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arV1831

Modern chromatics.

Cornell University Library

3 1924 031 167 889

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MIXTURE CWIRI

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MODERN

CHEOMATIOS

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AET AND II^DUSTRT.

OGDEN K^ EOOD,

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THE AUTHOR.

PEEFAOE.

It was not my intention to write a preface to this book, as I have usually found such compositions neither instructive nor amusing. On presenting the manuscript to my publishers, however, it was suggested that, al- though prefaces are of no particxdar use to readers, yet from a certain point of view they are not without value.

I accordingly beg leave to state that my object in this M'ork has been to present, in a clear, logical, and if possi- ble attractive form, the fundamental facts connected with our perception of colour, so far as they are at present known, or concern the general or artistic reader. For the explanation of these facts, the theory of Thomas Young, as modified and set forth by Helmholtz and Maxwell, has been consistently adhered to. The whole class of musical theories, as well as that of Field, have been discarded, for reasons that are set forth in the text.

Turning now from the more purely scientific to the sesthetic side of the subject, I will add that it has been my endeavour also, to present in a simple and comprehen- sible manner the underlying facts upon which the artistic use of colour necessarily depends. The possession of these

vi PREFACE.

facts will not enable people to become artists ; but it may to some extent prevent ordinary persons, critics, and even painters, from talking and writing about colour in a loose, inaccurate, and not always rational manner. More than this is true : a real knowledge of elementary facts often serves to warn students of the presence of difficulties that are almost insurmountable, or, when they are already in trouble, points out to them its probable nature ; in short, a certain amount of rudimentary information tends to save useless labour. Those persons, therefore, who are really iuterested in this subject are iirged to repeat for themselves the various experiments indicated in the text.

In the execution of this work it was soon found that many important gaps remained to be filled, and much tune has been consumed in original researches and ex- periments. The results have been briefly indicated in the text ; the exact means employed in obtaining them will be given hereafter in one of the scientific journals.

To the above I may perhaps be allowed to add, that during the last twenty years I have enjoyed the great privilege of familiar intercourse with artists, and duriag that period have devoted a good deal of leisure time to the practical study of drawing and painting.

O. K R.

OONTElfJ-TS.

CHAPTER I.

PACB

Transmission and Reflection op Light, 9

CHAPTER II. Production of Colodk by Dispersion, . . , . 17

CHAPTER III. Constants of Colour, . , .... 30

CHAPTER IV. Production of Colour by Interference and Polabization, . 43

CHAPTER V. Colours of Opalescent Media, .53

CHAPTER VI. Production of Colour by Fluorescence and Phosphorescence, . 62

CHAPTER Vn. Production of Colour by Absorption, . . 65

CHAPTER VIII. Abnormal Perception of Colour and Colour-blindness, 92

CHAPTER IX. Toung's Theory op Coloitr, 108

viii CONTENTS.

CHAPTER X.

PACE

Mixture of Colours, . . . . 124

CHAPTER XI. Complementary Colours, . . . ■ . 161

CHAPTER XII.

Effects produced on Colour by a Change in Luminosity and by MIXING it with White Light, 181

CHAPTER XIII. Duration of the Impression on the Retina, .... 202

CHAPTER XIV. Modes of arranging Colours in Systems, .... 209

CHAPTER XV. Contrast, ... . . . . 235

CHAPTER XVI.

The Small Interval and Gradation, . . . 2'73

CHAPTER XVIL

Combinations of Colours in Pairs and Triads, . . .286

CHAPTER XVin. Painting and Decoration 305

Note on two recent Theories op Colour, 324

Index, ... 326

MODERI CHROMATICS.

CHAPTER I.

THE REFLECTION AND TRANSMISSION OF LIGHT.

As long ago as 1795 it occurred to a German physicist to subject tlie optic nerve of the living eye to the influence of the newly discovered voltaic current. The result obtained was curious : the operation did not cause pain, as might have been expected, but a bright flash of light seemed to pass before the eye. This remarkable experiment has since that time been repeated in a great variety of ways, and with the help of the more efiicient electric batteries of mod- ern times; and not only has the original result of Pfaff been obtained, but bright red, green, or violet, and other hues have been noticed by a number of distinguished physicists. If, instead of using the electrical current, mechanical force be employed, that is, if pressure be exerted on the living eye, the optic nerve is again stimulated, and a series of bril- liant, changing, fantastic figures seem to pass before the experimenter. All these appearances are distinctly visible in a perfectly dark room, and prove that the sense of vision can be excited without the presence of light, the essential point being merely the stimulation of the optic nerve. In the great majority of instances, however, the stimulation of the optic nerve is brought about, directly or indii'ectly, by the

10 MODERN CHROMATICS.

aid of light ; and in the present work it is principally with vision produced in this normal manner that we have to deal. Back in the rear portion of the eye there is spread out a delicate, highly complicated tissue, consisting of a wonder- fully fine network woven of minute blood-vessels and nerves, and interspersed with vast numbers of tiny atoms, which under the microscope look like little rods and cones. This is the retina ; its marvellous tissue is in some mysteri- ous manner capable of being acted on by light, and it is from its substance that those nerve-signals are transmitted to the brain which awake in us the sensation of vision. For the sake of brevity, the interior globular surface of the retina is ordinarily called the seat of vision. An eye pro- vided only with a retina would still have the capacity for a certain kind of vision ; if plunged in a beam of red or green light, for example, these colour-sensations would be excited, and some idea might be formed of the intensity or purity of the original hues. Some of the lower animals seem to be endowed only with this rudimentary form of vision ; thus it has lately been ascertained by Bert that minute crustaceans are sensitive to the same colours of the spectrum which affect the eye of man, and, as is the case with him, the maximum effect is produced by the yellow rays. With an eye constructed in this simple manner it would, however, be impossible to distinguish the forms of ex- ternal objects, and usually not even their colours. We have, therefore, a set of lenses placed in front of the retina, and so contrived as to cast upon it very delicate and perfect pictures of objects toward which the eye is directed ; these pictures are coloured and shaded, so as exactly to match the objects from which they came, and it is by their action on the retina that we see. These retinal pictures are, as it were, mosaics, made up of an infinite number of points of light ; they vanish with the objects producing them — though, as we shall see, their effect lasts a little while after they themselves have disappeared.

THE REFLECTION AND TRANSMISSION OF LIGHT. H

This leads us in the next place to ask, " What is light, that agent which is able to produce effects which to a thoughtful mind must always remain wonderful ? " A per- fectly true answer to this question is, that light is some- thing which comes from the luminous body to us ; in the act of vision we are essentially passive, and not engaged in shooting out toward the object long, delicate feelers, as was supposed by the ancients. This something was considered by Sir Isaac Newton to consist of fine atoms, too fine al- most to think of, but moving at the rate of 186,000 miles in a second. According to the undulatory theory, however, light consists not of matter shot toward us, but of undula- tions or waves, which reach our eyes somewhat in the same way as the waves of water beat on a rocky coast.

The atoms, then, which compose a candle flame are themselves in vibration, and, communicating this vibratory movement to other particles with which they are in con- tact, generate waves, which travel out in all directions, like the circular waves from a stone dropped into quiet water ; these waves break finally upon the surface of the retina, and cause in some unexplained way the sensation of sight — we see the candle flame. Substances which are not seK- luminous cannot be seen directly or without help ; to ob- tain vision of them it is necessary that a self-luminous body also should be present. The candle flame pours out its flood of tiny waves on the objects in the room ; in the act of striking on them some of the waves are destroyed, but others rebound and reach the eye, having suffered certain changes of which we shall speak hereafter.

This rebound of the wave we call reflection ; all bodies in the room reflect some of the candle light. Surfaces which are polished alter the direction of the waves of light falling on them, but they do not to any great extent scatter them irregularly, or in all directions. It hence follows that pol- ished surfaces, when they reflect light, present appearances

12 MODERN CHROMATICS.

totally unlike those furnislied by surfaces which, though smooth, are yet destitute of polish ; the former are apt to reflect very much or very little light, according to their positions, but this is not true to the same extent with un- polished surfaces. The power which difllerent substances have under various circumstances to reflect light is not without interest for us ; we shall see hereafter that this is a means often employed by nature in modifying colour.

As a general thing polished metallic surfaces are the best reflectors of light, and may for the most part be con- sidered by the artist as reflecting all the light falling on them. Polished silver actually does reflect ninety-two per cent, of the light falling perpendicularly on it ; and though the percentages reflected by steel and other metals are smaller, yet the difference is not ordinarily and easUy dis- tinguished by an untrained eye.

The case is somewhat different with smooth water : if light falls on it, making a small angle with its surface, the amount reflected is as large as that from a metallic surface ; while, if the light falls perpendicularly on it, less than four per cent, is reflected. Thus with a clear blue sky and smooth water we find that distant portions of its surface appear very bright, while those at the feet of the observer are of an almost unbelievable dark-blue tint. In this par- ticular instance, the difference between the brightness of near and distant portions of the water is still further exag- gerated by the circumstance that the sky overhead is less luminous than that near the horizon ; and the distant por- tions of the sheet of water reflect light which comes from the horizon, the nearer portions that which has its origin overhead. The reflecting power of water is constantly used by artists as a most admirable means of duplicating in a picture a chromatic composition, and easily affords an op- portunity, by slight disturbances of its surface, ior the introduction of variations on the original chromatic design.

It may here be remarked that in actual landscapes con-

THE REFLECTION AND TRANSMISSION OF LIGHT. 13

taining surfaces of still water, it ordinarily happens that the reflected pictures are not exactly identical with those which are seen directly, and the difference may often be considerable. For example, it may easily be the case that an object beyond the water, and situated at some distance from it, is not seen in the reflected picture at all, light from it either not reaching the water, or reaching the water and not being reflected to the eye of the observer.

Polished surfaces, as we have seen, reflect light not only in large quantity, but they as it were press the light well together in rather sharply defined masses ; with unpolished surfaces the case is entirely different, the light which falls on them being scattered in all directions. Hence, where- ever the eye is placed, it receives some of this light, and a change of position produces far less effect on the quantity received than is the case with light reflected from polished surfaces. Owing to their power of scattering light in all directions, rough surfaces, however situated, never send very intense light to the eye.

If a surface of white linen drapery be illuminated by a dozen different sources, it will reflect to the eye a sample of each kind of light, and what we call its hue will be made up of as many constituents. When we remember that all the different objects in a room reflect some, and usually coloured light, we see that the final tint of our piece of linen drapery depends not only on the circumstance that its natu- ral colour is white, but also on the presence and proximity of curtains, books, chaii's, and a great variety of objects ; the final colour will hence not be exactly white, but some delicate, indescribable hue, difficult of imitation except by practiced artists. With objects which are naturally coloured, or which show colour when placed in white light, the case becomes still more complicated. Let us suppose that our drapery when placed in pure white light appears red ; its hue will

14 MODERN CHROMATICS.

still be modified by the light it receives from objects ia the room : for example, if it receives some green light from objects of this colour placed in its neighbourhood, the red hue will incline toward orange; if the added portion of light be yellow, the tendency to orange will be still more marked ; on the other hand, light received from blue or violet surfaces will cause the red to pass into crimson or even purple. The grandest illustrations of these changes we find in those cases where objects are illuminated simul- taneously by the yellow rays of the sun and the blue light of the clear sky : here, by this cause alone, the natural colours of objects are modified to a wonderful extent, and effects of magical beauty produced, which by their intricacy almost defy analysis. The nature of these changes will be considered in a subsequent chapter, after the principles upon which they depend have been examined.

Finally, it may not be altogether out of place to add that the majority of paintings and chromatic designs are seen by the aid of light which they reflect in a diflfused way to the eye of the observer ; transparencies, designs in stained or painted glass, etc., are, on the other hand, seen by light which passes entirely through their substance before reach- ing the eye. Corresponding to this we find that by far the larger proportion of natural objects act upon om- visual organs by means of reflected light, while a few only are seen by a mixture of reflected and transmitted light. It hence follows that Nature and the painter actually employ, in the end, exactly the same means in acting on the eye of the beholder. This point, seemingly so trite, is touched upon, as an idea seems to prevail in the minds of many per- sons that Nature paints always with light, while the artist is limited to pigments : in point of fact, both paint with light, though, as we shall hereafter see, the total amount at the disposal of the painter is quite limited.

THE REFLECTION AND TRANSMISSION OF LIGHT. 15

In concluding this matter of reflection, we may perhaps be allowed to add that the term reflection is quite frequent- ly confused with shadow — the reflected image of trees on the edge of quiet water being often spoken-of as the shadows of trees on the water. The two cases are of course essen- tially different, a genuine, well-defined shadow on water scarcely occurring except in cases of turbidity.

We have seen that all bodies reflect some of the light falling on them ; it is equally true that they transmit a certain portion. A plate of very pure glass, or a thin layer of pure water, will transmit all the light falling on it, ex- cept that which is reflected ; they transmit it unaltered in tint, and we say they are perfectly transparent and colour- less substances. Here we have one of the extremes ; the other may be found in some of the metals, such as gold or silver : it is only when they are reduced to very thin leaves that they transmit any light at all. Gold leaf allows a lit- tle light to pass through its substance, and tinges it bluish- green. Almost all other bodies may be ranged between these two examples ; none can be considered absolutely transparent, none perfectly opaque. And this is true not only in a strictly philosophical sense, but also in one that has an especial bearing on our subject. The great mass of objects with which we come in daily contact allow light to penetrate a little way into their substance, and then, turn- ing it back, reflect it outward in all directions. In this sense all bodies have a certain amount of transparency. The light which thus, as it were, just dips into their sub- stance, has by this operation a change impressed on it ; it usually comes out more or less coloured. It hence follows that, in most cases, two masses of light reach the eye : one, which has been superficially reflected with unchanged colour; and another, which, being reflected only after penetration, is modified in tint. Many beautiful effects of translucency are due to these and strictly analogous causes ; the play

16 MODERN CHROMATICS.

of colour on the surfaces of waves is made up largely of these two elements ; and in a more subdued way we find them also producing the less marked translucency of foliage or of flesh.

One of the resources just mentioned the painter never employs : the light which is more or less regularly reflected from the outermost surface, he endeavours to prevent from reaching the eye of the beholder, except in minute quanti- ty, his reliance being always on the light which is reflected in an irregular and diffused way, and which has for the most part penetrated, first, some little distance into his pigments.

The glass-stainer and glass-painter make use of the principle of the direct transmission of light for the display of their designs. Now, as painted or stained glass trans- mits enormously more light than pigments reflect in a prop- erly lighted room, it follows that the worker on glass has at his disposal a much more extensive scale of light and shade than the painter in oils or water-colours. Owing to this fact it is possible to produce on glass, paintings which, in range of illumination, almost rival Nature. The intensity and purity of the tints which can thus be produced by direct transmission are far in advance of what can be ob- tained by the method of reflection, and enable the designer on glass successfully to employ combinations of colour which, robbed of their brightness and intensity by being executed in oils or fresco, would no longer be tolerable.

CHAPTER II.

PRODUCTION OF COLOUR BY DISPERSION.

In the previous chapter we have seen that the sensation of sight is produced by the action of very minute waves on th& nervous substance of the retina ; that is to say, by the aid of purely mechanical movements of a definite character. When these waves have a length of about -5-5^57 •'^ ^^ inch, they produce the sensation which we call red — we see red light ; if they are shortened to xr^TTir of an inch, their ac- tion on us changes, they call up in us a different sensation — we say the light is coloured orange ; and as the lengths of the waves are continually shortened, the sensation passes into yellow, green, blue, and viplet. From this it is evident that colour is something which has no existence outside and apart from ourselves ; outside of ourselves there are merely mechanical movements, and we can easily imagine beings so constructed that the waves of light would never produce in them the sensation of colour at all, but that of heat.

The colour-sensations just mentioned are not the only ones capable of being produced by the gradual diminution of the wave-length : between the red and orange we find every variety of orange-red and red-orange hue ; the or- ange, again, changes by a vast number of insensible steps into yellow, and so of all the other tints. Types of all colours possible, except the purples, could be produced by this method. The colours generated in this way would not only pass by the gentlest gradations into each other, form- ing a long scries of blending hues, but they would also be

18

MODERN CHROMATICS.

perfectly pure, and, if the light was bright, very intense. The advantage of providing, in the beginning of our colour studies, a set of tints possessiug these precious qualities, is evident without much argument.

Now, white light consists of a mixture of waves pos- sessing every desirable degree of length, and it is only ne- cessary to select some instrument which is able to sort out for us the different kinds of light, and neatly arrange them side by side in an orderly series. Fortunately for us, we find in the glass prism a simple and inexpensive apparatus which is able to effect the desired analysis. "We may, Lf we are willing to take a little trouble, arrange matters so as to

Fi&. 1. — ^Prismatic Spectrum.

repeat the famous experiment made by Newton many years ago : viz., admit a small beam of sunlight into a darkened ' room, and allow it to fall on the prism, as indicated in Fig. 1. We shall notice, by observing the illuminated path of the sunbeam, that the prism bends it considerably out of its course ; ajid, on tracing up this deflected portion, we shall find it no longer white, but changed into a long streak of pure and beautiful colours, which blend into each other by gentle gradations. If this streak of coloured light be received on a white wall, or, better, on a large sheet of white cardboard, the following changes in the colours can

PRODUCTION OF COLOtJE BY DISPERSION. 19

be noticed : It commences at one end with a dark-crimson hue, which gradually brightens as we advance along its length, changing at the same time into scarlet ; this runs into orange, the orange becomes more yellowish, and con- trives to convert itself into a yellowish-green without pass- ing noticeably into yellow, so that at first sight yellow does not seem to be present. The orange-yellow and greenish- yellow spaces are brighter than any of the others, but the rise in luminosity is so gradual that the difference is not striking, unless we compare these two colours with those at a considerable distance from them. As we pass on, the ten- dency to green becomes more decided, until finally a full green hue is reached. This colour is still pretty bright,

I^a. 2.— Mode of isolating a Single Colour of the Prismatic Spectrum.

and not inferior to the red in intensity ; by degrees it changes into a greenish-blue, which will not at first attract the attention ; next follows a full blue, not nearly so bright as the green, nor so striking ; this blue changes slowly into a violet of but little brightness, which completes the series. If we wish to isolate and examine these tints separately, we can again follow the example of Newton, by making a small, narrow aperture in our cardboard, and use it then as a screen to intercept all except the desired tint, as is indi- cated in Fig. 2. In this manner we can examine separate

20 MODERN CHROMATICS.

portions of our spectrum more independently, and escape from the overpowering influence of some of the more in- tense tints. Under these circumstances the greenish-hlue becomes quite marked, and the blue is able to assert itself to a greater degree ; but the yellow will not be greatly helped, for in fact it is confined to a very narrow region, and it is only by greatly magnifying the spectrum that we can obtain a satisfactory demonstration of its existence.

These experiments, though very beautiful, are quite rough ; every two minutes the beam of sunlight strays away from the prism and needs again to be directed toward it ; and besides that, the colours blend into each other in such a subtile, puzzling way, that, without a scale or land- mark of some kind to separate them, it seems hopeless to undertake any exact experiments. In this diflSculty it is to the spectroscope that we must turn for aid ; it was certainly not originally contrived for such purposes as these, but nevertheless is just what we need. It is not necessary to. stop to describe the instrument, as this has been done by Professor Lommel in another volume of this series ; it is enough for us that it is a convenient instrument for sorting out the different kinds of light which fall on it, according to their wave-length, and that it performs this work far more accurately than a prism used according to Newton's plan. Just at this point we can take advantage of a sin- gular discovery made by Fraunhofer, and independently to some extent by Dr. Wollaston, early in the present cen- tury. These physicists found that when the coloured band of light just described is produced by a spectroscope, or by apparatus equivalent to jone, the band is really not con- tinuous, but is cut up crosswise into a great many small spaces. The dividing lines are called the fixed lines of the solar spectrum. Almost their sole interest for us is in the fact that they serve as admirable landmarks to guide us through the vague tracts of ill-defined colour. Fig. 3 shows the positions of some of the more important fixed lines of

PRODUCTION OF COLOmi BY DISPERSION.

