mine and bluish green is the same as that produced by a mixture of red and verdigris, then this grey will also re- sult from the mixture, in the original proportions, of all four colours. — This third law enables us to answer in the affirmative the question, raised above, whether it is legiti- mate to argue from our disc-colours to other forms of visual stimulus, and especially to the pure lights of the spectrum.
The answer is reached by way of the corollary that any unsat- urated colour may be produced by mixture of a saturated colour with grey. The disc-colours are relatively unsaturated colours; they are compounded, physically, of a number of different colours, with some one colour (or some small group of neighbouring colours) in the ascendant. Now the first law declares that the mixture of complementaries produces grey. By the third law, which asserts that all colour mixtures have a constant mixing value, any grey whatsoever may be considered as the result of mixture, in the right proportions, of some pair x, y of complemen- tary colours. If j is present in excess, then by the first law we obtain a colour sensation of low chroma and of the hue of y itself. Hence all that we have to do, in order to produce a particular unsaturated colour, is to search among the saturated colours for the fitting y, and, having found it, to add a certain amount of grey. This means, however, that our disc-colours may be regarded as spectral colours mixed with various amounts of white light, and therefore that our demonstration of the three laws holds for the spectrum as well as for coloured papers.
There are other important corollaries to these laws. Thus, it follows from the first and second, taken together, that the mix- ture of three colours, every pair of which embraces the complemen- tary of the third, will give colour sensations of every possible hue, and of all possible degrees of chroma from a certain maximum to zero. Take, for example, R, G and V. The mixture of R and G will, by the second law, give any hue from the O, Y and YG regions; the mixture of G and Fwill give BG and B; the mix- § i J. The Lazvs of Colour Mixture 71 ture of V and R will give P and C. Further, the complementary of Flies between R and G; that of R between Gand V; and that of G between Fand R. Hence, by a fittingly proportioned mix- ture of these three colours, it is possible to obtain a colour sensation of any required hue, and of any degree of chroma that is lower than that of the components. Similar triads of colours are R, Y, GB; O, G, V; P, YG, BG; etc. — This is a useful corollary to us, since it is seldom that the disc-colours are exactly complementary. Hence, to demonstrate the cotnplementarism of C and BG,wq take three coloured papers, C, B and G, and vary the propor- tions of the sectors until we find 3.BG that is antagonistic to the given C; and so on with other complementary pairs.
It follows, again, from the third law, as we have formulated it above, that colour-equations are independent of the energy of the physical stimulus. Suppose, for instance, that we have matched a grey derived from C and BG to a grey derived fromi? and Y. Now assume that the energy of these greys is doubled or tripled. This is the same thing as assuming that we have made the match two or three times over, and then added the greys together, mixed each grey once or twice with itself. The resulting double or triple greys ought, by the law, to match as well as the original, single greys. Here, however, we may come into conflict with the Pur- kinje phenomenon. If the equation is first made for a low degree of light-energy, and this energy is then considerably increased, the B- Y grey will evidently appear lighter than the C-BG grey. If, contrariwise, the equation is first made for a moderately high de- gree of light-energy, and the energy is then greatly diminished, the C-BG grey must appear lighter than the B- Y grey. And for certain shifts of energy the change of tint will be accompanied by a change of hue. Neither this corollary, therefore, nor the third law itself, in so far as it involves the corollary, can be regarded as valid under all conditions.
§ 18. The Dependence of Visual Sensation upon the Time and Space Relations of Stimulus. — The quality of visual sen- sation is dependent not only upon the wave-length, wave- 72 The Quality of Sensation: Vision amplitude and wave-form of light, but also upon the time during which the waves affect the eye and upon their dis- tribution in space. Under the former heading, we have to consider the facts of adaptation and of negative after-images; under the latter, the facts of light and colour contrast. — When the lamps are first lighted in the evening, we clearly realise that the illumination is not white but reddish-yellow. As time goes on, however, this colour dis- appears, and the objects about us look as they would look in a really white light. In ordinary language, we have grown accustomed to the artificial light; in technical terms, adaptation has set in. The law of adaptation is that all sen- sations of colour tend towards neutrality, and all sensa- tions of light towards a middle grey. Adaptation may be either general, extending over the whole field of vision, or local, extending over some part of the field to which our gaze is constantly directed.
The course of general adaptation to colour may be followed by help of an optician's trial frame and a set of coloured glasses. If, for instance, you wear a pair of yellow glasses even for five min- utes, you will find that adaptation has gone surprisingly far. Adaptation to dark and light is never so complete as adaptation to colour: the grey that you see after waking from sleep in a dark room and the grey that you see out of doors on a dull winter's day when the ground is covered with snow are distinctly different, the one lying on the black and the other on the white side of the middle grey. Nevertheless, there is a definite approach to this grey; after you have worn ' black ' glasses for a few hours, it is difficult to believe that the world looks darker than it did before you put them on.
