Colour vision

by Sir William de Wiveleslie Abney

Fig. 6.

The colour of the bichromate is therefore a complex or mixed colour according to our definition, for it is made up of a large number of simple colours. What I desire to show, however, is that this complex colour can be mistaken by the eye for a simple colour. First, let us interpose the cell with the bichromate in the path of the reflected beam, and throw the patch of light formed by it on a white surface A ([Fig. 6]), alongside the patch of light B formed by the spectrum. Next let us pass a single aperture ([Fig. 7]), which can be opened and closed by a screw arrangement, through the spectrum. By careful movement we at length come to an orange ray, which is spread out by the apparatus to form a patch on B, that to the majority (and the word majority is used with intention) of people exactly matches the colour of the bichromate. Thus we have a proof that, as far as the eye is concerned, the simple and the complex colours are identical. This illustration of the want of power of the eye to analyse colour might be repeated as often as we like. We may pass coloured wools, for instance, through the length of the spectrum and show that they have the property of appearing bright in, and therefore of reflecting, some colours and of almost disappearing in others—a sure indication that these colours are mixed colours as they are made up of the rays which are reflected. Yet when viewed in white light they can in many cases be matched with simple colours in the way we matched the colour of the bichromate solution. This tells us that there is something which requires investigating as to the constitution of the perceiving apparatus, and points to the probability that it is less complicated than it would be were it able to differentiate, without the aid of the spectrum, between simple and complex colours. If the eye had a separate apparatus—and when I say apparatus I use the word for want of a better—for taking up the impression of every simple colour, it might well be assumed that a differentiation must take place.

There is one class of colours, it must be remembered, which can never be mistaken for simple colours. I refer to the purples—mixtures of red and blue—for there are no spectrum colours which unmixed can possibly match them. All other colours, as no doubt will soon be apparent, can be referred to some one spectrum colour, either in its pure state or else mixed with some variable quantity of white light. We are all familiar with the fact that there are three primary colours, and we are naturally led to consider these in the light of the experiments just made. As good a definition as any other of a primary colour is that it is a colour which cannot be formed by the mixture of any two or more colours. The original investigators in colour phenomena were the artists, and they found that neither red, nor yellow, nor blue could be formed by any mixture of pigments on their palette, but that all other colours could be made by a mixture of two or more of these three. Hence to these three were given the name of primary colours. When, however, the physicist began to work with the simple colours of the spectrum, it was speedily found that, at all events, the yellow was not a primary colour, as it could be formed by a mixture of green and red, whilst a green could not be formed by a mixture of any other two colours. This we can prove with our apparatus.

Fig. 7.

Three apertures, all of which can be opened or closed as required (see [Fig. 7]), are placed in the spectrum, one in the red, one in the green, and one in the violet. The last we shall not require at present, so it is entirely closed; but we vary the width of the other two. We find that with a little red added to a bright green, a yellow green is produced; with more red added we have yellow; with still more red, an orange. The relative brightness of the two colours mixed together can be shown by removing the lens which recombines the spectrum to form the patch of light. Each colour issues through its slit and forms its own patch on a white screen which, for the purpose, we make rather larger than usual. The two patches overlap in the middle ([Fig. 8]), and the pure colours are seen one on each side of the mixed colours.

Fig. 8.

Now, placing one slit in the yellow and another in the blue of the spectrum, we find that whatever width of slit we take, no green is produced, but that, in fact, a yellowish or a bluish white results, and that when the two slits are properly adjusted, a pure white is produced. Evidently since none of the intermediate spectrum colours between the blue and the yellow can be made by their mixture, certainly green cannot. Hence, with pure colours a green and not a yellow is one of the primaries.