21

* It will be noticed that the term indigo, originally Introduced by Newton, haa been entirely rejected in thia work, and ultrama- rine aubatituted for it. Bezold auggested thia change aome time ago, baaing hia ob- jection to indigo on ita dingineaa ; the au- thor, however, finds a much more fatal ob- jection in the fact that indigo in solution, and as a pigment, ia a somewhat greenish- blue, being really identical with Prussian- blue in colour, only far blacker. In the dry atate thia tendency to greenneaa ia neu- tralized by the reddish tinge which the aub- Btanoe sometimes assumes : it was probably used by Newton in the dry state. A mix- ture of aix parta of artificial ultramarine- bluo, two parta white, and ninety-two parts black, when mingled according to the meth- od of Maxwell's disks, furnishes a colour quite like that of commercial indigo in the dry atate.

r

the spectrum. The figure is based on measurements made by the author on a flint-glass prism, with aid of a large spectroscope, or rather spectrometer, admirably constructed by Wm. Grunow, of New York. At the same time a series of observations was made on the extent of the coloured spaces in the spectrum ; these are indicated in the figure, and ac- curately given in one of the ta- bles that follow.* Let us sup- pose that the spectrum from A to H includes 1,000 parts ; then the following table indicates the po- sitions of the fixed lines :

* Eed-oran^e.

Orauge.

Orange-yellow.*

Yellow.

Green-yellow > and Yellow-green.

Green and Blue-green.

» Cyan-blue.

V Blue and / Blue-violet.

> Violet

Fio. 8.— Fixed Lines and Coloured Spaces of FriBuiatic Spectrum.

22

MODERN CHEOMATICS.

Fixed Likes of the Pkismatic Spectrum.

0

4005

74-02

C... 112-'71

D 220-31

A. a. B.

E 363-11

6 389-85

F 493-22

G VSS-BS

n 1000-00

The next table gives the positions of the coloured spaces in this spectrum, according to the observations of the author :

Oolouked Spaces in the Pkismatic SPEOTEtiM.

Red begins at 0

Red ends, orange-red begins at 149

Orange-red ends, orange begins at 194

Orange ends, orange-yellow begins at 210

Orange-yellow ends, yellow begins at 230

Yellow ends, greenish-yellow begins at 240

Tellow-greeB ends, green begins at 344

Blue-green ends, cyan-blue begins at 447

Cyan-blue ends, blue begins at 495

Violet-blue ends, violet begins at 806

Violet ends at ■ 1,000

The space out beyond 0 is occupied by a very dark red, which has a brown or chocolate colour, and outside of the violet beyond 1,000 is. a faint greyish colour, -which has been called lavender.

The third table shows the spaces occupied in the pris- matic spectrum by the several colours :

Red , 149

Orange-red 45

Orange X6

Orange-yellow , _ _ 20

Yellow IQ

Greenish-yellow and yellowish-green 104

Green and blue-green IO3

Cyan-blue 48

Blue and blue-viol<et 311

"Violet 194

1,000

PRODUCTION OF COLOUR BY DISPERSION. 23

In making these observations, matters were arranged so that only a narrow slice of the spectrum presented itself to the observer ; thus its hues could be studied in an isolated condition, and the misleading effects of contrast avoided. The figures given in the two latter tables are the mean of from fifteen to twenty observations. The hues of the spec- tral colours change very considerably with their luminosity ; hence for these experiments an illumination was selected such that it was only comfortably bright in the most lumi- nous portions of the spectrum, and this arrangement re- tained as well as possible afterward.

The colours as seen in the spectroscope really succeed each other in the order of their wave-lengths, the red hav- ing the greatest wave-length, the violet the least. But the glass prism does this work in a way which is open to criti- cism ; it crowds together some portions of the series of tints more than is demanded by their difference in wave-lengths ; other portions it expands, assigning to them more room than they have a right to claim. Thjis the red, orange, and yellow spaces are cramped together, while the blue and violet tracts stretch out interminably. Taking all this into consideration, it may be worth while to go one step further, and, without abandoning the use of the spectroscope, re- place its prism by a diffraction grating, or plate of glass ruled with very fine, parallel, equidistant lines, such as have been made by the celebrated Nobert, and lately of still superior perfection by Rutherfurd. In Lommel's work, previously referred to, the mode in which a plate of this kind produces colour is explained ; at present it is enough to know that the general appearance of the spectacle will be unchanged ; the same series of colours, the same fixed lines, will again be recognized ; but in this new spectrum all the tints will be arranged in an equable manner with reference to wave-length. According to this new allotment of spaces, the yellow will occupy about the centre of the spectrum,

24 MODERN CHROMATICS.

tlie red and different kinds of orange taking up more room than formerly; the dimensions of the blue and violet will he greatly reduced.

Let us suppose, as before, that the spectrum from A to H includes 1,000 parts ; then the following table, which is calculated from the observations of Angstrom, will indi- cate the positions of the principal fixed lines :

Fixed Lines in the Nokmal Spectrum.

A 0

a llS-n

B 201-61

C 285-05

D 468-38

E 688-92

b eei-M

F '?49-24

G 90207

H ....1000-00

The next table gives the positions of the coloured spaces in the normal spectrum, according to the observations of the author :

Coloured Spaces in the Normal Spectrum.

Red begins at 0

Pure red ends, orange-red begins at 330

Orange-red ends, orange begins at 434

Orange fends, orange-yellow begins at 459

Orange-yeUow ends, yellow begins at 485

Tellow ends, greenish-yellow begins at 498

Yellow-green ends, full green begins at ; . . . 596

Full green ends, blue-green begins at 682

Blue-green ends, cyan-blue begins at 698

Cyan-blue ends, blue begins at 749

Blue ends, violet-blue begins at 823

Blue-violet ends, pure violet begins at 940

The following table exhibits the spaces occupied by the several colours in the normal spectrum :

Pure red 830

Orange-red 104

Orange 25

Orange-yellow 26

PRODUCTION OF COLOUR BY DISPERSION.

25

Yellow IS

Greenish-yellow and yellow-green . 9"?

Full green 87

Blue-green 16

Cyan-blue 51

Blue 74

Violet-blue and blue-violet 117

Pure violet 60

1,000

Fig. 4 shows the normal spectrum with fixed lines and coloured spaces, corresponding to the tables just given.

If these tables are compared with those obtained by the aid of a prism of glass, it will be seen that the fixed lines and coloured spaces are arranged somewhat differently; the main cause of this difference has already been pointed out. When, however, we compare the spacing of the colours in the two spectra, it is also to be remembered that it is affected by another circumstance, viz., the distribution of the lumi- nosity in the two spectra does not agree, and this influences, as will be shown in Chapter XII., the appearance of the colours themselves ; very lu- minous red, for example, as- suming an orange hue, very dark blue tending to appear violet, etc. The normal spectrum employed by the autho

. Eed.

. Orange-red.

■ Orange. . Orange-yellow.

■ Yellow.

Greenish-yellow.

■ and Yellowish-green.

■ Green.

• Blue-green. . Cyan-blue.

. Blue.

Violet-blue.

Violet.

Fig. 4. — Fixed Lines and Coloured Spaces of Normal Spectrum.

26 MODERN CHROMATICS.

was obtained by using a superb plate for which he was indebted to Mr. Rutherfurd. The plate contained nearly 19,000 lines to the English inch, and was silvered on the back, so that the colours were as bright as those from a glass prism. The spectrum selected for use was nearly six times as long as that furnished by the glass prism — a cir- cumstance, of course, that favoured accurate observation.

The tables that have just been given enable us very easily to calculate the lengths of the waves of light, cor- responding to the centres of the coloured spaces in the nor- mal spectrum. It is only necessary to ascertain the number corresponding, for example, to the centre of the red space, then to multiply it by 3'653, and to subtract the product from 7,603 : the result will be the wave-length correspond- ing to that part of the normal spectrum, expressed in ten- millionths of a millimetre. The following table contains the wave-lengths corresponding to the centres of the col- oured spaces :

I U.OUO,(Tu u MM.

Centre of red V,000

" orange-red , 6,208

" orange , 5,972

" orange-yellow 5,8'79

" yellow '. 5,808

" full green 5,271

" blue-green 5,082

" cyan-blue 4,960

" blue 4,732

" violet-blue 4 383

" pure violet 4 059

The results here given differ somewhat from those obtained by Listing in 1867 ; the differences are partly due to the terms employed ; the author, for example, dividing up into orange-red, orange, and orange-yellow, a space which is called by Listing simply orange. According to the author

PRODUCTION OF COLOUR BY DISPERSION. 27

cyan-blue falls on the red side of the line F ; it is placed by Listing, however, on the violet side of this line. Other less important differences might be mentioned ; but, as a dis- cussion of them would be out of place in a work like the present, the curious reader is referred for further informa- tion to Listing's paper.*

A little study of the normal spectrum, Fig. 4, will enable us to answer some interesting questions. We have already seen that change in colour is always accompanied by change in the length of the waves of light producing it ; hence if we begin at one end of our normal spectrum where the colour is red, and the length of the waves equal to 7,603 ten- millionths of a millimetre, as we diminish this length, we expect to see a corresponding change in the colour of the light : small changes we anticipate will produce small effects on the colour, large changes greater effects.

Now, the question arises whether equal changes of wave- length actually are accompanied by equal alterations of hue in all parts of the spectrum. To take an example : in pass- ing from the orange-yellow, through the pure yellow and greenish-yellow well into the yellow-green region, we find it necessary to shorten our wave-length about 400 of our units ; now will an equal curtailment in other regions of the spectrum carry us through as many changes of hue ? The answer to this is not exactly what we might expect. In a great part of the red region a change of this kind produces only slight effects, the red inclining a little more or less to orange, and the same is true of the blue and violet spaces, the hue leaning only a little toward the blue or violet side, as the case may be. Hence it seems that the eye is far more sensitive to changes of wave-length in the middle regions of the spectrum than at either extremity. This circumstance, to say the least, is curious ; but, what is more to our purpose, it is a powerful argument against any theory

* Poggendorfifs " Anualen," cxxxi., p. 564.

38

MODERN CHROMATICS.

of colour whicli is founded on supposed analogies with music. But more of this hereafter.

In the prismatic spectrum and in our normal spectrum we found no representative of purple, or purplish tints. This sensation can not be produced by one set of waves alone, whatever their length may be ; it needs the joint action of the red and violet waves, or the red and blue. All other possible tints and hues find their type in some portion of the spectrum, and, as will be shown in the next chapter, this applies ' jast as well to the whole range of browns and greys, as to colonic like vermilion and ultramarine.

We have seen that the mixture of long and short waves which compose white light can be analyzed by a prism into its original constituents : the long waves produce on us the

Fig. 5.— Eecomposition of White Light.

sensation that we call red, and, as we allow shorter and shorter waves to act on the eye, we experience the sensa- tions known as orange, yellow, green, blue, and violet. "When, on the other hand, we combine or mix together these different kinds of light, we reproduce white light. There are a great many different ways of effecting this recom-

PEODTJCTION OF COLOUR BY DISPERSION. 29

position ; one of the most beautiful was contrived several years ago by Professor Eli Blake. Tlie spectrum is re- ceived on a strip of ordinary looking-glass, which is gently bent by the hands of the experimenter till it becomes some- what curved ; it then acts like a concave mirror, and can be made to concentrate all the coloured rays on a distant sheet of paper, as shown in Fig. 5. The spot where all the col- oured rays are united or mixed appears pure white.

CHAPTER in.

THE CONSTANTS OF COLOUR.

The tints produced by Nature and art are so manifold, often so vague and indefinite, so affected by their environ- ment, or by the illumination under which they are seen, that at first it might well appear as though nothing about them were constant ; as though they had no fixed proper- ties which could be used in reducing them to order, and in arranging in a simple but vast series the immense multitude of which they consist.

Let us examine the matter more closely. We have seen that when a single set of waves acts on the eye a colour- sensation is produced, which is perfectly well defined, and which can be indicated with precision by referring it to some portion of the spectrum. We have also found that when waves of light, having all possible lengths, act on the eye simultaneously, the sensation of white is produced. Let us suppose that by the first method a definite colour- sensation is generated, and afterward, by the second meth- od, the sensation of white is added to it : white light is added to or mixed with coloured light. This mixture may be accomplished by throwing the solar spectrum on a large sheet of white paper, and then casting on the same sheet of paper the white light which is reflected from a silvered mirror, or from an unsilvered plate of glass. Fig. 6 shows the arrangement. By moving the mirror M, Fig. 6, the white band of light may be made to travel slowly over the whole spectrum, and thus furnish a series of mixtures of

THE CONSTANTS OF COLOUR. 31

white light with the various prismatic hues. The general effect of this proceeding will be to diminish the action of the coloured light ; the mixtui'e will indeed present to the eye more light, but it will be paler ; the colour-element will begin to be pushed into the background. Conversely, if we now should subject our mixture of white and coloured light to analysis by a second prism, we should infallibly detect the presence of the white as well as of the coloured light ; or, if no white light were present, that would also

Fio. 6.— Mode of mixing White LigM with the Colours of the Spectrum.

be equally apparent. Taking all this into consideration, it is evident that, when a particular colour is presented to us, we can affirm that it is perfectly pure ; viz., entirely free from white light, or that it contains mingled with it a larger or smaller proportion of this foreign element. This furnishes us with our first clue toward a classification of colours : our pure standard colours are to be those found in the spectrum ; the coloured light coming from the surfaces of natural objects, or from painted surfaces, we must com- pare with the hues of the. spectrum. If this is done, in al- most every case the presence of more or less white light will be detected ; in the great majority of instances its

33 MODEEN CHROMATICS.

preponderance over the coloured light will be found quite marked. To illustrate by an example : if -white paper be painted with vermilion, and compared with the solar spec- trum, it will be noticed that it corresponds in general hue with a certain portion of the red space ; but- the two coloui-s never match perfectly, that from the paper always appear- ing too pal.e. K, now, white light be added to the pure spectral- tint, by reflecting a small amount of it from the mirror (Fig. 6), it will become possible to match the two colours ; and, if we know how much white light has been added, we can afterward say that the light reflected from the vermilion consists, for example, of eighty per cent, of red light from such a region of the spectrum, mixed with twenty per cent, of white light. If we make the experi- ment with a surface painted with " emerald green," we shall obtain about the same result, while we shall find that artificial ultramarine-blue, reflects about twenty-five per cent, of white light. In all of these cases the total amount of light reflected by the coloured paper is of course taken as 100, and the results here given are to be regarded only as approximations. In every case some white light is sure to be present ; its effect is to soften the colour and reduce its action on the eye ; when the proportion of white is very large, only a faint reminiscence of the original hue remains : we say the tint is greenish-grey, bluish-grey, or reddish- grey. If one part of red light is mixed with sixteen parts of white light, the mixture appears of a pale pinkish hue. The specific effects produced by the mixture of white with coloured light will be considered in Chapter XII. ; it is enough for us at present to have obtained an idea of one of the constants of colour, viz., its purity. The same word, it may- be observed, is often used by artists in an entirely dif- ferent sense : they will remark of a painting that it is no- ticeable for the purity of its colour, meaning only that the tints in it have no tendency to look dull or dirty, but not at all implying the absence of white or grey light.

THE CONSTANTS OF COLOUR. 33

Next let us suppose that in our study of these matters we have presented to us for examination two coloured sur- faces, which we find reflect in hoth cases eight tenths red light and two tenths white light. In spite of this, the tints may not match, one £i£ them being much brighter than the other ; containing, perhaps, twice as much red light and twice as much white light ; having, in other words, twice as great brightness or luminosity. The only mode of caus- ing the tints to match will be to expose the darker-coloured surface to a stronger light, or the brighter surface to one that is feebler. It is evident, then, that brightness or lumi- nosity is one of the properties by which we can define col- our ; it is our second colour-constant. This word luminos- ity is also often used by artists in an entirely different sense, they calling colour in a painting luminous simply because it recalls to the mind the impression of light, not because it actually reflects much light to the eye. The term " bi-ight colour " is sometimes used in a somewhat analogous sense by them, but the ideas are so totally different that there is little risk of confusion.

The determination of the second constant is practicable in some cases ; it presents itself always in the shape of a difficult photometric problem. The relative brightness of the colours of the solar spectrum is one of the most inter- esting of these problems, as its solution would serve to give some idea of the relative brightness of the colours which, taken together, constitute white light. Quite recently a set of measurements was made in different regions of the spectrum by Vieror(Jt, who referred the points measured to the fixed lines, as is usual in such studies.* Reducing his designations of the different regions of the spectrum to those of our spectral chart, which includes 1,000 parts from A to H (see previous chapter), and supplying the colours from the observations of the present writer, we obtain the following table :

* 0. Vierordt, Poggendorff'a " Annalen," Band cxxxvii., S. 200.

34

MODERN CHROMATICS.

Table showing the Luminosity of Different Regions op the Prismatic

SrEOTEUM.

Position.	Luminosity.	Colour.
From 40-5 to	57	80	Dark red.
" 104-5 "	112-71	493	Pure red.
" 112-'n "	138-6	1,100	Red.
" 158-5 "	168-5	2,773.	Orange-red.
" 189	220-31	6,985	Orange and orange-yellow.
" 220-31 "	231-5	7,891	Orange-yellow.
" 231-5 "	363-11	3,033	Greenish-yellow, yellow-green, and green.
" 389-85 "	493-22	1,100	Blue-green and cyan-blue.
" 493-22 "	558-5	493	Blue.
" 623-5 "	689-5	90-6	TJltramarine (artificial).
" 753-58 "	825-5	36-9	Blue-violet.
" 896-5 "	956	13-1	Violet.

The author finds that -with the aid of rotating disks the second, constant can often be determined.* Let us suppose that we -wish to determine the luminosity of paper painted ■with vermilion : a circular disk, ahout six inches in diame- ter, is cut from the paper and placed on a rotation apparatus, as indicated in Fig. 7. On the same axis is fastened a double disk of black and of -white paper, so arranged that the pro- portions of the black and -white can be varied at -will.f When the -whole is set in rapid rotation, the colour of the vermilion paper -will of course not be altered, but the black and white -will blend into a grey. This grey can be altered in its brightness, till it seems about as luminous as the red (Fig. 8). If we find, for example, that -with the disk three quar- ters black and one quarter white an equality appears to be established, we conclude that the luminosity of our red sur- face is twenty-five per cent, of that of the white paper. This

* " American Journal of Science and Arts," February, 1878. ■f See Maxwell's " Disks," chapter x.

THE CONSTANTS OP COLOUR.

35

is of course based on the assumption that the black paper reflects no light ; it actually does reflect from two to six per cent., the reflecting power of white paper being put at 100. The black disk used by the author reflected 5-2 per

Fig. 7.— Coloured Disk, with Small Blaok-and-White I

Disk.-

White Disk in Kotation.

cent, of white light ; to meet this a correction was intro- duced, and a series of measurements made, some of the more important of which are given in the following table :

Luminosity.

White paper 100

Vermilion (English)* 25'7

Pale chrome-yellow f 803

Pale emerald-green* 48'6

Cobalt-blue t 36-4

Ultramarine J: 1-6

These results were afterward tested by the use of a set of disks, the colours of which were complementary to those mentioned in the table, and these additional experiments and calculations showed that the original measurements differed but little from the truth. This agreement proved also the correctness of Grassmann's assumption, that the total

* la thick paste, f Washed on as a water-colour. % Artificial, as a paste.

36 MODERN CHROMATICS.

intensity of the mixture of masses of differently coloured 'ligtt is equal to the sum of the intensities of the separate components.

But to resume our search for colour-constants. We may meet with two portions of coloured light having the same degree of purity and the same apparent brightness, which nevertheless appear to the eye totally different : one may excite the sensation of blue, the other that of red ; we say the hues are entirely different. The hue of the colour is, then, our third and last constant, or, as the physicist would say, the degree of refrangibility, or the wave-length of the light. In the preceding chapter it has been shown that the spectrum offers all possible hues, except the purples, weU arranged in an orderly series, and the purples themselves can be produced with some trouble, by causing the blue or violet of the spectrum to mingle in certain proportions with the red.

Fis. 9.— Eye-piece with Dalton's Scale.

