust the knowledge of the province to which it is
confined. The high and complete result is then submitted to the theorist,
who, if he examines facts with accuracy, and reasons with candour, will find
such materials eminently useful as a basis for his conclusions.—Note AA.

RELATION TO PHYSIOLOGY AND PATHOLOGY.

733.

If the phenomena adduced in the chapter where colours were considered in
a physiological and pathological view are for the most part generally
known, still some new views, mixed up with them, will not be unacceptable
to the physiologist. We especially hope to have given him cause to be
satisfied by classing certain phenomena which stood alone, under analogous
facts, and thus, in some measure, to have prepared the way for his further
investigations.

734.

The appendix on pathological colours, again, is admitted to be scanty and
unconnected. We reflect, however, that Germany can boast of men who are
not only highly experienced in this department, but are likewise so
distinguished for general cultivation, that it can cost them but little to revise
this portion, to complete what has been sketched, and at the same time to
connect it with the higher facts of organisation.

RELATION TO NATURAL HISTORY.

735.

If we may at all hope that natural history will gradually be modified by the
principle of deducing the ordinary appearances of nature from higher
phenomena, the author believes he may have given some hints and
introductory views bearing on this object also. As colour, in its infinite
variety, exhibits itself on the surface of living beings, it becomes an

important part of the outward indications, by means of which we can
discover what passes underneath.

736.

In one point of view it is certainly not to be too much relied on, on account
of its indefinite and mutable nature; yet even this mutability, inasmuch as it
exhibits itself as a constant quality, again becomes a criterion of a mutable
vitality; and the author wishes nothing more than that time may be granted
him to develop the results of his observations on this subject more fully;
here they would not be in their place.

RELATION TO GENERAL PHYSICS.

737.

The state in which general physics now is, appears, again, particularly
favourable to our labours; for natural philosophy, owing to indefatigable
and variously directed research, has gradually attained such eminence, that
it appears not impossible to refer a boundless empiricism to one centre.

738.

Without referring to subjects which are too far removed from our own
province, we observe that the formulæ under which the elementary
appearances of nature are expressed, altogether tend in this direction; and it
is easy to see that through this correspondence of expression, a
correspondence in meaning will necessarily be soon arrived at.
739.

True observers of nature, however they may differ in opinion in other
respects, will agree that all which presents itself as appearance, all that we
meet with as phenomenon, must either indicate an original division which is
capable of union, or an original unity which admits of division, and that the
phenomenon will present itself accordingly. To divide the united, to unite
the divided, is the life of nature; this is the eternal systole and diastole, the

eternal collapsion and expansion, the inspiration and expiration of the world
in which we live and move.

740.

It is hardly necessary to observe that what we here express as number and
restrict to dualism is to be understood in a higher sense; the appearance of a
third, a fourth order of facts progressively developing themselves is to be
similarly understood; but actual observation should, above all, be the basis
of all these expressions.
741.

Iron is known to us as a peculiar substance, different from other substances:
in its ordinary state we look upon it as a mere material remarkable only on
account of its fitness for various uses and applications. How little, however,
is necessary to do away with the comparative insignificancy of this
substance. A two-fold power is called forth,[1] which, while it tends again to
a state of union, and, as it were, seeks itself, acquires a kind of magical
relation with its like, and propagates this double property, which is in fact
but a principle of reunion, throughout all bodies of the same kind. We here
first observe the mere substance, iron; we see the division that takes place
in it propagate itself and disappear, and again easily become re-excited.
This, according to our mode of thinking, is a primordial phenomenon in
immediate relation with its idea, and which acknowledges nothing earthly
beyond it.

742.

Electricity is again peculiarly characterised. As a mere quality we are
unacquainted with it; for us it is a nothing, a zero, a mere point, which,
however, dwells in all apparent existences, and at the same time is the point
of origin whence, on the slightest stimulus, a double appearance presents
itself, an appearance which only manifests itself to vanish. The conditions
under which this manifestation is excited are infinitely varied, according to
the nature of particular bodies. From the rudest mechanical friction of very
different substances with one another, to the mere contiguity of two entirely
similar bodies, the phenomenon is present and stirring, nay, striking and
powerful, and so decided and specific, that when we employ the terms or

formulæ polarity, plus and minus, for north and south, for glass and resin,
we do so justifiably and in conformity with nature.

743.

This phenomenon, although it especially affects the surface, is yet by no
means superficial. It influences the tendency or determination of material
qualities, and connects itself in immediate co-operation with the important
double phenomenon which takes place so universally in chemistry,—
oxydation, and de-oxydation.
744.

To introduce and include the appearances of colour in this series, this circle
of phenomena was the object of our labours. What we have not succeeded
in others will accomplish. We found a primordial vast contrast between
light and darkness, which may be more generally expressed by light and its
absence. We looked for the intermediate state, and sought by means of it to
compose the visible world of light, shade, and colour. In the prosecution of
this we employed various terms applicable to the development of the
phenomena, terms which we adopted from the theories of magnetism, of
electricity, and of chemistry. It was necessary, however, to extend this
terminology, since we found ourselves in an abstract region, and had to
express more complicated relations.

745.

If electricity and galvanism, in their general character, are distinguished as
superior to the more limited exhibition of magnetic phenomena, it may be
said that colour, although coming under similar laws, is still superior; for
since it addresses itself to the noble sense of vision, its perfections are more
generally displayed. Compare the varied effects which result from the
augmentation of yellow and blue to red, from the combination of these two
higher extremes to pure red, and the union of the two inferior extremes to
green. What a far more varied scheme is apparent here than that in which
magnetism and electricity are comprehended. These last phenomena may be
said to be inferior again on another account; for though they penetrate and
give life to the universe, they cannot address themselves to man in a higher
sense in order to his employing them æsthetically. The general, simple,

physical law must first be elevated and diversified itself in order to be
available for elevated uses.

746.

If the reader, in this spirit, recalls what has been stated by us throughout,
generally and in detail, with regard to colour, he will himself pursue and
unfold what has been here only lightly hinted at. He will augur well for
science, technical processes, and art, if it should prove possible to rescue
the attractive subject of the doctrine of colours from the atomic restriction
and isolation in which it has been banished, in order to restore it to the
general dynamic flow of life and action which the present age loves to
recognise in nature. These considerations will press upon us more strongly
when, in the historical portion, we shall have to speak of many an
enterprising and intelligent man who failed to possess his contemporaries
with his convictions.

RELATION TO THE THEORY OF MUSIC.

747.

Before we proceed to the moral associations of colour, and the æsthetic
influences arising from them, we have here to say a few words on its
relation to melody. That a certain relation exists between the two, has been
always felt; this is proved by the frequent comparisons we meet with,
sometimes as passing allusions, sometimes as circumstantial parallels. The
error which writers have fallen into in trying to establish this analogy we
would thus define:

748.

Colour and sound do not admit of being directly compared together in any
way, but both are referable to a higher formula, both are derivable, although
each for itself, from this higher law. They are like two rivers which have
their source in one and the same mountain, but subsequently pursue their
way under totally different conditions in two totally different regions, so
that throughout the whole course of both no two points can be compared.