Local adaptation may be demonstrated by the apparatus shown in Fig. 5. Gaze steadily, say for 1 min., at the button which lies at the centre of the line of junction of black and white. You soon § 1 8. The Law of Adaptation n see grey films or clouds, which appear first along this line and gradually spread, to right and left, over the whole surface. On the black, the cloud is dark and slowly lightens; on the white, it is light and slowly darkens. Both clouds are strongest at the centre, weaker towards the periphery. (The lines of brilliant white and intense black, that flash out from time to time, are irrelevant to the present observation; they are due to involuntary slips of fixa- tion.) — The black and the white are, evidently, tending both alike towards a middle grey. Indeed, if the gaze is maintained for a suffi- cient length of time, their differ- ence disappears, and the entire surface is seen as a uniform grey. Similar tests may be made with colours. FIG. 5. Adaptation Frame.
It is plain that this law of adaptation may be brought into relation with the laws of colour mixture. To say that, under adaptation, all sensations of colour tend towards neutrality is equivalent to saying_that, as time goes on, every colour in the field of vision is mixed with an increasing amount of its antagonistic colour. To say that, under adaptation, all sensations of light tend towards a middle grey is equivalent to saying that, as time goes on, the blacks in the field of vision are mixed with in- creasing amounts of white, and the whites with increasing amounts of black. Adaptation to colour suggests the first, adaptation to light suggests the second law of colour mixture.
Let us turn, now, to the after-effects of adaptation.
74 The Quality of Sensation: Vision When you come out into the daylight from a matinee performance, everything looks curiously bluish; when you pass from daylight into a darkened room, everything is oppressively black. Very soon, of course, the blue wears off and the black clears up; a novel adaptation is in progress. But the immediate after-effect of general adaptation is always this contrary trend of vision: if you were yellow-adapted, you are now blue-sighted; if green- adapted, now purple-sighted; if dark-adapted, now light- sighted.
The same thing holds of local adaptation. If, by steady fixation, you have brought a patch of colour to disappear- ance, and the colour stimulus is then removed, you see in place of it a patch of the antagonistic colour, a negative after-image. A yellow stimulus gives a blue after-image; a green stimulus, a purple after-image; a black stimulus, a white after-image.
The after-effect of general adaptation may be demonstrated with the coloured spectacles. When, for instance, the yellow glasses are taken off, all the blues in the field of vision look extremely saturated, all the yellows look whitish, and the other colours ap- pear as if mixed with blue.
To demonstrate the negative after-image, we may continue the observation made with the adaptation screen (Fig. 5). If, at the end of the 1 min., the half-black and half-white card is allowed to fall, and there is shown in its place a background of uniform grey, the observers will see an intense black where they previously saw white, and a brilliant white where they previously saw black. (The black and white lines, spoken of above, owe their depth and brilliancy to the fact that, as fixation slips, the white edge falls upon a black-adapted part of the retina, and conversely: the white is thus seen with a white-sighted eye, and the black with a black-sighted eye.) After-images of colour may be demon- 75 strated by the apparatus shown in Pig. 6. A disc of coloured glass is fixated, say, for 30 sec. Then a grey screen is dropped between glass and lamp, and the after-image de- velopes, in the antagonistic colour, upon this screen.
It is a little puzzling that, in all these phe- nomena of adaptation, black and white should behave, in the sphere of sensations of light, as complementary colours behave in the sphere of sensations of colour. We saw in § 14 that the series of light sen- sations, white-grey- black, resembles the R- Y, Y-G, G-B and B-R series of colour sen- sations; and we have just said that the course of adap- tation to light suggests, in consequence, the second law of colour mixture. But F-adaptation does not leave us c7-sighted, nor does ^-adaptation leave us F-sighted or ^-sighted: why, then, should white-adaptation leave us black-sighted, and conversely? We seek to answer this question, and so to bring all the facts of adaptation under a single principle, in § 22. In the meantime, we notice that, in the domain of contrast, black and white again appear in the same antagonistic or complementary relation. — FIG. 6. Wundt's Apparatus for the Observa- tion of Negative After-images.