For the determination of the hue, an ordinary one-prism spectroscope can be used ; it is only necessary to add a little contrivance which enables the observer to isolate at will any

THE CONSTANTS OF COLOITK

37

small portion of tlie spectrum. TMs object is easily at- tained by introducing into tbe eye-piece of the instrument

Fia. 10. — BufherfUrd's Automatlo Spectroscope.

a- diaphragm perforated by a very narrow slit (see Fig. 9). The observer then sees in the upper part of the field of view the selected spectral colour ; in the lower part of the field the scale is visible, and with its aid the precise position of the prismatic hue can be determined. Instead of using a

38

MODERN OHKOMATICS.

scale divided into equal parts, it is often advantageous to employ the plan suggested by Dr. J. C. Dalton, and used by him for determining the position of certain absorption bands. Dr. Dalton employs as a scale a minute photograph which shows the positions of the fixed lines, and divides up the spaces between them conveniently. Fig. 9 exhibits the appearance of the field of view and the scale. For more accurate work Rutherfurd's automatic six-prism spectro- scope can be employed (see Fig. 10*). A diffraction grat- ing can also be used — in those cases where the student of colour is so fortunate as to possess one.f With a very per- fect grating of this kind, for which the author was indebted to Mr. Rutherfurd, the third constant was determined for a number of coloured disks. The following table gives their positions in a normal spectrum haviug from A to H 1,000 parts ; the corresponding wave-lengths are also given :

Name of the Colour.

Vermilion (English)

Red lead

Pale chrome-yellow

Emerald-green

Prussian-blue

Cobalt-blue

Ultramarine (genuine)

Ultramarine (artificial)

Same, washed^ With Hoffmann's Tiolet B. B .\

Position in tlie Normal Spectram.

387 422 4S8 648 740 770 785 857

916

Wave-length . 11 Tsoiresire ™™-

6,290 6,061 ^,820 5,234 4,899 4,790 4,735 4,472

4,257

We have seen that the first colour-constant has reference to the purity of the colour, or indicates the relative amount of white light mixed with it. This constant is in all cases

* Fig. 10 is a facsimile of Rutherfurd's drawing of his six-prism spec- troscope ("American Journal of Science and Arts," 1865), f See Chapter iv. for an account of the grating.

THE CONSTANTS OF COLOUR. 39

difficult to determine ; probably something might be effected by carrying out practically the idea suggested in Pig. 6, by making the necessary additions to the apparatus there in- dicated. It would be necessary to measure the relative luminosity of the selected spectral hue and the white light, and then to mix them in proper proportions, till the mixture matched the colour of the painted paper, etc. The second and third colour-constants can be more easily determined.

It may be well here to refer to the terms used to indicate these constants. For the first constant, the word purity, in the sense of freedom from white light (or from the sensa- tion of white), is well adapted. The term luminosity will be employed in this work to indicate the second constant ; the third constant will generally be referred to by the term hue. Colours are often also called intense, or saturated, when they excel both in purity and luminosity ; for it is quite evident that, however pure the coloured light may be, it still will produce very little effect on the eye if its total quantity be small ; on the other hand, it is plain that its action on the same organ will not be considerable, if it is diluted with much white light. Purity and luminosity are, then, the factors on which the intensity or saturation de- pends. We shall see hereafter that this is strictly true only within certain limits, and that an inordinate increase of luminosity is attended with a loss of intensity of hue or saturation.

Having defined the three constants of colour, it will be interesting to inquire into the sensitiveness of the eye in these directions. This subject has been studied by Aubert, who made an extensive set of observations with the aid of coloured disks.* It was found that the addition of one part of white light to 360 parts of coloui'ed light produced a change which was perceptible to the eye ; smaller amounts failed to bring about this result. It was also ascertained

* Aubert, " Physiologie der Netzhaut," Breslau, 1866.

40 MODEBN CHROMATICS.

that- mingling the coloured light of a disk with from 120 to 180 parts of white light (from white paper) caused it to be- come imperceptible, the hue being no longer distinguishable from that of the paper.* Differences in luminosity as small as -j^ to ilir could also, under favourable circumstances, be perceived. It hence followed that irregularities in the illu- mination, or distribution of pigment over a surface, which were smaller than yfj- of the total amount of light reflected, could no longer be noticed by the eye. Experiments with , red, orange, and blue disks were made on the sensitiveness of the eye to changes of hue or wave-length ; thus, the combination of the blue disk with a minute portion of the red disk altered its hue, moving it a little toward violet ; on reversing the case, or adding a little blue to the red disk, the hue of the latter moved in the direction of purple.f Similar combinations were made with the other disks. Au- bert ascertained in this way that recognizable changes of hue could be produced, by the addition of quantities of coloured light, as small as from -j-J^ to ^^ of the total amount of light involved. From such data he calculated that in a solar spectrum at least a thousand distinguishable hues are visible. But we can ^till recognize these hues, when the light producing them is subjected to considerable variation in luminosity. Let us limit ourselves to 100 slight variations, which we can produce by gradually in-, creasing the brightness of our spectrum, tUl it finally is five times as luminous as it originally was. This will fur- nish us with a hundred thousand hues, differing perceptibly from each other. If each of these hues is again varied

* To obtain correct results it is of course necessary to know the lumi- nosity of the coloured disk aa compared with the white disk, for in the above results by Aubert they are considered to be equal. With the aid of the table of luminosities previously given, this correction can be made, and it will be found that four or five times as much white light is actually neces- sary as is indicated above.

f Compare Chapter x.

THE CONSTANTS OF COLOUK. 41

twenty times, by the addition of different quantities of white light, it cames the number of tints we are able to distinguish up as high as two millions. In this calculation no account is taken of the whole series of purples, or of colours which are very luminous or very dark, or mixed with much white light.

To the above we may add that interesting experiments on the sensitiveness of the eye to the different spectral colours have also been made by Charles Pierce, who found that the photometric susceptibility of the eye was the same for all colours. (See "American Journal of Science and Arts," April, 1877.)

With the aid of Vierordt's measurements previously given, and the determinations by the author of the spaces occupied by the different colours in the spectrum, a very interesting point can now be settled, viz. : we can ascertain in what proportions the different colours are present in white light. The amount of red light, for example, which is present will evidently be equal to the space which it oc- cupies in the spectrum, multiplied by its luminosity, and the same will be true of all the other colours. The author constructed a curve representing Vierordt's results, and from this, taken in combination with his own determina- tions of the extent of the coloured spaces, obtained the following table :

Table showing the Amounts or Coloured Light in 1,000 Pakis of White Sunlight.

Red 54

Orange-red 140

Orange 80

Orange-yellow 114

Yellow 84

Greenish-yellow 206

Yellowish-green 121

Green and blue-green 134

Cyan-blue. 32

43 MODERN CHROMATICS.

Blue ,■•■ 40

Ultramarine and blue-violet 20

Violet 5

1,000

Artists are in the habit of dividiag up colours into warm and cold. Now, if we draw the dividing line so as to in- clude among the warm colours red, orange-red, orange, orange-yeUow, yellow, greenish-yellow, and yellowish-green, then in white light the total luminosity of the warm colours will he rather more than three times as great as that of the cold colours. If we exclude from the list of warm colours yellowish-green, then they will be only about twice as luminous as the cold. We shall make use of this table hereafter.

It may have seemed strange that the chrome-yellow paper previously mentioned reflected eighty per cent, of light (the reflecting power of white paper being 100), while the table just given states that white light contains only a little more than five per cent, of pure yellow light. It will, however, be shown in a future chapter that chrome-yellow really reflects not only the pure yellow rays, but also the orange-yellow and greenish-yellow, besides much of the red, orange, and green light. By mixture, all these colours finally make a yellow, as will be explained in Chapter X. The high luminosity of some of the other coloured papers is to be explained in a similar manner.

CHAPTER IV.

PRODUCTION OF COLOUR BY mTEEFERENOE AND POLARIZATION.

Iw Chapter II. we studied the spectral tints produced by a prism and by a grating ; these were found to be pure and brilliant as well as very numerous, and consequently were adopted as standards for comparison. Most nearly allied to these central normal colours are those which are pro- duced by the polarization of light. In this class we meet with a far greater variety of hues than is presented by the solar spectrum ; and, instead of a simple arrangement of delicately shading bands, we encoxmter an immense variety of chromatic combinations, sometimes worked out with ex- quisite beauty, but as often arranged in a strange fantastic manner, that suggests we have entered a new world of colour, which is ruled over by laws quite different from those to which we have been accustomed. And it is indeed so ; the tints and their arrangement depend on the geo- metrical laws which build up the crystal out of its mole- cules, and on the retardation which the waves of light ex- perience in sweeping through them, so that in the colours of polarization we see, as it were, Nature's mathematical laws laying aside for a moment their stiff awkwardness, and gayly manifesting themselves in play.

The apparatus necessary for the study of these fasci- nating and often audacious colour-combinations is not necessarily complicated or very expensive. A simple form

44

MODERN CHROMATICS.

of polariscope is shown in Fig. 11. It consists merely of a plate of window-glass at P, which is so placed that the angle, a, is 33° as nearly as possible ; at N is a Nicol's prism, and at L a plano-convex lens with a focal length of about an inch. The distance of the prism from the plate

Fig. 11.— Simpio Polarizing Apparatus.

of glass is ten inches ; the lens is removable at .pleasure, the Nicol's prism is capable of revolving around its longer axis. If, now, light from a white cloud be reflected from the plate of glass toward the Nicol's prism, as indicated by the arrow, some of it will ordinarily traverse the prism and reach the eye at E ; the prism -should now be turned till this light is cut off, and the instrument is then ready for use. Thin slips of selenite or crystals of tartaric acid placed at 8, so that they are magnified, display the colours of polarization very beautifully. The arrangement just described constitutes a simple polarizing microscope ; if a compound polarizing microscope can be obtained, it will be still more easy to study the colours and combination of colours presented by the crystals bf many different salts. By dissolving a grain or two of the substance in a drop of watet, and allowing it to crystallize out on a slip of glass, it is possible very easily to make objects suitable for exami- nation.

Thin plates of selenite, obtained by removing successive layers with a penknife, answer admirably if we wish to study the phenomena of chromatic polarization in their simplest forms. It will often be found that nearly the

PRODUCTION OF COLOUR BY INTERFERENCE, ETC. 45

whole plate presents a single unshaded hue, which bears a close resemblance to a patch of colour taken from some por- tion of the spectrum. But, however bright the colour may be, it is never free from an admixture of white light, and it is the constant presence of this foreign element which causes the colours of polarization to appear a little less in- tense than those of the spectrum. With plates which are somewhat thicker the proportion of white light increases, washing out the colour, till only a faint reminiscence of it is left. The more powerful tints, however, quite equal and probably surpass in purity — that is, in freedom from white light — the most intense sunset hues. Among these colours we find many shades of red and purple-red ; all the red- orange hues are represented, and the same is true of the other colours found in the spectrum. Besides, there is a range of purples which bridges over the chasm between the violets and reds ; faint rose-tints are also present in abun- dance, and the same is true of the pale greens and bluish- greens. In addition to this, quite thin slips furnish a dis- tinct set of tints which are peculiar in appearance, and which, when once seen, are never forgotten ; a singular tawny yellow will be noticed in combination with a bluish- grey ; the yellow, such as it is, shading into an orange nearly allied to it, and this again into a brick-red ; black ai.d white will be associated with these subdued tints ; the general impression produced by these combinations being sombre if not dreary.

These slips of selenite furnish neither beautiful nor com- plicated patterns, the tints being for the most part arranged in parallel bands, with here and there angular patches, often in sharp contrast with the other masses of colour. There is no noticeable attempt at chromatic composition, except per- haps a little along fractured edges, where we frequently meet with pale grey or white deepening, into a fox-coloured yellow, followed by a red-violet, brightening into a sea- green dashed with pure ultramarine; or changing Suddenly

46 MODERN CHROMATICS.

into a full orange-yellow, after which may follow a broad field of purple. Just as often all the tints are pale, like those used on maps, with a narrow fringe on the edge, of rich variegated hues. The colour-combinations seldom rise into great beauty, though they often astonish and dazzle by their audacity and total disregard of all known laws of chromatic composition. The brilliancy and purity of these tints are so great, and they are laid on with such an unfal- tering hand, that all these wild freaks are performed com- paratively with impunity, and it is only when we proceed to make copies of these strange designs that we become fully aware of their peculiarities, and, from an artistic point of view, positive defects.

Crystals of tartaric acid present phenomena which are quite different : here the patterns are rich and often beautiful; the colour is full of gradation, touched on and retouched and wrought out with patience in delicate, complicated forms, which echo or faintly oppose the grand ruling ideas of the composition. We may have a wonderfully shaped mass radiating in curved lines over the entu-e field, tinted with soft grey and pale yellow, with here and there dashes of colour like the spots on a peacock's tail, glowing like coals of fire ; all this being set off by very dark shades of olive-green, dark browns and greys. If the crystals are thin this is their appearance ; but as the thickness increases so does the brilliancy of the hues, which are sure to be well contrasted with large masses of deep shade. The soft gra- dations, the sharp contrasts, the brilliant and pale colours, the dark shadows and the wonderful forms, all combine to lend to these pictures a peculiar charm which is not wholly lost even in copies executed in ordinary pigments.

Common sugar, if allowed time to crystallize out slowly, furnishes appearances somewhat resembling the above, but the designs are more formal and less interesting. Crystals of nitrate of potash present appearances, again, which are totally unlike those above mentioned ; here we have a great

PRODUCTION OF COLOUR BY INTERFERENCE, ETC. 47

number of delicately tinted threads of light ; there will be purples and golden greens or dull olive-greens and carmines, woven together so closely as almost to produce a neutral tint, which will brighten suddenly and display combinations of pui-ple-red with green, dashed here and there with pure ultramarine. These tinted threads of light will be disposed with regularity as though it had been intended to weave them into some wonderful cashmere-like pattern, and then warp and woof had been suddenly abandoned and for- gotten.

It would be useless to multiply these descriptions — every salt has its own peculiarities and suggests its own train of fancies ; some glow like coloured gems with polished facets, or bristle with golden spears like the advancing ranks of two hosts in conflict, or suggest a rich vegetation made of gold and jewels and bathed in sunset hues. Artists who see these exhibitions for the first time are generally very much impressed by their strange beauty, and not unfre- quently insist that their range of colour-conceptions has been enlarged. It has often seemed to the author that the cautious occasional study of some of these combinations might be useful to the decorator in suggesting new concep- tions of the possibilities within his reach.

When polarized light is made to traverse crystals in the direction of -their optic axes, phenomena of a different kind are presented. They were discovered in 1813 by Brewster, and, on account of their scientific interest and a certain beauty, have since then greatly attracted the attention of physicists and even of mathematicians. A series of rainbow- like hues, disposed in concentric circles, is seen on a white field ; a dark-grey cross is drawn across the gayly coloured circles, and, after dividing them in four quadrants, fades out in the surrounding white field. By a slight change in the adjustment of the apparatus, the grey cross can be made white ; the rings then assume the complementary tints. Other crystals, again, furnish double sets of rings, the daris

48 MODERN CHROMATICS.

cross being shared by tliem jointly, or so altered in form as no longer to be recognizable.

These appearances have been considered by many phys- icists to be extraordinarily beautiful ; it is, however, to be suspected that in this case the judgment was swayed by other considerations than those of mere beauty. The rarity of the phenomenon, the difficulty of exhibiting it; the bril- liant list of names identified with it, along with the insight it furnishes as to the molecular constitution of crystals, all combine to warp the judgment, and to seriously influence its final award. In point of fact, the formal nature of the figures, the constant repetition of the rainbow-tints in. the same set order, which is that of the spectrum, both exclude the possibility of the charming colour-combinations so fre- quently presented by many salts when simply crystallized on a 'slip of glass. The cross and rings are not for a mo- ment, in matter of beauty, to be compared with the appear- ances presented by crystals of tartaric acid.

Glass which has been heated and then suddenly cooled, or glass which is under strain, exhibits phenomena of coloxir closely related to the above ; we have as it were a set of distorted crosses and rings which sometimes lend themselves more kindly to the production of chromatic effects than is the case with the normal figures.

In ordinary life the colours of polarization are never seen ; the fairy world where they reign cannot be entered without other aid than the unassisted eye. This is not a matter for regret ; the purity of the hues and the audacious character of their combinations cause their gayety to appear strange and unnatural to eyes accustomed to the far more sombre hues appropriate to a world in which labour and trouble are such important and ever-present elements. The colours even of flowers have a thoughtful cast, when com- pared with those of polarization.

The colours which have just been considered are pro- duced in a peculiar manner ; the complete explanation is

PRODUCTION OF COLOUR BY INTERFERENCE, ETC. 49

long and tedious, and has for us no particular interest. The main idea, however, is this : white light is acted on in such a way that one of its constituents is suppressed ; the result is coloured light. For example, if we strike out from white light the yellow rays, what remains will produce on us the sensation of blue ; if we cut off the green rays, the remain- der will appear purple. The reason of this will be more fully appreciated after a study of the facts presented in Chapters XI. and XII. To effect this sifting out of certain rays a polarizing apparatus is employed ; when the crystals are removed from it, the colour instantly vanishes. Now, it so happens that there is a class of natural objects capable of displaying exactly the same hues without the intervention of any piece of apparatus — all objects that fulfill a certain condition may be reckoned in this class ; it is merely that their thickness should be very small. Thin layers of water, air, glass, of metallic oxides, of organic substances, in fact of almost everything, display these colours. The most fa- miliar example is furnished by a soap-bubble. When it first begins to grow, it is destitute of colour and perfectly trans- parent ; it gives by reflection from its spherical surface a distorted image of the window, with the bars all curved, but no unusual hues are noticed till it has become somewhat enlarged.* Then faint greens and rose-tints begin to make their appearance, mingling uneasily together as if subjected to a constant stirring process. As the bubble expands and the film becomes more attenuated, the colours gain in bril- liancy, and a set of magnificent blue and orange hues, pur- ples, yellows and superb greens replaces the pale colours which marked the early stages, and by their changing flow and perpetual play fascinate the beholder. If the bubble

* It is not very uncommon to meet with paintings in wliicli a bubble has been represented with window-bars on its surface, where nothing of the kind could have been visible. A friend has mentioned to the author four cases where dififerent ai-tists have introduced window-bars instead of slty and landscape, on the surfaces of bubbles which were in the open air.

50 MODERN CHROMATICS.

has the rare fortune to live to a good old age, at its upper portion a different series of tints begins to be developed ; the tawny yellow, before mentioned, begins to be seen in irregular patches, floating around among the more brilliant hues, a sign that the attenuation has nearly reached its ex- treme limit ; but, if by some unusual chance it should be a Methuselah among bubbles, pale white and grey tints also are seen, after which it is sure to burst. A long-lived soap- bubble displays every colour which can be produced by polarization. The thin film has a sifting action on white light, which in its final result is the same as in the case of the production of colour by polarized light : certain rays are struck out, and, as before, white light deprived of one of its constituents furnishes coloured light. This elimination is accomplished by the interference of the waves of light in- volved ; hence, colours produced in this way are called " in- terference colours." The colours of polarization are also just as truly interference colours, but they are not usually known under this name. From all this it follows that the colours produced by thin layers, or by very fine particles, always contain some white light, and consequently cannot quite rival in purity or intensity the spectral hues.

The colours of polarization, as we have seen, are never met with outside of the laboratory. Nature, on the other hand, here and there with a sparing hand, displays in small quantity, and as a rarity, the colours of interference. They are used as a wonderful kind of jewelry in the adornment of many birds ; lavishly so in the case of the common pea- cock, where the breast and tail feathers in full sunshine dis- play flashing, dazzling hues, which make our artificial or- naments appear pale and tame. In. contemplating these astonishing hues, or those of that tiny winged jewel, the humming-bird, we are struck by the circumstance that they actually have a metallic brilliancy, which we in vain at- tempt to rival with our brightest pigments. To compete with them successfully,, it is necessary to substitute a sur-

PRODUCTION OF COLOUR BY INTERFERENCE, ETC. 51

face of silver for the white paper, and to cover it with the purest and most transparent glazes. This appearance of metallic lustre depends on the circumstance that much col- oured light is reflected, mingled with only a small quantity of white light, the great bulk of the latter being absorbed by the dark pigment contained in the interior of the feath- ers. When this dark pigment is absent, we have as before colour ; but, being mingled with much reflected white lightj it- presents simply an appearance like that of mother-of- pearl.

