Colour vision

by Sir William de Wiveleslie Abney

Further investigation on these lines has placed the violet of the spectrum as a primary rather than the blue, but this is still a matter of debate. Suffice it to say that a red and a green in the spectrum are really two of the primary colours, and most probably the violet the third. Experiment shows that there is no other primary colour in the strict sense of the word. We thus arrive at the fact that, except the primary colours themselves, every colour in nature may be made by a mixture of two or three of these primaries.

Just a word of explanation as to why, with pigments, the primary colours appear to be red, yellow, and blue, and not red, green, and blue. The colour of a pigment, it must be recollected, is a complex one. If we analyse a yellow—a yellow glass will be just as good an example as anything else—we find it is made up of green, yellow, orange, and red. A blue is made up of blue and green. If a yellow is placed behind a blue glass, and we look at a white surface through them, the only light that can get through the glass is the green. If the light, coming through each glass separately, falls on the same spot on a white surface, it will be either colourless or bluish white, or yellowish white, whichever colour preponderates. As the light reflected from mixed pigments is made up principally by the light coming through the different particles, first coming through one and then through another, and only partially by mixed lights, it will be gathered why the primary colour, when deduced from experiments with pigments, was yellow, and not green.

With the spectrum colours there is this fact to remember, that though all intermediate colours between the pairs of primaries can be formed by their mixture, yet in some cases the resulting colours are slightly diluted with white, and that they thus appear less saturated than the spectrum colours themselves. The reason for this we shall be able to account for when we consider the colour sensations themselves.

When making matches to simple or other colours by the method of mixtures, we have to be careful of the conditions under which we experiment. This can be shown by a very simple experiment. I will make a match on B with the white light, which is thrown on the surface A ([Fig. 6]), by mixing the red, green, and violet that pass through the three adjustable apertures or slits already described. The apertures are altered till the match appears to myself perfect. From an appeal made to those of the audience who are at least 25 feet away from the patches of light, as to the correctness of the match, I gather that the match is to them imperfect. The mixed colours appear to them to give a pinkish white. The reason of this defect in the match is due to the fact that, as the lecturer is viewing the two square patches of 2 in. side from a distance of 2 ft. 6 in., their images on his retina extend beyond the boundary of the yellow spot, whilst the audience receives the whole of the image on that portion of the retina which is completely covered by it. To the lecturer only part of the blue and green is absorbed by the yellow spot, and the part of the retina outside it on which the image falls receives and records the full intensity of these colours. To the audience the full amount of absorption takes place, with the result that the patch of mixed colours must appear too red when it is correct to the lecturer. In this case habit makes the eye take an average of the different intensities which must exist at the various parts of the image. We can, however, cause a perfect agreement between all parties if the experimenter views the surfaces in a mirror placed some 12 feet away and then makes the match, for he is viewing the patches from what is practically a distance of 24 feet. If after making the match without the aid of the mirror the lecturer’s eyes are directed a little to one side of the illuminated surfaces, a match will no longer exist; the mixed colour, which is to the audience pinkish, will now appear a bluish green to him. The reason for this alteration in hue is that the whole of the images falls outside the yellow spot.

It will now be quite apparent that we must discount any assertion in regard to colour matches, unless we are told the distance of the eye from the surface on which the match is made, together with the size of that surface. This yellow spot is often provokingly tiresome in the study of colour mixtures, and one might almost be justified in doubting whether any absolutely exact matches can ever be vouched for, owing to the important region of the retina which it occupies.

The fatigue of the retina to colour after it has been presented to the eye for any length of time is a difficulty, but in a less degree. That the retina does experience fatigue can be shown by a very simple experiment. The lecture theatre is now illuminated by the incandescent light, and if we throw an image of the bright carbon points of the electric arc light on the screen and steadily fix the eyes on the image of the white-hot crater for some (say) twenty seconds, and then we suddenly withdraw it, a dark image of the points will be seen on the partially lighted screen, and will appear to travel with the eyes as they move away from the fixed point. This phenomenon is due to the fact that the perceiving apparatus for white light gets fatigued on the parts of the retina on which the bright image of the white carbon points thrown on the screen fell, and that when the source of brightness was removed, the less intense illumination of the screen failed to stimulate the vision apparatus at those parts to the same extent that they were stimulated over the rest of the field. We can vary the experiment by placing a red glass in front of the electric light, and, following the same course as before, we shall see a greenish-blue image of the carbon points upon the screen. In this case the retinal apparatus which has not been stimulated by the red sensation will be capable of the maximum stimulation by the feeble white light, whilst that part which has suffered fatigue will not respond so freely to the red contained in the white light. If we abstract a certain amount of red from the spectrum, its recombination will give a white tinged with greenish blue, which is a counterpart of the colour we feel when the eyes have been fatigued by the red light.

CHAPTER III.

Let me take you back again to matches of colour. We will now, however, make the matches with the primary colours in the guise of pigments. These colours themselves are complex colours, but as the eye cannot trace any difference, or at all events very little difference, between them and simple colours, a mixture of these complex colours should answer nearly as well as do mixtures of the simpler colours. We have here three discs, a red, a green, and a blue, and we can very closely match these colours by a red, a green, and a blue in the spectrum.

By having a radial slit cut to the centre of these card discs, we can slip one over the other so as to expose all three colours as sectors of a single disc. Then we can place the compounded disc on the axis of a rapidly rotating motor, and the colours will blend together, giving an uniform colour. Any proportions of the three colours can thus be mixed, and by a judicious alteration in them we now have them so arranged that they give a grey. By inter-locking together ([Fig. 9]) a black disc and a white disc, each with a diameter slightly larger than that of the other discs, but equal to each other, and rotating them on the same spindle behind the three colour discs, we can, by an alteration in the proportion of black to white, form a grey which will match that produced by the rotation of the three coloured sectors. In other words, white, though degraded in tone, can be produced by the three complex pigment colours, as we have seen can also be done by the mixture of the three simple spectrum colours.

Fig. 9.