The Quality of Sensation: Vision Contrast is the name given to the effects produced for sensation by the distribution of visual stimuli in space. Every patch of light and colour in the field of vision affects and is affected by all the rest in certain definite ways. The principal laws of contrast — that is, of this reciprocal induction of lights and colours — are as follows, (i) The contrast-effect is always in the direction of great- est opposition; a yellow makes its surroundings bluish, a black makes its surroundings light. (2) The nearer to- FlG. 7. Contrast Frame.
gether the contrasting surfaces, the greater is the contrast- effect. We may therefore distinguish between marginal contrast, in which the effect is maximal, and surface con- trast, in which it is less marked. (3) The contrast-effect is enhanced by the elimination of contours or boundary- lines. — There are two further laws of colour contrast: (4) that the effect is greatest when there is no simultane- ous light contrast; and (5) that the effect increases with increase of the saturation of the inducing colour.
A general idea of the phenomena of contrast may be gained from the contrast frame, shown in Fig. 7. The frame contains four sheets of coloured paper, — R, G, J' and B. Across the centre of these sheets is laid a horizontal strip of neutral grey paper. Each panel is faced with white tissue, which serves to § 1 8. The Laws of Contrast 77 bring colour and grey into the same plane, and also to obscure ll the outline of the grey strip. Under these conditions the grey appears in four different colours, which are complementary to the colours of the sheets, and whose tint varies inversely with the tint of the coloured background. The strip is so narrow that mar- ginal contrast is secured over its whole width.
Very beautiful contrast-effects may be obtained with coloured shadows. Fig. 8 shows two window-slits cut in the wall of a dark room, the one filled with a blue, the other with an ordinary ground glass. A black rod, standing on a table, casts two shadows upon a white screen. The farther shadow, due to the white light from the nearer slit, is illuminated by the blue light, and therefore appears blue. The nearer shadow, due to the blue light from the farther slit, is illuminated by the white light, and should therefore, in terms of its physical stimulus, appear light grey. In reality, it y appears, by contrast, in the yellow complementary to the blue j of its neighbour. — The rod should be moved to and fro, until the ', shadows are exactly juxtaposed; their lack of definite contour, their narrowness, and their identity of plane, all serve to enhance the contrast-effect. It is, indeed, easily possible, by varying the widths of the window-slits, to give the yellow a higher degree of chroma than is possessed by the blue shadow, so that a naive observer would unhesitatingly declare the blue to be the contrast, 78 The Quality of Sensation: Vision and the yellow the ' real ' colour. The bluish tinge of the back- ground shows, of course, that the yellow colour is due to contrast. ■ — Other coloured glasses may be substituted for the blue, with similar results.
It is clear, from all these facts, that the lights and colours of the field of vision, at any given moment, are not exclusively determined by the physical stimuli, the reflected light-waves, which affect the eye. What we see depends, in part, upon contrast; in part, also, upon the preceding adaptation of the eye, general and local. It is clear, fur- ther, that contrast and adaptation are in one sense op- posed, but in another sense mutually supplementary principles. Contrast is present, throughout the field of vision, as soon as we open our eyes; adaptation requires time. Contrast is a differentiating, adaptation a levelling principle. Hence contrast helps us to discriminate all the separate objects by which we are surrounded, while adap- tation prevents our being fatigued or disturbed by their variety after this discrimination has taken place.
§ 19. Daylight and Twilight Vision. — The human eye is a single sense-organ, and all its sensations are of one general kind. But it is also an extremely elaborate organ, the final product of a long course of development and differentiation. We must, therefore, consider visual sen- sation not only in its dependence upon external stimulus, but also in its dependence upon differences of structure and function within the eye. We shall not, in this way, discover any new sense-qualities; but we shall bring the sensations of light and colour into a novel perspective, and shall thus find uniformities which will help us, later on, towards their physiological explanation.
§ ig. Daylight and Twilight Vision 79 The facts of which we have to take account are, first, those of daylight and twilight vision; and, secondly, those of direct and indirect vision and of colour blindness. The former have already been touched upon, incidentally, in references to the Purkinje phenomenon.
It is, indeed, a curious thing that our sight undergoes a radical transformation as we pass from the light to the dark and back again. So long as the energy of the light- waves that strike the eye is maintained above a certain limit, we have daylight vision. We see the spectrum as a band of colours, with yellow as the lightest tint; we have all degrees of light sensation, from white to black; in a word, our vision is the vision that is summed up in the colour pyramid. When, on the other hand, the energy of the light-waves falls below this limit, we have twilight vision: the spectrum is seen as a band of greys, the lightest of which lies in the region occupied in daylight by the green, and sensations of colour are altogether lacking. Under certain conditions, the two modes of vision overlap. Twilight vision is greatly enhanced by adaptation of the eye to dark; so that, if there is sufficient light for us to distinguish colours, while at the same time the eye is partially dark-adapted, we see the Purkinje phe- nomenon superposed upon daylight vision. On the other hand, this overlapping is not possible over the whole extent of the retina. At the very centre of the eye, there is no twilight vision, and the Purkinje phenomenon does not appear. So far, then, the two modes of vision are locally separated: while the eye in general is composed, so to say, of two eyes, a nyctalopic and an hemeralopic, a small area in the middle of the retina is permanently hemeralopic.