There is yet another peculiarity of the colours now under consideration, which still more completely separates them from the hues furnished by pigments : it is their va- riability. These colours, as has been mentioned, are pro- duced by the interference of the waves of light which are reflected from the thin films : the nature of this interference depends partly on the angle at which this reflection takes place, so that, as we turn a peacock's feather in the hand, its colour constantly changes. The same is true of the tints of the soap-bubble, and of interference colours in general — the hue changes with the position of the eye ; as they are viewed more and more obliquely, the tint changes in the order of the spectrum, viz., from red to orange, to yellow, etc.

The brilliant metallic colours exhibited by many insects, particularly the beetles, belong also in this class, so also the more subdued steel-blues and bottle-greens displayed by many species of flies. So commonly does this occur that it suggests the idea that these humble creatures are not desti- tute of a sense for colour capable of gratification by bril- liant hues. If we descend into the watery regions we find their inhabitants richly decorated with colours of the same general origin, the pearly rainbow hues which they display all depending on the interference of light. The same is true of the iridescent hues which so commonly adorn shells externally and internally. In this case candour compels one

52 MODERN CHROMATICS.

to- admit that the colours, beautiful as they are, can hardly be a source of pleasure to the occupants or to their friends.

Leaving the animated world, we find the colours of in- terference shown frequently, but in an inconspicuous man- ner, by rather old window-glass ; some of the alkali seems to be removed by the rain, and in the course of time a thin film of silica capable of generating these hues is formed. In antique glass which has long remained bm-ied this pro- cess is carried much further, so that sometimes the whole plate or vase tends to split up into flates. Here, owing to successive reflections on many layers, the light which reach- es the eye is quite bright, and the colours intense. Crim- son, azure, and gold are found in combination ; blue melts into purple or flashes into red ; ruby tints contrast with emerald hues : each change of the position of the eye or of the direction of the light gives rise to a new and startling effect. In other cases broad fields of colour, with much gentle gradation and mingling of tender pearly hues, re- place the gorgeous prismatic tints, and fascinate the be- holder with their soft brilliancy.

The iridescent hues of many minerals fall into the same general class ; they are beautifully displayed by some of the feldspars, and the brilliant hues found on anthracite coal have also the same origin. The blue films often pur- posely produced on steel are due to thin layers of oxide of iron which suppress the yellow rays. Other cases might be mentioned, but these will sufiice for the present.

CHAPTER V. ON THE COLOURS OF OPALESCENT MEDIA.

If wHte light be allowed to fall on water which is con- tained in a clear, colourless glass vessel, some of it will be reflected from the surface of the liquid, while another por- tion will traverse the water and finally again reach the au-. These well-known facts are represented in Fig. 12. An eye placed at E will perceive the reflected light to be white, and the transmitted light will also appear white to an eye situated at O. But, if now a little milk be added to the water, a remarkable change will be produced : light will, as before, be reflected from the surface to the eye placed at E, and this surface-light will still be white ; but the little milk- globules ' under the surface and throughout the liquid will also reflect light to E — this light will be bluish. From this experiment, then, it appears that the minute globules sus- pended in the liquid have the power of reflecting light of a bluish tint. In Fig. 12 the light is represented as being reflected only in one direction ; but, when the milk-globules are added, they scatter reflected light in many directions, so that an eye placed anywhere above the liquid perceives this bluish appearance.

On the other hand, after the addition of the milk, the light at O (Fig. 12), which has passed through the milky liquid, will be found to have acquired a yellowish tint. From this it appears that fine particles suspended in a liquid have the power of dividing white light into two portions, tinted respectively yellowish and bluish. If more milk be

54

MODERN CHROMATICS.

added to the water, white light will mingle in and wiU final- ly overpower the bluish reflected light, so that it will hardly he noticed ; as the quantity of mUk is increased, the colour of the transmitted light will pass from yellow to orange, to red, and finally disappear, the liquid having become at last so opaque as to cease to transmit light altogether.

Fi8. 12.— Kefleotion and Transmission of Light by Water.

This very curious action is not confined to mixtures of milk and water, but is exhibited whenever very fine parti' cles are suspended in a medium different from themselves. If an alcoholic solution of a resin is poured with constant stirring into water, very fine particles of resin are left sus- pended in the liquid, and give rise to the appearances just described. BrUcke dissolves one part of mastic in eighty- seven parts, of alcohol, and then mixes with water, the water

ON THE COLOURS OF OPALESCENT MEDIA. 55

being kept in constant agitation. A liquid prepared in tHs way shows by reflected light a soft sky-like hue, the colour of the light which has passed through being either yellow or red, according to the thickness of the layer traversed. The suspended particles of resin are very fine, and remain mingled with the water for months ; they are often so fine as to escape detection by the most powerful microscopes.

Some kinds of glass which are used for ornamental pur- poses possess the same property, appearing bluish- white by reflected light, but tingeing the light which comes through them red or orange-red. The beautiful tints of the opal probably have the same origin, and the same is (rue also of the bluish, milky colour which characterizes many other varieties of quartz.

Not only liquids and solids exhibit this phenomenon of opalescence, but we find it also sometimes displayed else- where ; thus, for example, a thin column of smoke from burning wood reflects quite a proportion of blue light, while the sunlight which traverses it is tinted of a brown- ish-yellow, or it may be, even red, if the smoke is pretty dense.

AH these phenomena are probably due to an interference of light, which is brought about by the presence of the fine vparticles, the shorter waves being reflected more copiously than those which are longer ; these last, on the other hand, being more abundantly transmitted. An elaborate expla- nation of the mode in which the interference takes place would be foreign to the purpose of the present work ; we therefore pass on to the consideration of the more practical aspects of this matter.*

It will be well to notice, in the first place, certain con- ditions which favour not so much the formation as the per- ception of the tints in question : thus it will be found that

* Compare E. Briioke, in Poggendorff 's " Annalen," Bd. 88, S. 363"; alBO Bezold's " Farbenlehre," p. 89.

56

MODERN CHROMATICS.

the blue tint, in the experiments with the liquids, is best exhibited by placing the containing glass vessel on a black surface. This effectually prevents the-blue reflected light from being mingled with rays which have been directly transmitted from underneath. Indeed, the mere presence or absence of a dark background may cause the tint which

Fig, 13. — Smoke appears Blue on a Bark Backgroimd ; Brown on a Light Background.

finally is perceived by the eye to change from yellow to blue, the other conditions all remaining unaltered. Thus in Fig. 13 we have thin smoke seen partly against a dark background, and partly against a sky covered with white clouds : the lower portion is blue, from reflected light, while in the upper portion this tint is overpowered by the greater intensity of the transmitted light, which is yellow- ish-brown. As a general thing, the reflected and transmit- ted beams are both present ; dark backgrounds favour the former, luminous ones the latter.

If a thin coating of white paint, such as white lead or zinc-white, is spread over a black or dark ground, the touches so laid on will have a decidedly bluish tint, owing to the causes which have just been considered. If a draw- ing on dark paper be retouched with zinc-white used as a water-colour, the touches will appear bluish and inharmo- nious, unless especial care is taken to prevent the white

ON THE COLOURS OF OPALESCENT MEDIA. 57

pigment from being to some extent translucent ; this disa- greeable appearance can only be prevented by making each touch dense, and quite opaque. For the production of such effects it is not even necessary to go through the formality of laying the white pigment on a dark ground ; white lead mingled with any of the ordinary black pigments gives not a pure but a bluish grey. This is very marked in the case of black prepared from cork, which hence has sometimes been called "beggars' ultramarine." If yellow pigments are mixed with black, the effect is not simply to darken the yellow, as would be expected, but it is converted into an olive-green. This is particularly the case with those pig- ments which approximate to pure yellow in hue, such as gamboge and aureolin ; the least admixture of dark pig- ment carries them over toward green. But if these black- and-white or black-and-yellow pigments are combined by the method of rotating disks (see Chapter X.), we' obtain pure greys or darker yellow tints, showing that the blue hue is not, as many suppose, inherent in the black pigment, but an accident due to the mode of its employment. The above-mentioned cases are marked examples of the applica- tion of these principles to painting ; but in a more subtile way the whole theory of the process of oil-painting takes cognizance of them, and is so adjusted as to avoid difficul- ties thus introduced, or, more rarely, to utilize them. It is perhaps scarcely necessary to add that the somewhat bluish tone of drawings made with body colour, or of frescoes, is due to these same causes.

Having hinted at the influence which this peculiar op- tical action exerts on the infancy of a picture, we pass on to consider some of its effects on a painting in its old age. It is well known that old oil-paintings frequently become more or less covered by what seems to be a coating of grey or bluish-grey mould, which, spreading itself particularly over the darker portions, obscures them, so that all details aje lost, and the work of the artist entirely destroyed. In-

58 MODERN CHROMATICS.

vestigation has shown that this trouble is caused by an iiu' mense number of fine cracks in the painting itself, which seem to act somewhat in the same way as the mixtures which have just been considered, so that the observer is practically looking at the picture through a rather dense haze. By filling these invisible cracks up with varnish the matter is somewhat helped, but much moi'e perfectly by. the "regenerations process" of Pettenkofer. This celebrated chemist once by accident used an old, worn-out oil-cloth mat, from which the pattern had mostly disappeared, to cover a beaker containing hot alcohol. On removing the mat, some hours afterward, he was surprised to find that the portion acted on by the alcoholic vapours had been reju: venated and the pattern restored. It was soon ascertained that the vapours had softened the pigment, and the separated grains had again been fused together. Experiments on old oil-paintings gave similar results, and the process is now in use in some of the largest European galleries.

In many other objects besides those that have been mentioned these peculiar tints can be observed ; among minor examples may be mentioned the bluish-grey or green- ish-grey tint which marks the course of veins under the skin ; the blue or greenish colour of the human eye also owes its tint to the same cause. In these cases an opalescent membrane is spread over a dark background, and the colour is produced in the same manner as in the experiments de- scribed at the commencement of this chapter. In blue eyes there is no real blue colouring-matter at all.

It is, however, the sky that exhibits this class of tints on the grandest scale, as well as in the greatest perfection. Our atmosphere, even when perfectly clear, contains sus- pended in it immense numbers of very fine particles which never settle to the earth, and which the rain has no power to wash down. When they are illuminated by sunlight they reflect white light mixed with a certain proportion of blue, and this blue is seen on a black background, which is

ON THE COLOURS OF OPALESCENT MEDIA. 59

nothing less than the empty space in which the earth is hung. Hence, the blue colour of the sky. This tint on clear days can be traced up tolerably near the sun, indeed, until the brightness of the sky. begins to be blinding. An examination of the deepest blue portions of the sky with the spectroscope reveals the presence of much white light, so that the blue colour is very far from being pure or satu- rated— a fact that young landscape-painters soon have forced on their attention. In clear weather, as long as the sun is at a considerable distance from the horizon, the yel- low colour which accompanies the transmission of light through an opalescent medium is not much noticed ; but, as the sun sinks lower, its rays traverse an always increasing thickness of the atmosphere, and encounter a greater num. ber of fine particles, so that the transmitted light, late in the afternoon, becomes decidedly yellow or rather orange- yellow.

Having thus briefly considered the production of or- dinary sky-tints, let us pass to the aspects assumed by a landscape under the influence of the minute suspended par- ticles. These atoms will of course reflect light toward the observer, and this light will add itself to that which comes regularly from objects in the landscape, producing thus important changes in their appearance. The very thick layer of air intervening between the observer and the most distant mountains will send to the eye a very large amount of whitish-blue light, which will not greatly differ from a sky-tint. This will entirely overpower the somewhat feeble light reflected from portions of the mountain in shade, so that as a result we shall have all the shadows of the moun- tain represented by more or less pure sky-tints, and these tints will be far more luminous, far brighter, than the ori- ginal shadows were. No details will be visible in these wonderfully shaped bluish patches. Those portions of the mountains, on the other hand, which are in full sunlight will still visibly send light to the eye through the haze, and

60 MODEEN CHROMATICS.

their prevailing tint will be yellowish or orange, or some other -warm tone. Not many details will be visible, and the actual colours or local colours of the mountain will not at all. ai^pear, or at most will only blend themselves with the soft, warm tints due to the medium. In a word, the con- trast between the light and shade will be enormously dimin- ished, so that the general luminosity of the mpuntain. will be hardly less than that of the sky itself, and its colour will be worked out mainly in tints which have the same origin and character with those of the sky. As we approach nearer the. mountain, these effects begin slowly to- diminish, and in the sunlit portions delicate greens, varied and soft greys, begin to make their appearance, while the shadows lose theh' heavenly blue, and, darkening, become bluish- grey. Afterward all those parts lying in sunlight display their local tints, somewhat softened, and the coloured light from the shadows begins to make itself felt, and, mingling with the blue reflected light, to produce soft purplish-greys, greenish-greys, and other nameless tints. The sunlit por- tions of the pine-trees will be of an olive-green or of a low greenish-yellow, the shadows on the same trees being pure grey or bluish-grey without any suggestion of green. On nearer objects these effects are less traceable, and the natural relation between light and shade more and more preserved, so that contrast of this last kind becomes progressively stronger as we turn from the most distant to the nearest objects. All these effects are readily traceable on any or- dinary clear day ; they change, of course, with the condition of the atmosphere, and as it becomes misty the blue reflected light changes to grey, the transmitted light not being equally affected.

Late in the afternoon, when the sun is low, its rays trav- erse very thick layers of the atmosphere, and wonderful chromatic effects are produced. Near the sun the transmit- ted light is yellowish, but so bright that the colour is not very perceptible ; to the right and left the colour deepens

ON THE COLOURS OF OPALESCENT MEDIA. 61

into an orange, often into a red, whicli, as the distance from the sun increases, fades out into a purplish-grey, greyish- blue, passing finally into a sky-blue. The warm tints are produced mainly by transmitted light, the cold ones by re- flected light, and the neutral hues by a combination of both. Above the sun there is, in clear sunsets, a rather regular transition upward, from the colours due to transmitted, to those produced by reflected light. As the sun sinks lower its rays encounter a greater mass of suspended particles, and the warm tints above mentioned move toward the red end of the spectrum, and also gain in intensity. The pres- ence of clouds breaks up the symmetry of these natural chromatic compositions, and gives rise to the most magnifi- cent effects of colour with which we are acquainted. The landscape itself sympathizes with the sky, and near the sun, chameleon-like, assumes an orange or even red hue ; while at greater distances its cold tints are wanned, even the greens being changed into olive or yellowish hues. Simultaneous- ly the shadows lengthen enormously, bringing thus the com- position into grand and imposing masses, and investing even the most commonplace scenery with an air of great noble- ness and beauty.

The complete series of sunset hues, from the brightest light to the deepest shade, runs as follows :

1-. Yellow. I 3. Red. I S. Violet-blue.

2. Orange. I 4. Purple. ' 6. Grey-blue.

This, as it were, normal series is often interrupted by the omission of one or more of the intermediate hues, and sometimes begins as low as the red or even purple.

CHAPTER YI.

PRODUCTION OF COLOUR BY FLUORESCENCE AND PHOSPHORESCENCE.

In all the cases thus far examined, colour has heen pro- duced either by the analysis of white light or by subjecting it to a process of, subtraction, as in the examples mentioned in the last chapter. The very astonishing discovery of Stokes, however, has proved that colour can be produced in a new and entirely different way. If, in a darkened room the pure violet light of the spectrum be allowed to fall on a plate or wineglass made of uranium glass, these articles will not reflect violet light to the eye as would be expected, but will glow with a bright-green light, looking in the dark- ness almost as though they had suddenly become self-lumi- nous. This kind of glass has, then, the extraordinary prop- erty of entirely altering the colour of the light that falls on it, and of causing the light to assume a quite different hue. But, as colour depends on wave-length, we are led to ask whether this property of the original beam of light is also affected by the uranium glass. Stokes proved conclu- sively that this is the case, and that in all such experiments the length of the wave is made greater. It would appear that the waves of light act on the atoms which make up (or surround) the molecules of the glass, and set them in vibra- tion ; they continue in vibration for some little time after- ward, at a rate of their own selection, which is always less than that of the waves of light which gave the first impulse. Being in vibration, they act as luminous centres, and com-

PRODDCTION OF COLOUR BY FLUORESCENCE, ETC. 63

municate vibrations to the external ether, and this is the green light that finally reaches the eye. The action takes place not only on the surface of the glass, but deep in its interior, so that, if the experiment be made with a thick cube of the glass, it actually appears milky and almost opaque, owing to the abundant flood of soft green light which it pours out in all directions. It is not even neces- sary to employ as a source of illumination the pure violet of the spectrum ; sunlight streaming through blue cobalt glass answers as well, and the sharp change from the violet-blue to the milky-green is quite as astonishing.

Under ordinary daylight uranium glass scatters in all directions a bluish -green light, which is due to the cause above mentioned, but the light which passes through its substance is merely tinged yellow. Both these tints make their appearance in daylight, and by their combination communicate to articles made of this glass a peculiar and rather beautiful appiearance. Candle-light or gas-light fur- nishes but a scanty supply of blue and violet rays, hence this kind of illumination robs uranium glass entirely of its charm, and the articles made of it assume a dull yel- lowish hue which is neither striking nor attractive. There are many salts which have this property in a high degree : among the best known is the platino-cyanide of barium, which presents appearances similar to those above men- tioned. Thallene, an organic substance derived from coal- tar, and described by Morton, must also be classed with uranium glass.* Drawings made with this substance on white paper, by daylight appear yellowish, but when placed under a violet or blue illumination flash into sudden bril- liancy, and scatter in all directions a strong greenish light. There are many liquids which have the same property, and which display different colours when acted on by violet light, but for an account of them we must refer the curi-

* See "Chemical News," December, 1S72.

64 MODERN' CHKOMATICS.

ous reader to. Dr. Pisko's work on the fluorescence of light.*

Before passing from this subject it maybe as well to. add .that phosphorescence also often gives rise to colours which more or less closely resemble those of fluorescence. If tubes filled with the sulphides of barium, strontium, calcium, etc., be placed in a dark room and illuminated for an instant by a beam of sunlight, by the electric light, or by bumiag magnesium wire, they will display a charming set of tints for some minutes afterward. Some will shine with a soft violet light, others will display an orange or yellow glow ; delicate blues will make their appearance, and will contrast well with the red hues, the latter resembling in the dark- ness living coals of fire. The tints, as such, are very beau- tiful and suggestive, though of course no direct application can be made of them to artistic purposes.

* " Die Fluorescenz des Lichtes," F. J. Pisko, Yicnna.

CHAPTER VII.

ON THE PRODUCTION OF COLOUR BY ABSORPTION.

