Colour vision

by Sir William de Wiveleslie Abney

“Now suppose light to enter at E, to pass through the lens, and to be refracted by the two prisms at P, a pure spectrum, showing Fraunhofer’s lines, is formed at A B, but only that part is allowed to pass which falls on the three slits, X Y Z. The rest is stopped by the shutters. Suppose that the portion falling on X belongs to the red part of the spectrum; then, of the white light entering at E, only the red will come through the slit X. If we were to admit red light at X, it would be refracted to E, by the principle in optics that the course of the ray may be reversed.

“If, instead of red light, we were to admit white light at X, still only red light would come to E; for all other light would be either more or less refracted, and would not reach the slit at E. Applying the eye at the slit E, we should see the prism P uniformly illuminated with red light, of the kind corresponding to the part of the spectrum which falls on the slit X, when white light is admitted at E.

“Let the slit Y correspond to another portion of the spectrum, say the green; then if white light is admitted at Y, the prism, as seen by an eye at E, will be uniformly illuminated with green light; and if white light be admitted at X and Y simultaneously, the colour seen at E will be a compound of red and green, the proportions depending on the breadth of the slits and the intensity of the light which enters them. The third slit Z, enables us to combine any three kinds of light in any given proportions, so that an eye at E shall see the face of the prism at P, uniformly illuminated with the colour resulting from the combination of the three. The position of these three rays in the spectrum is found by admitting the light at E, and comparing the position of the slits with the position of the principal fixed lines; and the breadth of the slits is determined by means of the wedges.

“At the same time, white light is admitted through B C to the mirror of black glass at M, whence it is reflected to E, past the edge of the prism at P, so that the eye at E sees through the lens a field consisting of two portions, separated by the edge of the prism; that on the left hand being compounded of three colours of the spectrum refracted by the prism, while that on the right hand is white light reflected from the mirror. By adjusting the slits properly, these two portions of the field may be made equal, both in colour and brightness, so that the edge of the prism becomes almost invisible.

“In making experiments, the instrument was placed on a table in a room moderately lighted, with the end A B turned towards a large board covered with white paper, and placed in the open air, so as to be uniformly illuminated by the sun. In this way the three slits and the mirror M were all illuminated with white light of the same intensity, and all were affected in the same ratio by any change of illumination; so that if the two halves of the field were rendered equal when the sun was under a cloud, they were found nearly correct when the sun again appeared. No experiments, however, were considered good unless the sun remained uniformly bright during the whole series of experiments.

“After each set of experiments light was admitted at E, and the position of the fixed lines D and F of the spectrum was read off on the scale at A B. It was found that after the instrument had been in use some time these positions were invariable, showing that the eye-hole, the prisms, and the scale might be considered as rigidly connected.”

Fig. 15.

With this instrument he made mixtures of three colours, to match with white. By shifting the slits into various positions and taking as his three standard colours a red near the C line, a green near E, and a blue between F and G (see frontispiece), he obtained a variety of matches, from which he formed equations. After eliminating, or rather reducing the errors to the most probable value by the method of least squares, he got from his matches with white a table of colour values in terms of the three standard colours, from which the diagram of the spectrum ([Fig. 15]) was made. (The heights of the dotted curves are derived from the widths of the slits, and the continuous curve is the sum of these heights.) Now what appears to be a properly chosen colour does not necessarily stimulate only one sensation. Indeed the probabilities are against it, except in the extreme red and extreme violet. If colours intermediate to the standard colours be matched by a mixture of the latter, we do not arrive at any solution of the amount of stimulation of each sensation, since the chosen standard colours themselves may be due to a stimulation of all three sensations. As a matter of fact, Clerk Maxwell chose colours which do not best represent the colour sensations. The red is too near the yellow, as is also the green. The blue should also be nearer the violet end of the spectrum than the position which he chose for it. We may take it, then, that except as a first approximation, Clerk Maxwell’s diagrams need not be seriously taken into account. The diagram itself shows that the colour sensations are not represented by the colours he chose. Supposing any one in whom the sensation of green is absent were examining the spectrum, there would, according to the diagram, be no light visible at the green at E. Anticipating for a moment what we shall deal with in detail shortly, it may be stated that in cases where it is proved that a green sensation is absent, there is no position in any part of the spectrum where there is an absence of light. Had he chosen any other green, the same criticism would have been valid. The diagram as it stands is really a diagram of colour mixtures in terms of three arbitrarily chosen colours, and not of colour sensations. It merely indicates what proportions were needed of the three colours, which he took as standards, to match the intermediate spectrum colours. The negative sign in some of the equations—given in the appendix, [page 201]—may be somewhat puzzling to those who have not made colour matches, but not to those who have actually made experiments. It means that where it is present no match of colour by a mixture of the standard colours is possible; and that it would be only possible if a certain quantity of the colour to which is attached a negative sign were to be abstracted—an impossible condition to fulfil, but one which may often occur in colour-matching experiments. Later you will find that when colours are chosen as standards so that the resulting equations give no negative sign for any colour, we have a criterion as to the colours which give the nearest approach to the true sensations. The next diagram ([Fig. 16]) of colour sensations is due to Kœnig, who investigated the subject with Von Helmholtz. By a modified method, which perhaps need not be explained in detail here, he produced them, and they must be apparently not far from the actual state of things, supposing this theory be proved to be true. For my own part, I am under the impression that the positions of the colours which most nearly approach the colour sensations might be slightly altered in regard to the green and the blue, for reasons that will subsequently be given when the later experiments of General Festing and myself come to be described. For the immediate purpose of the lecture, the curves are sufficiently accurate, and I will ask you to notice what they tell us. It is presupposed in these diagrams that, if the three colour-perceiving apparatus are equally stimulated, a sensation of white will be produced; and the reverse, of course, is true, in that white will give rise to equal stimulation of the three apparatus. It follows, then, that in the parts of the spectrum where all three curves of sensation are seen to take a part in the production of a colour, such as at the E line, the colour is really due to the extra stimulation of one or two of the apparatus above that required to produce a certain amount of white. The colour in every part of the spectrum may be represented by not more than two sensations, with a proportion of white. In the orange and scarlet there are only two sensations excited, without any sensible amount of white, as the amount of violet sensation is extremely small. At the extreme ends of the spectrum only one sensation—the red or the violet—is excited; but in the region of the green the colour must be largely diluted with the sensation of white. As an example, we may take the part of the spectrum where the red and the violet sensation curves cut each other. At this point the green sensation curve rises higher than the intersection of the other curves. The red and the violet sensations have only to be mixed with an equal amount of the green sensation to make white, so that the height of the green sensation curve above the point of intersection represents the amount of pure green sensation which is stimulated. The colour is therefore caused by the green sensation, largely diluted with white. A scrutiny of the curves will show that at no point is the green sensation so free from any other as at this point, if we regard white by itself as a neutral colour. Looking at these figures, we can readily see what effect the removal of any one or two of the three sensations would have upon the colour vision of the individual. The probabilities, however, against two of the three sensations being absent must evidently be very much smaller than that there should be an absence of only one of the sensations, either red, green, or violet.

Fig. 16.