8o The Quality of Sensation: Vision We said in § 16 that the Purkinje phenomenon might be ob- served by looking at red and blue papers through a pinhole in a card. It may also be observed by looking at the colours through nearly closed eyelids, or by taking them from a light into a dark room. In all three cases, since the colours are still visible, there is a mixture of daylight and twilight vision; that is to say, the phenomenon does not appear at once, but only after a little while, when dark-adaptation has gone a certain distance. A similar mixture of the two types of vision occurs as you watch the reds and blues of the carpet in a deepening twilight. If, on the other hand, you go straight from bright daylight into a perfectly dark room, in which is exposed a spectrum of such low energy that no colour can be seen, then, as soon as you are able to observe at all, you observe that this spectrum shows the Purkinje phenomenon. Twilight vision is primarily dependent, not upon dark-adaptation, but upon the reduction of the energy of light. What dark-adap- tation does is to make the greys of twilight vision much clearer and stronger than they are without it.
The absence of the Purkinje phenomenon at the centre of the retina can be demonstrated only by aid of refined physical instru- ments. It is, however, easy to convince oneself that this central area, which in daylight is preferred for all the most delicate uses of vision, is not stimulable by light-waves below a certain limit of energy. Look directly, on some dark night, at a faint star or a distant lamp that is just visible as the eye travels over the field of vision, and the point of light disappears. Shift your gaze ever so little from this direct fixation, and it flashes out again.
We can now understand the exceptions to the third law of colour mixture, mentioned in § t 7. Colour equations made in day- light vision will hold in daylight vision: they cease to hold when we exchange this for twilight vision, or when in consequence of dark-adaptation twilight encroaches upon daylight vision.
§ 20. Indirect Vision and Colour Blindness. — Under ordi- nary circumstances, we pay but little attention to the outlying parts of the field of vision. What we want to § 20. Indirect Vision and Colour Blindness 81 see, we look at, and so bring upon the centre of the retina; and we take it for granted that the visible objects which lie far out in the field, round about this region of direct regard, retain the colours which they show when we turn the eye upon them. Nevertheless, the colour vision of the peripheral retina is very different from that of the centre.
FlG. 9. Perimeter for Mapping the Retinal Zones.
Suppose that the left eye is shaded, and that the right gazes steadily at some fixation-mark placed directly before it, or a little to the right. Suppose, further, that a small red object is moved into the field of vision from the nasal side, so that its image falls upon the temporal half of the right retina. The object first becomes visible as a patch of black; then it shows as B or Y; then, as it advances, it looks P or O; finally, as it approaches the fixation-point, 82 The Quality of Sensation: Vision it appears in its true colour, as a carmine or vermilion. Other colours give like results; so that we are finally led to the conclusion that the retina consists of three dis- tinct zones. The outermost zone is totally colour blind, and accordingly furnishes only sensations of light, what- ever the stimulus may be. The intermediate zone is partially colour blind and furnishes, besides the sensa- tions of light, only sensations of B and Y, in all tints and in all degrees of chroma. The middlemost or central area furnishes all the sense-qualities that are represented in the colour pyramid.
We have called the three zones distinct, and it is true that they may be distinguished in any experiment such as that just described. At the same time, as the P or O phase of the ob- servation shows, they are not sharply separated, but pass gradually into one another. Hence a red object of large area will still be seen as R where a smaller object would appear as P or O, and will still be seen as coloured where a smaller object would look dark grey or black. Similarly, a stimulus of high energy and brief duration will retain its colour farther from the centre than a stimulus of low energy and longer duration. It is, therefore, im- possible to map the retinal zones in any hard and fast way. They are regions of relatively, not absolutely different colour sensitivity. Indeed, if the energy of the stimuli were made exceedingly great, it is probable that they would be seen in their true colours over the whole extent of the retina. Under the usual conditions of stimulation, however, the zones are distinct.