The colours produced by the dispersion, interference, and polarization of light have great interest from a purely scientific point of view, and are also valuable in helping us to frame a true theory of colour, but it is to the phenomena of absorption that the colours of ordinary objects are almost entirely due. The pigments used by painters, the dyes em- ployed by manufacturers, the colouring-matter of flowers, trees, rocks, and water, all belong here. Let us begin our study of this subject with a fragment of stained glass. When we place the glass flat on a surface of black cloth, and expose it to ordinary daylight, we find that it reflects light to the eye just as a piece of ordinary window-glass would under similar circumstances, and this light is white, not coloured. In this experiment, the rays of light which reach the eye have been reflected from the mere surface of the plate of glass, those rays which penetrate its interior being finally absorbed by the black cloth underneath, and never reaching the eye at all. If we now raise the glass and allow the light of the sky to pass through it and fall on the eye, we find that it has been coloured ruby-red. The light of a candle- or gas-flame is affected in the same way, and a beam of sunlight streaming through the plate of glass falls on the opposite wall as an intensely red, luminous spot. Our first and very natural impression is, that the stained glass has the power of altering the quality of light — that the white light is in some way actually transmuted

66

MODERN CHROMATICS.

rCito red light. This seems to he the universal impression among those who have not particularly examined the mat- ter. We saw, in Chapter II., that with a prism we could analyze white light, and sort out the waves composing it according to their length, and that the sensation which- the waves produced on the eye varied with their length, the long- est giving red, the shortest violet. The prism can also be ap- plied to the study of the matter now under consideration. A

Fig. 14.— Eed Glass placed over Slit in Black CardbQard,

screen of black pasteboard is to be prepared with a narrow slit cut in its centre ; over the slit a piece of stained glass is to be fastened, as indicated in Fig. 14. If, now, this arrange- ment be placed in front of a window, matters can be so contrived that white light from a cloud shall fall upon the slit and traverse the stained glass ; it will afterward reach

Pio. IB.— Spectrum of Light transmitted through Eed Gllass.

the prism which will analyze it. On mating this experi- ment we find that the result is similar to what is indicated in Fig. 15 : the prism informs us that the transmitted beam

ox THE PRODUCTION OP COLOUR BY ABSORPTION. 67

consists mainly of red light ; a little orange light is also present. The experiment can, howe%''er, be made in a more instructive way, by covering only half of the slit with the plate of glass. On repeating it with this modification, we obtain side by side an analysis of the white light direct from the cloud, and of the light which has traversed the ruby glass ; the result is indicated .in Fig. 16, and we see

r ■ RIO	D	/EL (	E REEN	F BLUE	VIOLET 1
1 RED	il
mm^		^^1	im^H	^^HHHi

Fia. 16.— Spectrum of White and Ked Light compared.

at a glance that the solution of the whole matter is sim- ply this : the ruby glass is able to transmit the red rays, but it stops all the others ; these last it absorbs — hence we say it produces its colour by absorption. The other rays are in fact converted into heat, and raise the temperature of the glass to a trifling extent. The experiment can be varied somewhat without affecting the result ; if a solar spectrum be projected on a screen, as described in Chapter II., we shall find, when we look at it through the ruby glass, that we can see only the red space, light from the other col- oured spaces not being able to pen'etrate the glass ; and finally, when we hold our plate of glass directly in the paths of the coloured rays, we shall notice that it stops all ex- cept those that are red. These simple fundamental experi- ments prove that the ruby glass does not transmiite white light into red, but that it arrests certain rays, and converts them into a kind of force which has no effect on the eye ; the rays which are not arrested finally reach the eye and produce the sensation of colour.

68

MODERN CHROMATICS.

For more careful examinations of the coloured light transmitted by stained glass a spectroscope with one flint- glass prism can advantageously be used. The stained glass is to be fastened so that it covers one half of the slit, and then we shall have, placed side by side, the spectrum due to the glass and a prismatic one for comparison. In this latter the fixed lines will be present, and we can use them as a kind of natural micrometer for mapping down our results. There is, however, another point to be attended to. When we come to examine the red glass carefully with the spec- troscope, we find that it not only transmits the red rays powerfully, but that a little of the orange rays also passes through with still smaller portions of the gi-een and blue rays. Hence we are dealing not only with spaces in the spectrum, but with the relative intensities of the coloured light filling those spaces. It is difficult, or rather impos-

RED

YCL. GBEEM

BLUE

yiQlET

l\

Fio. 17.— Spectrum, showing the Extent and Intensity of the Golonred Light transmitted by Eed Glass. The shaded portion represents the transmitted hght.

sible, to represent the different intensities by shading on paper ; hence physicists have adopted a certain convention which removes this trouble, and enables them to express differences in luminosity readily and accurately. All this is accomplished by drawing a curve, and agreeing that dis- tances measured upward to it shall represent different de- grees of luminosity. We agree, then, to let the entire rec- tangle A H O N, Fig. 17, represent a solar spectrum, with

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 69

its different colours properly arranged, and having their natural or normal luminosities, and in this rectangle we draw the curve furnished by the red glass (Fig. 17). We find that it is highest in the red space ; but even here it reaches only about half way up, showing that the luminos- ity of the transmitted red light is only half as great as that of the same light in the spectrum ; in the orange space it falls rapidly off, the curve sinking with a steep slope ; after that it runs out into the green and blue, almost to the vio- let, in such a way as to indicate that the red glass transmits minute quantities of these different kinds of coloured light. The luminosity, then, of all the transmitted rays, except the red, being quite feeble, the light which comes through appears pure red. Making an examination of an orange- yellow glass in the same way, we obtain the curve shown in Fig. 18 : this glass, it appears, transmits most of the red.

RED

YEL.

GREEN

BLUE

VIOLET

A,

Fio. 18.— The shaded portion shows the amount of light transmitted by an orange- coloored glass.

orange, and yellow rays, with much of the green and a little of the blue. Here, of course, the orange and yellow rays after transmission make up an orange-yellow hue, and the green and red rays by their union reproduce the same colour, as we shall see in Chapter X. Hence the final colour is orange-yellow, without the least tint of red or green. Taking next a green glass, we obtain another curve. Fig. 19, showing that much green light is transmitted, but along

70

MODERN CHROMATICS.

nEO

YEL.

GREEN

BLUE

VIOLET

B C

^ 0

Fig. 19. — The shaded portion represents the amount of light transmitted hy green glass.

with it .some red and some' blue. Blue glass shows the cyan-blue weakened, the ultramarine-blue and violet strong; the green is very weak, so also are yellow and orange ; the red is mostly absent, except a feeble extreme red. The re- sult is of course a violet-blue (Fig. 20). A purple glass is

RED

YEL.

GREEN

BLUB

VIOLET

Fio. 20.— The shaded portion represents the amount of light transmitted by blue glass.

found to absorb the middle of the spectrum, i. e., the yel- low, green, and cyan-blue ; the red and violet are also en- feebled, but are at all events far stronger than the other transmitted rays. We have, then, as a final result, red, ultramarine-blue, and violet, which being mingled make "purple. It is evident from these experiments that the col- ours produced by absorption are not simple, like those fur- nished by the prism, but are resultant hues, produced by the mixture of many different kinds of coloured light hav-

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 71

ing varying degrees of brightness. On this account, and by reason of the tendency of many kinds of stained glass to absorb to a considerable extent all kinds of coloured light . presented to them, it happens that stained glass furnishes us with coloured light inferior in purity and luminosity to that obtained by the use of a prism. Nevertheless these colours are the purest and most intense that we meet with in daily life, and far surpass in brilliancy and saturation those produced by dyestuflfs or pigments.

There is one property which probably all substances possess which produce colour by absorption, upon which a few words must be now bestowed. If we cause white light to pass through a single plate of yellow glass, the rays which reach the eye will of course be coloured yellow. Add now a second plate of the same glass, and the light which traverses the double plate assumes a somewhat dif- ferent appearance ; it evidently is not so luminous, and its colour is no longer quite the same. Using six or eight plates of the yellow glass, we find that the transmitted light appears orange. If the same experiment be repeated, using a considerable number of plates of the same glass, the colour will change to dark red. From this it appears that the colour of the transmitted beam of light depends somewhat on the thickness of the absorbing medium. This change in the case of some liquids is very considerable : thin layers, for example, of a solution of chloride of chro- mium transmit green light mainly, and so imitate the ac- tion of a plate of green glass ; thick layers of the same liquid transmit less light in general, but the dominant colour is red, and objects viewed through them look as they would, seen through a plate of dark-red glass. This curious property is easily explained by an examination of the action of the liquid on the prismatic spectrum. In Fig. 21 the curve represents the relative intensity of the coloured light in different portions of the spectrum. If we cut off

successively slices of the rectangle, as is done in Figs. 23 4

72 MODERN CHROMATICS.

A B C D E F G

RED

YEU

SREEH

BUIE.

VIOLET

Fig. 21. — ^The sliaded portion represents the amount of light transmitted by chloride of

chromium.

and 23, we obtain the curves corresponding to a greater and greater thickness of liquid, and it is plain that at last we shall have the state of things indicated in Fig. 23 ; the

ABC

H

RED YEL. GREEN BLUE VIOLET

Fig. 22. — Chloride of Chromium; Effect produced by a Thick Layer.

curve is about the same as for red glass (Pig. 17), and the final colour is red. This is an extreme case, but in stained glasses, pigments, dyestuffs, etc., there is generally a ten-

A B C

RED YEL. GREEN BLUE VIOLET

Fia. 28.— Chloride of Chromium ; Kfiect produced by a very Thick Layer.

dency toward the production of effects of this kind, some of which will hereafter be noticed.

The colours of painted glass are similar to those of stained glass in origin and properties ; both are intense, rather free from admixture with white light, and capable

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 73

of a high degree of luminosity. In these respects they far surpass the colours of pigments, which compared with them appear feeble and dull, or pale. Oiying to this circum- stance, chromatic combinations may be successfully worked out in stained glass, which would prove failures if attempt- ed with pigments or dyestuflEs. Hence also the wonderfully luminous appearance of paintings on glass viewed in a prop- erly darkened room : they surpass in some respects oil or water-colour paintings to such a degree that the two are not to be mentioned together. There is no doubt but that glass-painting offers advantages for the production of real- istic eflEects of colour and light and shade, such as the very narrow scale of oil and water-colour utterly denies ; and yet great artists seem to reject this process, and severely confine themselves to work on canvas or paper, choosing to depend for their effects rather on pure technical skiU and artistic feeling.

If we place on a sheet of white paper a fragment of pale- blue glass, it will display its colour, though not so brilliantly as when held so that the light of the window streams through it directly. The reason is very evident : the light which penetrates the glass falls on the paper and is reflected by it back through the glass to the eye. The light then traverses the glass twice, but this is not the only cause of its inferior luminosity, for a double plate of the same glass held before the window appears still far brighter than the single glass on the paper. The other reason is that the paper itself re- flects only a small amount of the light falling on it. Upon examining the matter more closely we find also that the blue glass reflects from its surface quite a quantity of white light, which, when mingled with the coloured light, renders it somewhat pale. If, now, we grind up into a very fine pow- der some of the blue glass, we obtain the pigment known as smalt, and, after mixing it with water, we can wash our white paper with a thin layer of it. When it dries the

74 MODERN CHROMATICS.

paper will be coloured blue, but the hue will be neither so luminous nor so intense as that of the light directly trans- mitted by the blue glass when held before a window. Its origin, however, is similar : the white light after traversing a layer of the minute blue particles reaches the paper, and is reflected backward once more through them toward the eye. In this process many coloured rays suffer absorption, and only a small portion of the constituents of the original beam finally reach the eye. In the original experiment, where the blue glass was simply laid on the white paper, it sometimes happened that the white light regularly reflected from its first surface mingled itself with the coloured light and caused it to look paler, but it was always possible to arrange matters so that this damaging coincidence did not occur.- In the experiment with the blue powder spread on the paper this is impossible, for the surfaces of the little particles lie with all possible inclinations, so that, hold the paper as we will, it is sure to reflect much white along with its coloured light. What we have, then, to expect when pigmeiltB in dry powder are spread on white paper is, that they will reflect only a moderate quantity of colom'ed light to the eye, and that it will be rendered somewhat pale by admixture with white light.

With the aid of a little hand spectroscope these points are readily demonstrated : when we direct the instrument toward our blue paper, we find that all the colours of the spectrum are present in considerable quantities — hence some white light must be reflected from the paper ; we also notice that the red, yellow, orange, and green rays are present in less quantity than in an ordinary prismatic spectrum — Whence the curve for the smalt-paper is like that given in Fig. 24. In making examinations with the spectroscope of the col- oured light reflected from painted surfaces, it is advanta- geous to use simultaneously, along with the strip of painted paper, one which is white and a third which is black. It has been found by the author that paper painted dead-black

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 75

with lampblack, to which has been added just enough spirit varnish to prevent its rubbing off, but not enough to cause it in the least degree to shine, reflects yf^ as much light as white paper. Hence if we set the luminosity of white paper as 100, that of dead-black paper will be 5. Now, when a

ABC

H

RED

Fio. 24.—

,fl??^^^

'/hmv^/'m/////wr/i/,

y£L.

Gfl££N

BUit

VIOLZT

Curve for Bmalt-paper ; the shaded portion represents the light reflected by smalt-paper.

strip of this black paper is placed before the slit of the spec- troscope it acts like white paper seen under a feeble illumi- nation, and consequently furnishes a complete though not a very luminous spectrum. By using, then, a black-and-white strip along with the one which has been painted, we can ascertain several facts which may best be explained with the help of an example. Let us first select vermilion in dry powder, and undertake an examination of its optical proper- ties in this way. We find that the red of its spectrum is about as powerful as the red in the spectrum from white paper, and that the other colours, though all present, are not much if any stronger than those from the black paper. This is all we can demand from any pigment : it reflects to the eye its full share of the rays it professes to reflect, and they are not mingled with more white light than is reflected by dead-black paper. Emerald-green when tested in this way proves sensibly inferior to vermilion : examined in dry powder the green space was bright, but less bright than that from white paper ; the other colours had about the same degree of luminosity as those from the black paper,

76 MODERN CHROMATICS.

except the violet, which was not present. Chrome-yellow reflected the red, orange, yellow, and green rays about as brilliantly as white paper ; the cyan-blue, ultramarine, and violet, about like black paper. Hence the great luminosity of this pigment, for it reflects not only the yellow rays abundantly, but also all the other rays of the spectrum which are distinguished for luminosity. As before re- marked, the sum of these rays makes up yellow. It is plain from these experiments that a painted surface can never be as luminous as one which is white ; the most that can be demanded from a painted surface is, that it should reflect its peculiar coloured light as powerfully as a white surface does ; the very cause of its furnishing coloured light is, that it fails to reflect all the coloured rays equally well. Hence coloured surfaces are always darker than those which are white. If we set the luminosity of white paper as 100, that of vermilion will be about 25, emerald-green 48, and chrome- yellow as high as 75 or 80.

These experiments can now be repeated with the same pigments covered by a layer of water. The surface of the water being quite flat, the spectroscope can be held in such a way as to avoid the light directly reflected from the water, and it then becomes possible to observe certain changes which the presence of the water brings about. In the case of vermilion we find that the blue and violet portions of the spectrum almost entirely vanish, a little of the yellow, orange, and green spaces remains, and the red is nearly as powerful as before. This proves that the presence of the water has greatly diminished the amount of white light re- flected from the surfaces of the particles of pigment, but has not much affected the brilliancy of the reflected col- oured light. Experiments with emerald-green and chrome- yellow give corresponding results ; less light in general is reflected, but it is somewhat purer, there being not so much white light mingled with it. ■ By immersing our pigments in oil or varnish we push these effects still further : the

ON THE PRODUCTION OF COLOUE BY ABSORPTION. 77

pigments appear darker, but the colour is richer, and more nearly free from "white light. The explanation of these changes is well known to physicists : they depend upon the fact that light moving in a rare medium like the air is abun- dantly reflected when it strikes on a dense substance like a pigment ; but if the pigment be placed under water we have then light moving in a dense medium (water), and striking on one which is only a little more dense (pigment) : hence but little white light will be reflected from the sur- face of the small particles. The coloured light which is so abundantly furnished by the pigment, even under water, has its source in reflections which take place in the interior of the somewhat coarsely grained particles of the pigment itself. If the pigment is naturally fine-grained, and also is mixed with a liquid like oil, having about the same optical density as itself, scarcely any light will be reflected from it, coloured or otherwise. Prussian-blue and crimson-lake, ground in oil, are good examples. In order to exhibit their colours it is necessary either to spread them in thin layers over a light surface, or to mix them with a white pigment ; alone by themselves they appear very dark, the Prussian- blue, indeed, almost black. Many other pigments are more or less affected in the same way by the presence of oil or varnish.

From what has been stated above it follows that the medium with which pigments are mixed has an important influence on their appearance. In drawings executed in coloured chalks, and in oU-paintings, we have the two ex- tremes, works in water-colour being intermediate. Hence oil-painting is characterized by the richness of the colouring and the transparency and depth of its shadows, while in pastel drawings the tints are paler, the shadows less intense, and over the whole is spread a soft haze which lends itself readily to the accurate imitation of skies and distances. Changes in the medium are sometimes a source of embar- rassment to the painter. This is particularly true in the

78 MODERN CHROMATICS.

process of fresco-painting, and also to some extent in that of water-colour : as long as the pigment is moist it appears darker than afterward when dry, and it is necessary for the artist in laying on each wash to make a proper allowance for these changes ; this is one of the minor causes that ren- der the process of painting in water-colours tnore difficult than that in oils.

As has already been stated, when we obtain our coloured light from pigments, it is apt to be more mingled with white light than when stained glass is used ; but, besides this, it is inferior to that from stained glass in the matter of lumi- nosity. The range of illumination in our houses is small, so that practically the scale of light at the disposal of the painter in oils or water-colours is quite limited ; in point of fact he is obliged by the necessities of the case to employ means which are quite inadequate : hence the extraordinary care with which he husbands his resom-ces in the matter of light and shade, and his constant struggle for excellence and decision in colouring. Muddy and dirty colours are instantly recognized to be such under a feeble illumination, even though they have passed muster under the blaze of full sunlight. Almost any surface looks beautiful if very brightly illuminated ; the eye is dazzled, and remains un- conscious of defects that are instantly exposed under the feebler light of a gallery.

The colours which are exhibited by woven fabrics are due, like those of stained glass, to absorption. In the case of silk and wool the dye penetrates the fibres through and through, so that under the microscope they have much the same appearance as fine threads of stained glass. When white light falls upon a bundle of such coloiu*ed fibres, a portion is reflected uncoloured from the surface of the top- most fibres, while another portion penetrates to the rear surfaces of these same fibres and there is again subdivided, some rays penetrating still deeper into the bundle, while others returning to the upper surface emerge coloured.

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 79

This process is repeated on each deeper-lying set of fibres, and the result is that a good deal of strongly coloured light is sent to the eye, mingled with a portion due to the surface layers, which is more faintly coloured ; there is in addition a small portion which is quite white. It will be seen that the reflective power of the fibres is an important element in this process, for all the coloured light which reaches the eye is sent there by reflection. If we take similar structures of silk and wool, we can compare directly the lustre or reflective power of the individual fibres, with the aid of a lens mag- nifying ten or fifteen diameters. A silk-cocoon and a piece of white felting answer very well for this pui-pose, and when they are compared under the microscope it is very evident that the natural lustre of the silk is greatly superior to that of the wool. On comparing in this way the felting with white cotton batting, it will be found that the wool surpasses the cotton in lustre, the latter appearing almost dead-white and free from sparkle. It follows from this that the coloured light which is reflected from sUk is more satu- rated or intense, and appears richer, than that from wool. The fibres of silk also can be made to lie in straight, paral- lel, compact bundles, which enables them to reflect the white light in definite directions, whereas woollen fabrics reflect it equally well in all directions. It results from this that a fabric of silk is capable, according to circumstances, of ex- hibiting a rich saturated colour nearly free from white light, or it may reflect much white light and exhibit a pale colour. This sparkling play of colour is beautiful, and causes the more uniform appearance shown by woollen fabrics to appear dull and tame. On the other hand, the superior transparency of the dyed fibres of wool over those of cotton give to the colours of the former material a cer- tain appearance of richness and saturation, and cause the tints of the cotton to appear somewhat opaque.

In velvet the attempt is made to suppress all surface-light, and to display only those rays which have penetrated deeply

80 MODERN CHROMATICS.

among the fibres, and have become highly coloured. This is accomplished by presenting to the light a surface which is entirely composed of the ends of fibres, and consequently which has little or no capacity for reflecting light. The rays then penetrate between the fibres thus set up on end, and, after wandering among them, finally again in some small quantity reach the surface as richly coloured light, which produces its full effect undiminished by any admix- ture of white light from the surface. In the case of sUk- velvet the desired effect is for the most part actually real- ized : the colours are rich, and an examination with a lens shows that scarcely any of the fibres reflect white light, even when the fabric is held in unfavourable positions. If cotton-velvet is subjected to a similar examination under a lens, it will be found to reflect much surface-light, particu- larly when not quite new, and the surface will present a broken, rough appearance, quite different from that of its ' more aristocratic rival.

It would appear that at present it is actually possible to employ for woven fabrics dyes which furnish coloured light having a degree of intensity and purity which is actually undesirable. This is the case ynth some of the aniline dyes. Dresses dyed with some of them, when seen in full daylight, act on the eye so powerfully that mere momentary inspec- tion gives rise to the phenomenon of accidental colours (see Chapter VIII.). These harsh effects are interesting as con- veying certain information that our dyers have already touched, and indeed gone beyond, the greatest allowable limits in the matter of the intensity and purity of then- hues. At least this applies to large surfaces, such as com- plete dresses, etc. In the case of smaller articles, such as ribbons, etc., these intense colours are more allowable, just as the flash of diamonds is more tolerable on account of their insignificant size.