A red stimulus, as it travels out from the centre, changes first to P or O, and then to B or Y, only because it is not a physio- logically pure red. If we can find a red stimulus that has no B or Y effect, then the red will change to black or grey as soon as it leaves the middlemost zone. This red has, as a matter of fact, been determined; it is not a spectral red or vermilion, but a slightly purplish or carmine red. What holds of it holds also of § 20. Indirect Vision and Colour Blindness 83 its complementary, a spectral hue of about 495 fi/x; this, too, passes directly into grey as it leaves the middlemost zone. If these two colours are equated as regards area, tint and chroma, they become colourless at the same distance from the centre of the retina, so that the zone of red-vision is coextensive with the zone of green-vision. The same thing is true of a B of about 470 fji/x and a Y of about 575 /x/x: the zone of ^-vision is co- extensive with that of Kvision. The retina thus appears as made up of an outermost Bk- W zone, an intermediate Bk-W-\- B-Y, and an innermost Bk-W '+ B-Y-\- R-G zone.
If a spectrum is thrown upon the Bk- W zone, it appears, of course, as a band of greys. It is noteworthy that, in light-adapta- tion, the lightest of these greys occupies the region of the yellow, so that the relative distribution of tint in the colourless spectrum is unchanged. — Most of us use our eyes for a lifetime, without discovering these differences of zonal sensitivity. The reason is that, in indirect vision, it is very difficult to make out the form, size or contour of objects in the visual field. This sort of discrimination is, however, of great importance for the organism. Hence we habitually turn our eyes toward that which we wish to observe; attention goes with direct vision, and the phenomena of indirect vision are disregarded.
We must say, then, that the normal eye is normal only for purposes of direct vision, while in indirect vision it is partially or totally colour blind. There is also an abnormal colourblindness; certain persons show these defects of vis- ion over the whole extent of the retina. Thus, some 3 per cent, of the male population are, from birth, partially colour blind; their eyes lack the middlemost or R-G zone. The physiologically pure red and the physiologically pure green, of about 495 (jl/jl, appear to them as grey; the left or long-wave end of the spectrum is yellow, and the right or short-wave end is blue. In other words, the spectrum 84 The Quality of Sensation: Vision looks in direct vision as it normally looks in indirect vision with the Y-B zone, or as it normally looks in direct vision when the energy of the light-waves is very great (§ 16); and the whole visual world consists of blacks, whites and greys, together with blues and yellows in all possible variety of tint and chroma.
It follows from this that partially colour blind persons will con- fuse a pure red and a pure green, if tint and chroma are the same.
They will also, under these conditions, con- fuse O and YG, P and BG, rose and blue, vermilion and brown. In everyday life they make very few mistakes, partly because they have learned the names of coloured objects from their normal-sighted acquaintances, partly because objects of the con fu sable col- ours usually differ in tint or chroma or both, as well as in hue, and in certain cases because difference of hue is connected with a difference of grain or texture.1 1 This, no doubt, explains the otherwise curious fact that only in compara- tively recent times has partial colour blindness attracted any widespread atten- tion. Scattered references to it go hack to the seventeenth century; but it did not obtain general recognition from scientific men until 1798, when the chemist John Dalton published a paper on Extraordinary Facts relating to the Vision of Colours {Edinburgh Journal of Science, ix., 97). Dalton was him- self partially colour blind, and for some time the defect was known as Dalton- FiG. 10. Hering's Apparatus for the Investigation of Partial Colour Blindness.
§ 21. The Primary Colours 85 There are two types of partial colour blindness. In the first and commoner form, the distribution of tints in the spectrum is the same as in normal vision; the lightest grey lies in the region of yellow. In the second, the lightest grey has shifted towards the short-wave end, and lies in the region of yellow-green. This and certain other anomalies of congenital partial colour blindness still await explanation. — A partial colour blindness due to lack of the intermediate or i?-F zone is found only as a pathological condition of the eyes, not as a congenital defect.
A much more serious congenital defect of vision is that known as total colour blindness, in which the eye lacks both the R-G and the B-Y zones, and the world of colour ap- pears in monotone as an arrangement of blacks, whites and greys. The defect is rare; only some fifty cases have been examined. The totally colour blind eye is nyctalopic; that is to say, its vision, in any state of adaptation, is twilight vision, and the spectrum as seen by it always shows the Purkinje phenomenon. Further, the small central area which, in the normal eye, is permanently hemeralopic is in the totally colour blind eye either wholly or almost wholly blind, so that direct fixation of an object in the field of vision is impossible, and the eye twitches and jerks in the effort after clear vision.
§ 21. The Primary Colours. — We have seen that, psy- chologically regarded, all colours are equally simple; it is impossible, for instance, by introspective analysis to split up orange into yellow and red. On the other hand, certain colours have exceptional positions in the colour pyramid, — those colours, namely, which lie at the four corners of