We have seen, thus far, that the colours of pigments and dyestuffs are due to absorption, and to this same cause

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 81

we must attribute the colours of most objects which occur in landscapes. Two of these are so important that it will be worth while to devote a few moments to their separate consideration : we refer to the colour of water, and to that of vegetation. The colour of large masses of water, such as lakes and rivers, is so much influenced by that of the sky that many persons consider it to depend wholly on it, and are disposed to doubt whether water has any proper colour of its own. It is quite true that a small quantity of pure water, such as is contained in a drinking-glass, appears perfectly colourless, and that the light from white objects passes through it without suffering sensible absorption. K, however, we allow the white light from a porcelain plate to traverse a layer of pure distilled water two metres in thick- ness, it will be found to be tinged bluish. This experiment, which was first made by Bunsen, proves that an absorption takes place along the red end of the spectrum, and that water is really coloured in the same sense as a weak solution of indigo. The water of the lake of Geneva is quite pure, being produced mainly by the melting of glaciers ; the granite meal mingled with the water, being coarse, soon settles to the bottom, and leaves it free from turbidity. Hence along the wonderful shores of this lake it is easy to repeat the experiment of Bunsen, and to study the colour of this liquid. White objects, resting on the bottom in the shallow places where the depth is six or eight feet, show very plainly a greenish-blue hue, and the tint can be exam- ined at different depths by lowering a piece of white porce- lain with a string. Even on cloudy days, when the sky is overcast and grey, the lake itself displays a wonderfully in- tense cyan-blue colour, while on clear days, on looking down into its waters, one is tempted to believe that it is a vast natural dyeing-vat. When vegetable matter is present in small quantity the colour of water changes to a bluish-green ; many excellent examples occur among the beautiful lakes of the Tyrol. Decaying organic matter,

82

MODERN CHROMATICS.

on the other hand, tinges water brownish, and lakes or rivers of this colour are apt to assume on cloudy days a silver-grey appearance, while under a clear sky they often appear very decidedly blue. There seems to be some reason to believe that the absorptive action of pure water on white light changes with its temperature, and that warm water is actually more deeply coloured than cold water. Heat has an action of this kind upon many coloured substances, and Wild with his photometer actually found that both distilled and pump water showed somewhat stronger colours on being heated. He accounts in this way for the niore in- tense colour which it is claimed mountain lakes display during the summer months.

The green colour of vegetation offers a rather peculiar . case. When we examine with the spectroscope any ordi- nary green pigment, we find that the red is absent and the blue and violet much weakened, as was the case with em-

A B CPE

SED YEL. GREEN BLUE VIOLEF

Fig. 25.-1116 shaded portioTi represents the light reflected by green leaves.

erald-green. Green leaves, however, furnish a spectrum of a different character : the extreme red is present ; then occurs a deficiency of coloured light, which is followed by ah orange-red space ; next comes the orange, then the yel- low, greenish-yellow, and yellowish-green ; . after this fol- lows a little full green ; the rest of the spectrum decreases rapidly in luminosity. Fig. 25 represents this spectrum. The sum of all these colours is a somewhat yellowish green, which is accordingly the colour presented by green leaves

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 83

in white light. It will be shown in Chapter X. that a mix- ture of red and green light furnishes yellow light, which explains the production of a yellowish-green in this some- what singular way. It follows, from the analysis just given, that green leaves are capable of reflecting a considerable quantity of red Kght, where surfaces painted with green pigments would not have this power, and conseqpiently would appear black or grey. Hence under the red light of the setting sun foliage may assume a red or orange-red hue. Corresponding to this, when the illumination is of an orange colour, foliage will partake more of this hue than would be the case with ordinary -green pigments. Connected with this is also the great change of colour which foliage expe- riences according as it is illuminated by direct sunlight or by light from the blue sky, the tint in extreme cases varying ' from a yellow or slightly greenish yellow up to a bluish-green. Simler has constructed a simple and beautiful piece of apparatus, based on the singular property which living leaves have of reflecting abundantly the extreme red rays of the spectrum ; it is called an erythroscope. A plate of blue glass, stained with cobalt, is to be procured, having a thickness such that it will allow the extreme red of the spectrum to pass, but no orange or yellow ; it should also transmit the small band of greenish-yellow just before the fixed line E, and all the green from b to F> also all the blue and violet. A plate of rather deeply coloured yellow glass is also needed ; this should be capable of transmitting all the light of the spectrum from the farthest red up to G ; that is to say, it should cut off the violet and blue-violet only. When a sunny landscape is viewed through these two glasses, it assumes a most wonderful appearance : all green trees and plants shine with a coral-red colour, as though they were self-luminous ; the sky is cyan-blue,* the clouds purplish-violet ; the earth and rocks assume various

* Cyan-bluc is a greenish-blue.

84 MODEEN CHROMATICS.

tints of violet-grey. Pine-trees appear of a dark-red hue ; orange or yellow flowers become red or blood-red ; greens, other than those of the foliage, are seen in their natural tints, or at least only a little more bluish ; lakes preserve their fine blue-green colouring, and the play of light and shade over the landscape is left undisturbed ; the whole effect is as though a magician's -wand had passed over the scene, and transformed it into an enchanted garden. For the full realization of these effects it is essential that stray light should be prevented from reaching the eyes, and ac- cordingly the glasses should be mounted in an arrangement of wood or pasteboard -which adapts itself to the contours of the face, and excludes as much as possible diffuse light. On comparing the spectrum given by the blue and yellow glasses with that of green leaves, it will be found that the two glasses cut off almost all the green light furnished by the leaves, but allow those green rays of light to pass which the leaves are incapable of supplying.

The colours which metals such as copper, brass, or gold display, are due to absorption. A quantity of white light is reflected from the real surface, but along with it is min- gled a certain amount which has penetrated a little distance into the substance of the metal, and there has undergone reflection ; this last portion is coloured. If we cause this mixture of white and coloured light to strike repeatedly on a metallic surface — for example, such as gold — we constant- ly increase the proportion of light which has penetrated under the surface, and has become coloured. A process of this kind takes place in the interior of a golden goblet ; hence the colour in the inside is deeper and more saturated than on the outside. Some metals, like silver or steel, hard- ly show much colour till the light has been made to strike repeatedly on their surfaces ; when this is done with sUver, the light gradually assumes a yellow colour, while with steel it becomes blue.

ON THE PRODUCnON OF COLOUR BT ABSORPTION. 85

The true colour of metals must not be confounded with that which is often given to them by the presence of a sur- face-film of oxide or sulphide ; such films cause for the most part a bluish appearance, though all the colours of the spectrum may be produced on metals in this way. In fact, the. hue in these cases is due to an interference of light caused by the thin layer of oxide, and is quite distinct from the actual colour of the metal. (See Chapter IV.)

Metals, whether coloured or white, are chiefly remark- able for the large quantities of light which they are capable of reflecting. Measurements made by Lambert have shown that the total amount of light reflected by white paper is about forty per cent, of the light falling on it. Silver, however, is capable of reflecting ninety-two per cent. ; steel sixty per cent., etc.

Polished surfaces, particularly of metals, have another property which adds to their apparent brilliancy, and in- creases their lustrous appearance. Those portions of the surface which are turned away from the light often reflect but little, and look almost black. This sharp contrast en- hances their brilliant, sparkling appearance, and raises them quite above the rank of surfaces coloured by pigments. In consequence of this, metals cannot be used along with pig- ments in serious or realistic painting ; they are quite out of harmony, and produce the impression that the painter has sought to help himself by a cheap trick rather than by em- ploying the true resources of art. In those cases where gold was so extensively used during the middle ages for the backgrounds of pictures of holy personages, or even for the adornment of their garments, the object was far more to produce symbolic than realistic representations, and here the presence of the gold was actually a help, as tending to convey the idea that the painting was not the portrait of an ordinary mortal, but rather a childlike attempt to depict and lavishly adorn the ideal image of a venerated and saintly character. On the other hand, this brilliancy of

86 MODERN CHROMATICS.

gold, with its rich colour, preeminently adapts it for the purposes of inclosing paintings and isolating them from sur- rounding objects. A painted frame or wooden frame, inas- much as its colour belongs to the same order as those con- tained in the picture, becomes as it were an extension of it, and is apt to injure the harmony of its colouring ; and, be- sides this, its power of isolation is inferior to that of gold, on account of its greater resemblance to ordinary surround- ing objects.

Having now considered with some detail the colours that are produced by absorption, it may be well to add a few words concerning the attempts that have been made to reproduce colour by the aid of photography. Photographs render accurately the light and shade, why should they not also record the colours, of natural objects ? In 1848 E. Becquerel announced that he had been able to photograph the colours of a prismatic spectrum falling on a silver plate which had been treated with chlorine. These colors were quite fugitive, lasting only a few minutes. In 1850 Nifepce de Saint- Victor and in 1852 J. Campbell claimed that they had rendered these colours more permanent. In 1863 the former experimenter, by washing the finished plates with a solution of dextrin containing chloride of lead, obtained coloured pictures that lasted twelve hours. In the following year he still further improved his process, the colours last- ing three or four days in rather strong daylight. An ex- amination of the details of these memoirs and of the pic- tures indicates that the colours thus obtained are due to a greater or less reduction of the film of chloride of silver, and are, in fact, produced merely by the interference of light, and consequently have no necessary connection with the hues of the natural objects to which they seem to owe their origin. Hence we must regard this problem as un- solved, and in the present state of our knowledge there'

ON THE PRODUCTION OF COLOUR BY ABSORPTION. 87

does not seem to be any reason to suppose that it ever will be solved. Why should the red rays when acting on a cer- tain substance produce a red compound, the green and vio- let rays green and violet compounds, and so on with all the other coloured rays ? But photography in colour implies exactly this.

This problem has more recently been handled in an en- tirely different manner, and with a more hopeful result, from a practical point. of view. Suppose we place a red glass before a photographic camera, and photograph some object with brilliant colours — a carpet, for example. We shall obtain an ordinary negative picture, which will be en- tirely due to the red light sent by the carpet toward the instrument. Portions of the carpet having a different col- our will not be photographed at all. Next let us hold be- fore the camera a glass which transmits only the yellow rays (if such glass could be found), and we shall obtain a picture of the yellow constituents of the carpet ; the same is to be done with a blue glass. From these three ordinary negatives, three positive pictures are to be made in gelatine, the first being colored with a transparent red pigment, the second with a yellow, the third with a blue pigment. The first sheet of gelatin will contain a red picture, due to the red parts of the carpet ; the second and third, similar yel- low and blue pictures. When these transparent coloured sheets are laid over each other, we shall have a picture cor- rect in drawing, which will roughly reproduce the cplom-s of the carpet. This gives an idea of the plan proposed in 1869 by C. Cross and Ducos du Hauron, for the indirect reproduction of colour by photography. In actual practice the negatives were taken with glasses coloured orange, green, and violet ; these negatives were then made to yield blue, red, and yellow positive pictures. This process has been greatly improved by Albert, of Munich, and by Bier- stadt, of New York. In the final picture the gelatine is dis- pensed with, films of colour, laid on by lithographic stones,

88

MODERN CHROMATICS.

being substituted. The selection of the pigments is neces- sarily left to the judgment of the operator, and in its pres- ent state the process is better capable of dealing with the decided colours of designs made by the decorator than with the pale, evanescent tints of Nature.

APPENDIX TO CHAPTER VII.

We give below a list of pigtoents whioli, according to Field and Linton, are not affected by the prolonged action of light, or by fonl air:

WliUe. Zinc-white. True pearl-white. Baryta-white. Tin-white.

Sed.

Vermilion. Indian red. Venetian red. Light red. Red ochre.

Yellow.

Cadmium-yeilow. Lemon-yellow. Strontia-yellow. Yellow ochre. Raw Sienna. Oxford ochre. Roman ochre. Stone ochre. Brown ochre.

Illach. Ivory-black. Lampblack. Indian ink. Graphite.

Orange vermilion. Jaune de Mars. Orange ochre. Burnt Sienna. Burnt Roman ochre.

Green. Oxide of chromium. Rinman's green. Terre-verte.

Mile. Ultramarine. Blue ochre.

Violet. Purple pchre. Violet de Mars.

Brown. Rubens's brown. Vandyck brown. Raw umber. Burnt umber. Cassel earth. Cologne earth. Bistre. Sepia. Asphalt.

APPENDIX TO CHAPTEK VIL

89

White lead, smalt and cobalt-Wue ave not affected by light, but are by foul air. The last two are considered permanent in water- colour painting.

According to Field, the tints of the following pigments are not affected by mixture with lime, consequently they are adapted for use in fresco-painting :

White.

Baryta. Pearl. Gypaiim. Pure earths.

Sed.

Vermilion. Red lead. Red ochre. Light red. Venetian red. Indian red. Madder red.

Yellow.

Indian yellow. Yellow ochre. Oxford ochre. Roman ocbre. Stone ochre. Brown ochre. Raw Sienna. Naples yellow.

Black.

Ivory-black. Lampblack. Black chalk. Graphite.

Orange. Orange vermilion. Chrome-orange. Orange ochre. Jaune de Mars. Burnt Sienna.

Green. Terre verte. Emerald green. Mountain green. Cobalt-green. Chrome-green.

Blric. Ultramarine. Smalt. Cobalt.

Fwple. Madder purple. Purple ochre.

Brown. Vandyck brown. Bubens's brown. Raw umber. Burnt umber. Cassel earth. Cologne earth. Antwerp brown. -Bistre.

As the effect of light on pigments is a matter of considerable importance to artists, particularly to those working with the thin washes used in water-colour painting, a careful experiment on this

90 MODERN CHROMATICS.

matter was made by the present writer. The washes laid on or- dinary drawing-paper were exposed during the summer to sunlight for more than three months and a half, and the effects noted ; these were as follows :

Watek-colouk Pigments that are not affected by Light:

Red.	Orange.	Yellow.
Indian red.	Jaune de Mars.	Cadmium-yellow.
Light red.		Yellow ochre. Roman ochre.
(h-een.	Blue.	Brown.
Terre verte.	Cobalt.	Burnt umber.
	French blue.	Burnt Sienna.
	Smalt.
	New blue.

The following pigments were all more or less affected ; those that were very little changed head the list, which is arranged so as to indicate the relative amounts of damage suffered, the most fugi- tive colours being placed at its end :

Chrome-yellow becomes slightly greenish.

Red lead becomes slightly less orange.

Naples yellow becomes slightly greenish brown. •

Raw Sienna fades slightly ; becomes more yellowish.

Vermilion becomes darker and brownish.

Aureoline fades slightly.

Indian yellow fades slightly.

Antwerp blue fades slightly.

Emerald green fades slightly.

Olive green fades slightly, becomes more brownish.

Rose madder fades slightly, becomes more purplish.

Sepia fades slightly.

Prussian blue fades somewhat.

Hooker's green becomes more bluish.

Gamboge fades and becomes more grey.

Bistre fades and becomes more grey.

Burnt madder fades sSmewhat.

Neutral tint fades somewhat.

Vandyok brown fades and becomes more grey.

Indigo fades somewhat.

Brown pink fades greatly.

APPENDIX TO CHAPTER VII. 91

Violet carmine fades greatly, becomes brownish. Yellow lake fades greatly, becomes brownish. Crimson lake fades out. Carmine fades out.

To this we may add that rose madder, burnt or brown madder, and purple madder, all, are a little affected by an exposure to sun- light for seventy hours. Pale washes of the following pigments were completely faded out hy a much shorter exposure to sunlight :

Carmine, Yellow lake, Italian pink.

Full red. Gall-stone, Violet carmine.

Dragon's blood. Brown pink.

CHAPTER VIII.

ON THE ABNORMAL PERCEPTION OF COLOUR, AND ON COLOUR-BLINDNESS.

We have considered now, with some detail, the various ordinary modes of producing the sensation of colour ; but, in order to render our account more complete, it is necessary to touch on some of the unusual or extraordinary methods. In every case examined thus far, the sensation of colour was generated by the action on the eye of coloured light — that is, of waves of light having practically a definite length. As colour, however, is only a sensation, and has no existence apart from the nervous organization of living beings, it may not seem strange to find that it can be produced by white as well as by coloured light, or even that it can be developed in total darkness, without the agency of light of any kind whatever. If the eyes be directed for a few m.oments to- ward a sheet of white paper placed on a black background and illuminated by sunlight, on closing them and excluding all light by the hands or otherwise, it will be found that the absence of the light does not at once cause the image of the paper to disappear. After the eyes are closed it will stiU be plainly visible for several seconds, and will at first be seen quite correctly, as a white object on a black ground ; the colour with some observers then changes to blue, green, red, and finally back to blue, the background remaining all the while black. After this first stage the background changes to white, the colour of the sheet of paper appearing blue-green, and finally yellow. Most of these colours are

ON THE ABNORMAL PERCEPTION OF COLOUR, ETC. 93

as distinct and decided as those of natural objects. If the experiment be made for a shorter time, and under a less brilliant illumination, the eyes being first well rested by- prolonged closure, the series of colours will be somewhat different. Fechner, Seguin, and Helmholtz observed that the original white colour passed rapidly through a greenish blue (Seguin, green). into a beautiful indigo-blue ; this af- terward changed into a violet or rose tint. These colours were bright and clear, afterward followed a dirty or grey orange ; during the presence of this colour the background changed from black to white, and the orange tint altered often into a dirty yellow-green which completed the series. If, instead of employing white, a coloured object on a grey ground is regarded intently for some time, the eyes will be so affected that, on suddenly removing the coloured object, the grey ground will appear tinged with a complementary

Fio. 26.— Disk with Black and Fig. 27.— Black and White Spiral

White Sectors for the Produo- on Disk, for the Production of

tion of Subjective Colour. Subjective Colour.

tint ; for example, if the object bo red, the after-image will be bluish green. It is not necessary to dwell longer on these phenomena at present, as a portion of Chapter XV. will be especially devoted to them. In both the cases men- tioned above, the colour develops itself after the eyes are.^. closed, or at least withdrawn from the illuminated surface. ■ There are, however, cases where very vivid colours can be seen while the eyes are exposed to full daylight. If a disk

94

MODERN CHEOMATICS.

of cardboard painted witli alternate white and black sectors, like that shown in Fig. 26, be set in rotation while exposed to full daylight, colours will be seen after a few trials. It will be found that a certaui rate of rotation communicates to the disk a green hue, a somewhat more rapid rate causing it to assume a rose colour. According to Helmholtz, these effects are most easily attained by using a disk painted with a black spiral, like that in Fig. 27. These phenomena may be 9.dvantageously studied by a method which was used by the author several years ago. A "blackened disk with four open sectors seven degrees in width was set in revolution by clockwork, and a clouded sky viewed through it. With

Fig. 28.— Subjective Colours seen in Sky, with aid of

Kotating Bisk.

Fia. 29.— Subjective Colours, Kinj, etc., seen in Sky witli aid of Eo- tating Disk.

a rate of nine revolutions per second, the whole sky often appeared of a deep crimson hue, except a small spot in the centre of the field of view, which was pretty constantly yel- low. Upon increasing the velocity to eleven and a half revolutions per second, the central spot enlarged somewhat, and became coloured bluish green, with a narrow, faint, blue border, indicated by the dotted line ; the rest of the sky appeared purple, or reddish purple. (See Fig. 28.) With the exception of fluctuation's in the outline of the spot, this appearance remained tolerably constant as long as the rate of revolution was steadily maintained. When the velocity

ON THE ABNORMAL PERCEPTION OP COLOmi, ETC.

95

of the disk was increased, the bluish-green spot expanded into an irregularly shaped blue-green ring, which with a rate of fifteen turns per second mostly filled the whole field of view. (See Fig. 29.) With higher .rates all these ap- pearances vanished, and the sky was seen as with the naked eye.* More than one elaborate attempt has been made to found on phenomena of this class a theory of the production of colour, though it may easily be shown that in all such cases the disk really transmits not coloured but white light, and that the effects produced are due to an abnormal state of the retina caused by alternate exposure to light and darkness.

A current of electricity is also capable of stimulating the optic nerve in such a way that brilliant colours are per- ceived, although the experiment is made in perfect dark- ness. If the cm-rent of a strong voltaic battery be caused to enter the forehead, and travel hence to the hand, accord- ing to Ritter, a bright-green or bright-blue colour is per- ceived, the hue depending on whether the positive cm-rent enters the hand or forehead. Helmholtz, in repeating this operation, was conscious simply of a wild rush of colours without order. The experiment is, however, interesting to us, as proving the possibility of the production of the sen- sation of colour without the presence or action of light.

Recently a substance has been discovered which, when swallowed, causes white objects to appear coloured greenish yellow, and coloured objects to assume new hues. Persons under the influence of santonin cannot sec the violet end of the spectrum ; and this fact, with others, proves that they have become temporarily colour-blind to violet.

An observation of Tait's, and others by the author, have shown that a shock of the nervous system may produce momentarily colour-blindness to green light. White objects then appear of a purplish red, and green objects of a much

• "American Journal of Science and Arts," September, 1860. 5

96 MODERN CHROMATICS.

duller green hue than ordinarily.* These effects are eva- nescent, though quite interesting, as we shall see presently, from a theoretical point of view.

Investigations during the present century have shown that many persons are born with a deficient perception of colour. In some the defect is slight and hardly noticeable, while in others it is so serious as to lead to quite wonderful blunders. This imperfection -of vision is often inherited from a parent, and may be shared by several members of the same family. It is remarkable that women are com- paratively free from it, even when belonging to families of which the male members are thus affected. The occupations of women, their attention to dress and to various kinds of handiwork where colour enters in as an important element, seem to have brought their sense for colour to a higher de- gree of perfection than is the case with men, who ordinarily neglect cultivation in this direction. Out of forty-one young men in a gymnasium, Seebeck found five who were colour-blind ; but during his whole investigation he was able to learn of only a single case where a woman was to some extent similarly affected. It not unf requently happens that persons with this defect remain for years unconscious of it. This was the case with some of the young men in- vestigated by Seebeck ; and in one remarkable instance a bystander, in attempting to help a colour-blind person who was under investigation, showed that he was himself colour- blind, but belonged to another class ! The commonest case is a deficient perception of red. Such persons make no dis- tinction between rose-red and bluish-green. They see in the spectrum only two colours, which they call yellow and blue. Under the name yellow they include the red, orange, yellow, and green spaces : the blue and violet they name, with some correctness, blue. In the middle of the spectrum

* "American Journal of Science and Arts," January, ISY'?. A similar observation by Charles Pierce was communicated to the author while this work was going through the press.

ON THE ABNORMAL PERCEPTION OF COLOUR, ETC. 97

there, is for them a neutral or grey zone, which has no colour ; this, according to Preyer, is situated near the line F. For the normal eye it is greenish-blue ; for them, white. The extreme red of the spectrum, when it is faint, they fail to distinguish ; the rest of the red space appears to them of a saturated but not luminous green ; the yellow space has for them a colour which we should call bright green ; and finally, they see blue in the normal manner. Maxwell found that by the aid of his disks, using only two colours, along with white and black, he was able, by suitable variations in their proportions, to match for them any colour which pre- sented itself ; while the normal eye requires at least three such coloured disks, besides white and black. His experi- ments led to the result that persons of this class perceive two of the three fundamental colours which are seen by the normal eye. Helmholtz also arrived at the same result. It is possible to render the nonnal eye to some extent colour- blind to red in the manner followed by Seebeck in 1837, and afterward by Maria Bokowa. These observers wore for several hours spectacles provided with ruby-red glasses ; and this prolonged action of the red light on the eye finally, to a considerable extent, tired out the nerve fibrils destined for the reception of red, so that on removing the glasses they saw in the spectrum only two colours. The second observer called them yellow apd blue. Furthermore, the extreme red end of the spectrum was not visible to her, just as is the case with those who are actually blind to red ; all red objects appeared to her yellow, and dark red was not distinguishable from dark green or brown.

Dalton, the celebrated English chemist, suffered from this defect of vision, and was the first to give an accurate description of it ; hence this affection is sometimes named after him, Daltonism. It is very remarkable that, accord- ing to the observations of Schelske and Helmholtz, even in the normal eye there are portions which are naturally colour- blind to red, and when this zone of the eye is used the same

98 MODERN CHROMATICS.

mistakes in matching colours are made. Such experiments are somewhat difficult to make without considerahle prac- tice, as it is necessary that the colored objects should be viewed, not directly, but by the eye turned aside somewhat. There is a simple means by which persons who are colour- blind to red can to some extent help themselves, and prevent the occurrence of coarse chromatic blunders, such as con- fusing red with green. Green glass does not transmit red light ; hence, on viewing green and red objects through a plate of this glass it will be found, even by persons who are colour-blind, that the red objects appear much more dark- ened than those which are green. On the other hand, a red glass will cause green objects to appear darker, but will not affect the luminosity of those having a tint similar to itself. The exact tints of the glasses are important, and they should of course be selected with the aid of a normal eye.

The kind of colour-blindness just described is rather common, and it has been estimated that in England about one person in eighteen is more or less afflicted with it. We pass on now to the consideration of a class of cases that is more rare. Persons belonging to this second class see only two colours in the spectrum, which they call red and blue. They set the greatest luminosity in the spectrum in the yellow space, as is done by the normal eye ; and they easily distinguish between red and violet, but confuse green with yellow and blue with red. In two cases examined by Preyer, yellow appeared to them as a bright red ; this same observer also found that in the spectrum, near the line h, the two colours into which they divided the spectrum were separated by a small neutral zone, which was for them identical with grey. A sufficient number of observations have not been accumulated to furnish means of ascertaining with certainty the exact nature of the difficulty under which they labour, though it is probable that they are colour-blind to green light. There are also observations on record of cases of temporary colour-blindness of a third kind, where the violet

ON THE ABNORMAL PERCEPTION OF COLOUR, ETC. 99

end of the spectrum was seen shortened to a very remark- able extent ; and if it should prove that the cause was of a nervous character, rather than due to a deeper yellow col- ouration of the axial portions of the retina, this would demonstrate the existence of violet colour-blindness.

The subject of colour-blindness is one of considerable iniportance from a practical point of view, and this defect has no doubt been the occasion of railroad accidents. In 1873-75 Dr. Favre, in France, examined one thousand and fifty railroad officials of various grades, and found among them ninety-eight pei-sons who were colour-blind — that is, 9"33 per cent. In 1876 Professor Holmgren, in Sweden, examined the entire personnel of the Upsala-Gefle line, and out of two hundred and sixty-six persons ascertained that thirteen were colour-blind. These were found in every grade of the service, many of them being required daUy to make use of coloured signals. It is singular that in no case nad there been previously any suspicion of the existence of the defect. For further information with regard to the practical side of this matter, the reader is referred to the essay of Holmgren, which will be found in the Smithsonian Report for 1877 : a French translation also exists.

In concluding this subject, it may not be amiss to allude to the very remarkable case described by Huddart, of a shoemaker, an intelligent man, where only a trace of the power to distinguish colours seemed to remain.* According to the observations, he was colour-blind to both red and green, and in general seems to have had hardly any percep- tion of colour, as distinguished from light and shade. Curi- ously enough, recent observations of Woinow show that even in the normal eye there is a condition like this at the farthest limit of the visible field of view ; here all distinc- tions of colour vanish, and objects look merely white or black, or grey. It is probable that between the case of

* " Philosophical Transactions," Ixvii.

100 MODERN CHROMATICS.

Harris, just mentioned, and that of a normal eye possessed of the maximum power of perceiving and distinguishing colours, a great number of intermediate gradations will be found to exist. Slight chromatic defects of yision generally receive no attention, or are explained in some other way. The writer recalls the case of an excellent physicist who for many years had a half suspicion that he was to some extent colour-blind, but rather preferred to explain the dis- crepancies by the assumption of a difference in nomencla- ture. Taking up the matter at last seriously, be made an investigation of his own case, and found that he actually was to some extent colour-blind to red. It has been sug- gested that the very inferior colouring of some otherwise great artists can be accounted for by supposing them to have been affected with partial colour-blindness ; this idea is plausible, but, as it appears to us, totally without proof. There are great numbers of persons who are able to hear distinctly all the notes employed in music, who yet have no talent for it and no enjoyment of it. On the other hand, we know of cases of persons who from infancy have been afflicted with partial deafness, and have nevertheless been musicians, and even composers. It is the same in painting as in music : the possession of a perfect organ is not by any means the first necessity, and it can be proved that even artists who are actually colour-blind to red may still, with but slight external aid, produce paintings which are univer- sally prized for their beautiful colouring. Their field of operations is of course more restricted, and they are com- pelled to avoid certain chromatic combinations. During the evening, by gas- or lamp-light, we are all somewhat in the condition of persons who are colour-blind to violet ; but yet, with precautions and some patience, it is possible to execute works in colour, even at this time, which afterward stand the test of daylight. It would appear probable, then, that the difficulty with the inferior colourists above alluded to was not so much anatomical or physiological as psychical.

ON THE ABNORMAL PERCEPTION OF COLOUR, ETC. 101

According to a theory recently proposed by Hugo Mag- nus, our sense for colour has been developed during the last four or five thousand years ; previous to this period it is assumed that our race was endowed only with a perception of light and shade. The evidence which is offered is of a philological character, and tends to show that the ancients either distinguished or described colours less accurately than the modems. The same kind of reasoning might be applied to proving that children at the present day have but little power of distinguishing tints, as they usually scarcely notice any but the most intense colours. When, however, we study the prehistoric races at present existing on the globe, and living in the same style as their ancestors, we find them quite capable of discriminating colours, and often very fond of them. Going many steps lower, we meet with animals that have the power of perceiving and even imitating a series of colours with accuracy. This is the case with the chameleon, as shown by P. Bert, and also, according to Pouchet and A. Agassiz, with certain kinds of flounders. The skin of the chameleon is provided with an immense number of minute sacs filled with red, yellow, and black liquids ; the animal has the power of distending these star-like vesicles at pleasure, and thus adjusts its colour in a few minutes (after a series of trials) so as to match that of the surface on which it is placed. Its chromatic scale covers red, orange, yellow, and olive-green, and the mix- tures of these colours with black, which includes of course an extensive series of browns. The olive-green is made by distending the yellow and black sacs, the effect being simi- lar to that obtained by combining a black and yellow disk. . (See Chapter XII.)

Corresponding to this, A. Agassiz has often noticed, when a young flounder was transferred from a jar imitating in colour a sandy bottom to one of- a dark-chocolate hue, that in less than ten minutes the black-pigment cells would obtain a great preponderance, and cause it to appear wholly

102 MODERN CHROMATICS.

unlike the yellowish-grey speckled creature which a few moments before had so perfectly simulated the appearance of sand. When removed to a gravelly bottom, the spots on the side became prominent.

If our sense for light and shade is old, but that for colour recent and still undergoing development, we should perhaps expect that it would require more time to recog- nize colour than appearances dependent simply on light and shade ; but, according to the experiments of the writer, forty billionths of a second answers as well for one as for the other act of perception.*

In closing this chapter, it may be well, to mention a very simple but beautiful experiment, by which we all can easily place ourselves in a condition somewhat like that of Harris, where all or nearly all sensation of colour had vanished. If some carbonate of soda be ignited in the flame of a Bun- sen burner, it will furnish an abundance of pure homoge- neous light of an orange-yellow hue. This light is quite bright enough to illuminate objects in a darkened room, but all distinctions of colours vanish, light and shade only remaining. A red rose exhibits no more colour than its leaves ; gayly painted strips of paper show only as black or white or grey ; their colours can not even be guessed at. The human face divested of its natural colour assumes an appearance which is repulsive, and the eye in the absence of colour dwells on slight defects in the clearness and smoothness of the complexion. If now an ordinary gas- burner be placed near the soda flame, and allowed at first .to burn with only a small flame, objects will resume their natural tints to some slight extent, and begin again faintly to clothe themselves with pleasant hues, which will deepen as more light is furnished, till they finally seem fairly to

* The amount of time necessary for Tision. " American Journal of Sci- ence," September, 1871.

APPENDIX TO CHAPTER VIII.

103

glow with radiant beauty. Those who have never wit- nessed an experiment of this kind have but little conception how great would be to us the loss of our sense for colour, or how dreary the world would seem, divested of the fasci- nating charm which it casts over all things.

APPENDIX TO CHAPTER VIII.

Maxwell has published an account of his rather elaborate examination of the case of one of his students, who was colour- blind to red.* ■ An apparatus was employed by which the pure colours of the spectrum could be mixed in any proportion ; these colours were then mingled by the colour-blind person, in such a proportion as to produce to his eyes the effect of white. In this way the following equation was obtained: 33-7 green + 33-1 blue = white. Maxwell then, employing the same colours, obtained his own or a normal equation, which was ; 26 green + ST'-i blue + 22'6 red = white. If we combine these equations by subtraotipn, we obtain : 32'6 red — 7'7 green + 4'3 blue = D ; D being the missing colour not perceived by the colour-blind. The sensation, then.

^

/6R^

/'blX

^

ABCDEF G H

Fig. so.— Curves of a Colour-blind Person. (Maxwell.)

which Maxwell had in addition to those of the colour-blind person was somewhat like that of a full red, but different from it in that the full red was mixed with 7'7 green, which had to be removed from it, and 4'3 of blue substituted. The missing colour, then, ac- cording to this experiment, was a crimson-red. Even normal eyes vary a little, and, if this examination had been made by Maxwell's

* " Philosophical Transactions " for 1860, vol. cl., p. 78.

104

MODERN CHKOMATICS.

assistant (observer K), the result would have been a red mixed with less blue, consequently a colour much more like the red of the spectrum. From experiments of this kind Maxwell was able to construct the curves of intensity of the two fundamental colours which are perceived by those who are colour-blind to red ; these curves are shown in Fig. 30. The letters A, B, 0, D, etc., mark the positions of the fixed lines in the solar spectrum ; the curved line marked GE exhibits the intensity of the green element, the line marked BL that of the blue or violet. It wUl be noticed that the green sensation attains its maximum about half way between the lines D and E, that is, in the yellowish green; whUe the high- est point of the other curve is about half way between F and G, that is, in the blue space. Maxwell also constructed similar inten- &ity curves for a normal eye; they are represented in Fig. 31, the

BED r

\ Gff.

^

BL.

Fig. 81. — Curves of Normal Eye. (Maxwell.)

curve for red being indicated by a heavy Une, the others as above. The green and blue curves have about the same disposition as with the colour-blind person, whUe the red attains its maximum between 0 and D, but nearer D — ^that is, in the red-orange space.

A set of observations was also made by Maxwell on the same colour-blind gentleman, with the aid of coloured disks in rapid ro- tation ; and, from the colour equations thus obtained, the positions of the colours perceived by him were laid down in Newton's dia- gi-aro, in a manner similar to that explained in the appendix to Chapter XIV. In Fig. 32, V shows the position assumed for red or vermilion ; U, that of ultramarine- blue ; and G,. that of emerald-

APPENDIX TO CHAPTER VIII. 105

green. They are placed according to Maxwell's method, at the three angles of an equilateral triangle. W would be the position of white for a normal eye, and Y that of chrome-yellow. D is the position of the defective colour, which Maxwell was able to imitate by mingling, by the method of rotating disks, 86 parts of vermilion and 14 of ultramarine-blue. A line drawn from D through W con- tains along its length the various shades of grey and the white of the colour-blind. The grey which they perceive when green and

Fia.82.— Newton'B Dlwram for a Person Fio. 88.— Newton's Diagram for a Per-

Colour-blind to Red. (Maxwell.) son Oolour-bllnd to the Fundamental

Bed.

blue are mixed lies at w; the white of white paper, i. e., a more luminous grey, was on the same line but considerably farther along outside.

It may pei'haps be as well to add to the above one or two re- marks concerning the construction of Newton's diagram for the colour-blind. Let us suppose that the pure colours of the spectrum are employed, and that the missing colour is the fundamental red ; we then place the fundamental green at G, Fig. 33, the fundamental blue or violet at U, and the missing red at D. Then along the line U Q- will lie mixtures of blue and green, and at w will be the white of the colour-blind person. Along the line D G will be situated various shades of green, from dark green to bright green, the latter colour predominating as we approaolj G. Along the line D U we shall have various shades of blue, from bright blue to dark blue, the colour being very dark near D and very bright near U. A line like the dotted one (Fig. 33) will contain various shades of green, fi'om light green to dark green, but none of them so intense as

106 MODERN CHROMATICS.

those situated along the line D G- ; in other words, they all will be mingled with what the colour-blind call white.

If the defective colour-sensation is supposed still red, but to be only partially absent, the diagram takes the form indicated in Fig. 34: ; that is, red, instead of occupying the position at one of the angles of an equilateral triangle, wiU be moved up toward the centre to E'. White will also be shifted from W to w, and the white of a person thus affected would appear to the normal eye of a somewhat green- ish-blue hue. Between D and K' lie, so to speak, mixtures of red with darkness, and along the line E' G- wiU be various mixtures of red and green, in which, according to a normal eye, the green element quite predominates ; that is to say, their orange is more

Fig. 84. — Newton^B Diagram for a Per- Fig. 35. — ^Newton's Diagram for Lamp-

fion partially Colour-blind to Bed. light Illuxninatlon.

like our yellow, their yellow like our greenish yellow, etc. Along the line E' U will be their mixtures of red and blue, or a series of purples, which will be more bluish than ours.

The condition of the normal eye by lamp-light is shown in Fig. 35. The blue or violet is moved from U, its position by daylight, up to M ; white is moved from W to w — that is, into a region that would be called by daylight yellow. Yellow itself, T, is not far Irom this new representative of white, and consequently by candle- light appeal's always whitish. In the purples, along the line E u, the red element predominates ; and in the mixtures of green and blue, along the line G u, the green constituent has the upper hand.

If we were colour-blind to every kind of light except red, then the colour diagram would assume a form similar to that shown in Fig. 86, D representing the darkest red perceptible to eyes so consti- tuted. This sensation would be brought about by pure feeble red

APPENDIX TO CHAPTER VIH.

107

light, or by a mixture of intense green and blue light, or by either of the latter. As we advance from D toward E, the red light gains in brightness, and out at v> becomes very bright and stands for white. When a red glass is held before the eyes, something approximating to this kind of vision is produced.

Fio. 86,— Newton's Diagram for Persons Colonr-bltod to Green and Violet.

CHAPTER IX.

THE COLOUR THEORY OF YOUNG AND HELMHOLTZ.

It is well known to painters tliat approximate represen- tations of all colours can be produced by the use of very- few pigments. Three pigments or coloured powders wUl suifiee, a red, yellow, and a blue ; for example, crimson- lake, gamboge, and Prussian blue. The red and yeUow mingled in various proportions will furnish different shades o^ orange and orange-yeUow ; the blue and yellow will give a great variety of greens ; the red and blue aU the purple and violet hues. There have been instances of painters in water-colours who used only these three pig- ments, adding lampblack for the purpose of darkening them and obtaining the browns and greys. Now, though it is not possible in this way to obtain as brilliant representatives of the hues of nature as with a less economical palette, yet substitutes of a more or less satisfactory character can actu- ally be produced in this manner. These facts have been known to painters from the earliest ages, and furnished the foundation for the so-called theory of three primary colours, red, yellow, and blue. The most distinguished defender in modern times of this theory was Sii- David Brewster, so justly celebrated for his many and brilliant optical discov- eries. He maintained that there were three original or fun- damental kinds of light, red, yellow, and blue, and that by their mixture in various proportions all other kinds of col- oured light were produced, in the manner just described for pigments. Brewster in fact thought he had demonstrated

THE COLOUK THEORY OF YOUNG AND HELMHOLTZ. 109

the existence in the spectrum itself of these three sets of fundamental rays, as well as the absence of all others ; and his great reputation induced most physicists for more than twenty years to adopt this view, Airy, Melloni, and Draper alone dissenting. This theory of the existence of three fundamental kinds of light, red, yellow, and blue, is found in all except the most recent text books on physics, and is almost universally believed by artists.- Nevertheless, it will not be diiEcult to show that it is quite without founda- tion. If we look at the matter from a theoretical point of view, we reach at once the conclusion that it can not be true, because outside of ourselves there is no such thing as colour, which is a mere sensation that varies with the length of the wave producing it. Outside of and apart from ourselves, light consists only of waves, long and short — in fact, of mere mechanical movements ; so that Brewster's theory would imply that there were in the spectrum only three sets of waves having three different lengths, which we know is not the case. If we take up the matter experi- mentally, we meet with no better result. According to the theory now under consideration, green light is produced by mixing blue and yellow light. This point can be tested

Fig. 87.~MaxweIl'8 Disks. Blue and Yellow Disks in the Act of being combined.

with Maxwell's coloured disks. A circular disk, painted with chrome-yellow and provided with a radial slit, is to be combined with one prepared in the same way and painted with ultramarine-blue. Fig. 37 shows the separate disks, and in Pig. 38 they are seen in combination. If the com- pound disk be now set in quite rapid rotation, the two kinds

110

MODERN CHKOMATICS.

of coloured light will be mmgled, and the resultant tint can be studied. It will not be green, but yellowish grey or

Fig. 88— Blue and Yellow Disks in Combination.

reddish grey, according to the proportions of the two col- ours. These disks of Maxwell are ingeniously contrived so as to allow the experimenter to mingle the two colours in

Fio. 89.— Apparatus of Lambert for mixing Coloured Light.

any desired proportion ; but, vary the proportions as we may, it is impossible to obtain a resultant green hue, or in-

THE COLOUR THEOKY OF YOUNG AND HELMHOLTZ. Ill

deed anything approaching it. Another way of making this experiment is simply to use a fragment of window-glass of good quality, as was done by Lambert and Helmholtz. This apparatus is shown in Fig. 39. The glass is supported in a vertical position about ten inches above a board painted black, and on either side of it are placed the coloured papers. The blue paper is seen directly through the glass, while the light from the yellow paper is first reflected from the glass and then reaches the eye. The result is that the two images are seen superimposed, as is indicated in Fig. 40. The relative luminosity or brightness of the two im-

1

Fio. 40.— Eesutt furnished by the Apparatus.

ages can be varied at will ; for instance, moving the papers further apart causes the blue to predominate, and bringing them nearer together produces the reverse effect. In this manner the resultant tint may be made to run through a variety of changes, which will entirely correspond to those obtained with the two circular disks ; but, as before, no tendency to green is observed. Helmholtz has pushed this matter still further, and has studied the resultant hues pro- duced by combining together the pure colours of the spec- trum. The following experiment, which is easy to make, will give an idea of the mode of proceeding : A blackened screen of pasteboard is pierced with two narrow slits, ar- ranged like those in Fig. 41. The light from a window is allowed to shine through the two slits and to fall on a prism of glass placed just before the eye, and distant from the slits about a metre. Then, as would be expected, each slit

112

MODEEN CHROMATICS.

furnishes a prismatic spectrum, and owing to the disposition of the slits, the two spectra will overlap as shown in Fig. "42, which represents the red space of one spectrum falling

Fig. 41.— Two Slits arranged for mixing Two Spectra.

on the green space of its companion. By moving the slits fur- ther apart or nearer together, all the different kinds of light which the spectrum contains may thus be mingled. Using a more refined apparatus, Helmholtz proved that the union of the pure blue with the pure yellow light of the spectrum produced in the eye the sensation, not of green, but of white light. Other highly interesting results were also obtained by him- during this investigation ; these will be

RED	0. >	GREEN ORMQE	BLUE	^^^^^1
	is.fr.		B.miET
■	1	RED	p.	K; 6REEN	BLUE VIOLET

Wm. 42.— Two Overlapping Spectra. Bed and Green are mixed, also Violet and Blue, etc.

considered in the following chapter, but in the mean while it is evident that this total failure of blue and yellow light to produce by their mixture green light is necessarily fatal

THE COI/OUR THEOEY OF YOUNG AND HELMHOLTZ. 113

to the hypothesis of Brewster. Helmholtz also studied the nature of the appearances which misled the great English optician, and showed that they were due to the fact that he had employed an impure spectrum, or one not entirely free from stray white light.

As has been remarked above, there can be in an objec- tive sense no such thing as three fundamental colours, or three primary kinds of coloured light. In a totally differ- ent sense, however, something of this kind is not only pos- sible, but, as the recent advances of science show, highly probable. We have already seen in a previous chapter that in the sp^lar spectrum the eye can distinguish no less than a thousand different tints. Every small, "minute, almost invisible portion of the retina of the eye possesses this power, which leads us to ask whether each atom of the ret- ina is supplied with an immense number of nerve fibrils for the reception and conveyance of this vast number of sensa- tions. The celebrated Thomas Young adopted another view : according to him, each minute elementary portion of the retina is capable of receiving and transmitting three different sensations ; or we may say that each elementary portion of its surface is supplied with three nerve fibrils, adapted for the reception of three sensations. One set of these nerves is strongly acted on by long waves of light, and produces the sensation we call red ; another set re- sponds most powerfully to waves of medium length, produc- ing the sensation which we call green ; and finally, the third set is strongly stimulated by short waves, and generates the sensation known as violet. The red of the spectrum, then, acts powerfully on the first set of these nerves ; but, according to Young's theory, it also acts on the two other sets, but with less energy. The same is true of the green and violet rays of the spectrum : they each act on all three sets of nerves, but most powerfully on those especially designed for their reception. All this will be better un- derstood by the aid of the accompanying diagram, which is

114

MODERN CHROMATICS.

taken from Helmholtz's great work on " Physiological Op- tics." In* Fig. 43, along the horizontal lines 1, 2, 3 are

Fig. 43. — Curves showing the Action of the Different Colours of the Spectrum on the Three Sets of Nerve Fibrils. (Helmholtz.)

placed the colours of the spectrum properly arranged, and the curves above them indicate the degree to which the three kinds of nerves are acted on by these colours. Thus we see that nerves of the first kind are p'owerf ully stimu- lated by red light, are much less affected by yellow, still less by green, and very little by violet light. Nerves of the second kind are much affected by green light, less by yellow and blue, and still less by red and violet. The third kind of nerves answer readily to violet light, and are suc- cessively less affected by other kinds of light in the follow- ing order : blue, green, yellow, orange, red. The next point in the theory is that, if all three sets of nerves are simultaneously stimulated to about the same degree, the sensation which we calt white will be produced. These are the main points of Young's theory, which was published as long ago as 1802, and more fully in 1807. Attention has within the last few years been called to it by Helmholtz, and it is mainly owing to his labours and those of Maxwell that it now commands such respectful attention. Before making an examination of the evidence on which it rests,

THE COLOUR THEORY OP YOUNG AND HELMHOLTZ. 115

and of its applications, it may be well to remember, as Helmholtz remarks, tbat the choice of these three particular colours, red, green, and violet, is somewhat arbitrary, and that any three could be chosen which when mixed together would furnish white light. If, however, the end and mid- dle colours of the spectrum (red, violet, and green) are not selected, then one of the three must have two maxima, one in the red and the other in the violet ; which is a more complicated, but not an impossible supposition. The only known method of deciding this point is by the investigation of those persons who are colour-blind. In the last chapter it was shown that the most common kind of this affection is colour-blindness to red, which indicates this colour as being one of the three fundamental sensations. But, if we adopt red as one of our three fundamental colours, of necessity the other two must be green and violet or blue- violet. Red, yellow, and blue, for example, will not pro- duce white light when mingled together, nor will they under any circumstances furnish a green. Red, orange, and blue or violet would answer no better for a fundamental triad. In the preceding chapter it was also shown that colour-blindness to green exists to some extent, though by no means so commonly as the other case. Hence, thus far, the study of colour-blindness has furnished evidence in favour of the choice of Young, and its phenomena seem explicable by it.

Let us now examine the explanation which the theory of Young furnishes of the production of the following colour- sensations, which are not fundamental, viz. :

Orange-red. Eed-ovange. Orange-yellow.

Yellow.

Greenish-yellow.

Yellowish-green.

Bluish-green.

Cyan-blue.*

Ultramarine-blue.

Starting with yellow, we find that,. according to the theory under consideration, it should be produced by the joipt

* Cyan-blue is a greenish-blue.

116 MODERN CHKOMATICS.

stimulation of the red and green nerves ; consequently, if we present simultaneously to the eye red and green light, the sensation produced ought to be what we call yellow. This can be most perfectly accomplished by mixing the red and green light of the spectrum ; it is possible in this way to produce a fair yellow tint. The method of rotating disks furnishes, when emerald-green and vermilion are em- ployed, a yellow which appears rather dull for two reasons : first, because the pigments which we call yellow, such as chrome-yellow or gamboge, are, as wUl hereafter be shown, relatively more brilliant and luminous than any of the -red, green, blue, or violet pigments in use ; so that these bright- yellow pigments stand in a separate class by themselves. This circumstance influences our judgment, and, finding the resultant yellow far less brilliant than our (false) standard, chrome-yellow, we are disappointed. The second reason is, that green light stimulates, as before mentioned, the violet as well as the green nerves ; hence all three sets of nerves are set in action to a noticeable extent, and the sensation of yellow is mingled with that of white, and consequently is less intense than it otherwise would be. When the green and red of the spectrum are mingled, we have at least not to contend with a false standard, and only the second reason comes into play, and causes the yellow thus produced to look as though mingled with a certain quantity of white. It was found by the lamented J. J. Mtiller that green light when mingled with any other coloured light of the spec- trum diminished its saturation, and caused it to look as though at the same time some white light had been added. This is what our fundamental diagram (Fig. 43) would lead us to expect ; it is quite in consonance with the theory of Young and Helmholtz.

Having now accounted for the fact that the yellow pro- duced by mixing red and green light is not particularly brilliant, it will be easy to show how several of the other colour-sensations are generated. If, for instance, we dhnin-

THE COLOtTR THEORY OF YOTTNG AND HELMHOLTZ. 117

ish the intensity of the green light in the experiment above mentioned, the resultant hue will change from yellow to orange, red-orange, orange-red, and iSnally to pure red. These changes are best followed by using the coloured light of the spectrum, but may also be traced by the help of Maxwell's disks (Fig. 38), or by the aid of the glass plate of Helmholtz (Fig. 39). On the other hand, if, in the experiment now under consideration, the green light be made to preponderate, the resultant yellow hue will pass into greenish yellow, yellowish green, and finally green. This accounts for the production of more than half the col- our-sensations in the list above given, and the remaining ones, such as ultramarine, cyan-blue, and bluish green, can be produced in the same way by mingling in proper pro- portions green and violet light, using any of the methods above mentioned.

In the cases thus far considered we have presented to the eye mixtures of two different kinds of coloured light, or, to speak more accurately, two kinds of light differing in wave-length ; it now remains for us to account for the pro- duction of colour-sensations in those cases where the eye is acted on only by one kind of coloured light, or by light having one wave-length. In the case of red, green, or vio- let light, the explanation of course lies on the surface : the red light stimulates powerfully the red nerves and produces the sensation we call red, and so of the others. But this does not quite exhaust the matter ; for, according to the theory of Young and Helmholtz, this same red light also acts to some extent on the green and violet nerves, and si- multaneously produces to some small degree the sensations we call green and violet. The result then, according to the theory, ought to be the production of a strong red sensa- tion, mingled with much weaker green and violet sensa- tions ; or, in other words, even when the eye is acted on by the pure red light of the spectrum, this red light ought to appear as though mingled with a little white light, even if

118 MODERN CHROMATICS.

none is actually present. Experiment confirms this theo- retical conclusion, and here again decides in favour of the correctness of our theory. The simplest way of making the experiment would be to temporarily remove, were it possible, the green and violet nerves froin a portion of the retina of the eye, and then to throw on the whole retina the pure red light of the spectrum. This red light ought then to appear more intense and saturated when falling on the spot from which the green and violet nerves had been re- moved than when received on the rest of the retina, where all three kinds of nerves were present. Now, though we can not actually remove the green and violet nerves from a spot in the retina, yet we can by suitable means tire them out, or temporarily exhaust them, so that they become to a considerable extent insensitive. If a small spot of the retina be exposed for a few moments to a mixture of green and violet light so combined as to appear bluish green, the green and violet nerves will actually become to a consider- able extent inoperative ; and, when the eye is suddenly turned to the red of the spectrum, this spot of the retina will, if we may use the expression, experience a more powerful and purer sensation of red than the surrounding unfatigued portions, where the red will look as if diluted with a certain amount of white light. From this experi- ment of Helmholtz it appears, then, that it is actually pos- sible to produce by artificial means colour-sensations which are more powerful than those ordinarily generated by the light of the spectrum — a point to which we shall return in the following chapter.

Having accounted now for the production of the colour- sensations red, green, and violet by red, green, and violet light, and noticed an interesting peculiarity connected with this matter, we pass on to the others. Taking up the yel- low of the spectrum, we find that it can be produced by the action on the eye of waves of light intermediate in length between those which give the sensations red and green.

THE COLOUR THEORY OF YOUNG AND HELMHOLTZ. ng

These waves are too short to act very powerfully on the red nerves, and too long to set into maximum activity the green nerves, but they set both into moderate action ; the result of this joint action of the two sets is a new sensation, which we call yellow. Furthermore, it may be remarked that the waves of the light called yellow are far too long to produce any but a feeble effect on the violet nerves ; they affect them less than green light does. Prom this it results that the sensation of yellow, when directly produced by the yellow light of the spectrum, is less mingled with that of white, and is purer than is the case when it is brought about by mixing red light with green in the manner before described. And this explanation may serve to account for the fact that it is impossible, by mixtures of red and green light taken from the spectrum, to produce a yellow light as pure and brilliant as the yellow of the spectrum. Let iis suppose, in the next place, that, instead of presenting to the eye the yellow of the spectrum, we act on it by the light belonging to one of the other spaces — the blue, for example. The explanation is almost identical with that just given for the yellow : the waves constituting blue light being too short to powerfully affect the green nerves, and too long to accomplish this with the violet nerves, both green and violet nerves are moderately affected, giving the sensation we call blue. Meanwhile the blue light produces very little action on the red nerves, and hence very little of the sensation of white is mingled with that of blue ; and consequently this blue hue is more saturated than when produced by the actual mixture of green and violet light. In fact, J. J. Mtlller found that green light, when mingled with light from any other part of the spectrum, produced a hue which was less saturated and more whitish than the corresponding tint in the spectrum which the mixture imi- tated. The production of all the other colour-sensations obtained by looking at the spectrum is explained in the same way by our theory. From all this one interesting 6

120 MODEEN CHROMATICS.

conclusion can he drawn, viz. : that there are two distinct ways of producing the same colour-sensation ; for we have seen that it may be accomplished either by presenting to the eye a mixture of green and yiolet light, or simply one kind of light, the waves of which are intermediate in length between those of green and violet. The eye is quite unable to detect this difference of origin, although a prism reveals it instantly.

Having examined thus, with a degree of detail which may have seemed tedious, the mode in which colour-sensa- tions are accounted for by the theory of Young and Helm- holtz, we pass to another point. In order to give more exactness to this theory, it is necessary to define with some degree of accuracy the three fundamental colours ; for there is a great variety of reds, greens, and violets. Helmholtz, as the result of his first investigation, selected a red not far from the end of the spectrum, a full green and violet ; in other words, the tints chosen were the middle and end col- ours of the spectrum. Maxwell, who made a series of beau- tiful researches on points connected with Young's theory, was led to adopt as the fundamental colours a red which in the spectrum lies between the fixed lines C and D, and is distant from C just one third of the distance between C and D. This is a scarlet-red with a tint of orange, and is represented by some varieties of vermilion. His green is situated between E and F, being distant from E by one quarter of the distance between E and P. This colour finds among pigments an approximate representative in emerald-green. Instead of adopting a full violet, Maxwell selected a violet-blue midway between the lines F and G, which is represented tolerably by artificial ultramarine-blue. By subjecting the results of experiments on the spectrum to calculation, it is possible to fix on the position of one of the fundamental colours, viz., the green. Thus Charles S. Pierce, using data given in Maxwell's paper, obtained for this colour a slightly different result from that just men-

THE COLOUR THEORY OF YOUNG AND HELMHOLTZ. 121

tioned.* According to his calculations, the fundamental green has a wave-length of 524 ten-millionths of a milli- metre, and is situated between the lines E and 5, being one third of the distance E b from E, whereas Maxwell's green is just beyond b. J. J. Mtlller, who conducted an impor- tant investigation on this subject by a quite different meth- od, arrived at a somewhat different result for the position of the green, and assigned to it a wave-length of 506-3 ten- millionths of a millimetre. This position in the spectrum is nearer the blue than the positions given by Maxwell and Pierce, and the tint is more of a bluish green. Again, von Bezold, basing his calculations on the experimental results of Helmholtz and J. J. MflUer, reached a conclusion not differing much from those of Maxwell or Pierce. He se- lects a green in the middle of the normal spectrum between E and b, but nearer b. /None of these results differ very greatly ; in fact, the differences can hardly be well indicated in a spectrum of the size of this page. All these greens may be imitated by using the pigment known as emerald- green, alone or mixed either with a small quantity of chrome-yellow or cobalt-blue. Hence all these green hues are of the most powerful or, as artists would say, over- powering character.

The exact determination of the other two fundamental colours is a more difficult matter, so that even the advocates •of Young's theory have not entirely agreed among them- selves upon the exact colours, Maxwell taking ultramarine- blue, Helmholtz and J. J. Mtlller violet, as the third funda- mental. These fundamental colours are among the most saturated and intense of those furnished by the spectrum. Compared with them, the blue of the spectrum is a feeble tint, so that it has often been remarked by Rutherfurd that, in comparison with the other colours, it appears of a slaty hue. The greenish yellow is also feeble ; and, as is well

* "Proceedings of the American Academy of Arts and Sciences, 1873."

122 MODERN CHROMATICS.

known, pure yellow is found in tlie spectrum in very small quantity and of no great intensity. The orange-yellow is also much weaker than the red, and the orange only be- comes strong as it approaches redness in hue. From this it very naturally follows that, if a normal spectrum is cast on a white wall in a room not carefully darkened, scarcely more than the three fundamental colours will be discerned, red, greefi, and blue-violet ; the other tints can with some difficulty be made out, but at first sight they strike the un- prejudiced observel- simply as the places where the three principal colours blend together. The representatives of the fundamentals among pigments are also those which surpass all others in strength and saturation. One of the fundamental colours, red, is used without much difficulty in painting and decoration ; the other two are more difficult to manage, particularly the green. The last colour, even when subdued, is troublesome to handle in painting, and many artists avoid 'it as far as possible, or admit it into their work only in the form of low olive-greens of various shades. When the tint approaches the fundamental green, and is at the same time bright, it becomes at once harsh and brilliant, and the eye is instantly arrested by it in a disagreeable manner.

NOTE TO CHAPTER IX.

YouifG does not appear to be the first who proposed red, green, and violet as the three fundamental colours. As far back as 1792 "Wilnsch was led to the same result by his experiments on mixtures of the coloured rays of the spectrum. The title of his -work is "Versnohe und Beobachtungen tlber die Farben des Lichtes" (Leipsic, 1792). An abstract of the contents is contained in the " Annales de Chimie," LXIV., 135.

A. M. Mayer has recently called attention to the way in which Young was led to adopt red, green, and violet as the three funda- mental colours, and has shown that Young at first " selected red,

NOTE TO CHAPTER IX.

123

yellow, and blue as the three simple colour-sensations ; second, that he subsequently modified his hypothesis, and adopted red, green, and violet as the three elementary colour-sensations, showing that up to the date of this change of opinion all of his ideas on the sub- ject were hypothetical, and not based on any observations of his own or others ; third, that this change of opinion as to the three elementary colours was made on the basis of a misconception by WoUaston of the nature of his celebrated observation of the dark lines in the solar spectrum, and also on the basis of an erroneous observation made by Young in repeating WoUaston's experiment ; fourth, that Young subsequently tested his hypothesis of colour- sensation, and found that it was in accord with facts reached by experiment, and that these experiments then vindicated his hy- pothesis and raised it to the dignity of a theory." ("American Journal of Science and Arts," April, 1875.)

Fig. 43 (page 114) shows the intensities of the three primary sensa- tions, red, green, and violet, as estimated by Helmholtz. The intensi- ties were afterward measured by Maxwell, and found to differ slightly in the case of diflFerent eyes. In Fig. 44 the letters G D E F G de- note the