§ 8. Effects of the Mixture of Lights of Different Wavelengths.—When lights of all wavelengths are intermingled in due proportion, the result is grey or white. If in the mixture there is a relative predominance of some one light, such as green or blue, the result is a whitish green or a whitish blue.

* Foster, TextBook of Physiology, part iv., pp. 12401241.

If we select any colour of the spectrum, it is possible to find some other colour which, mingled with it in due proportion, will yield a neutral tint. If one of the components of the mixture is present in greater quantity than is required to produce a grey, the predominant light gives its own colour to the mixture. The other light diminishes the degree of saturation. Thus, if golden yellow and blue be mixed in proper proportions, they yield the sensation of white. As the proportion of blue is increased, the white becomes more and more a bluish white; as the proportion of yellow is increased, the white becomes more and more a yellowish white. Colours which, intermixed with each other, yield white, are called complementary. Yellow is complementary to blue. The red of the spectrum is not complementary to green, but to a bluish green. It should be remembered, however, that the red of the spectrum is not pure red, but yellowish. As every discernible colour of the spectrum possesses its complement, either within the spectrum or in the purple series, the pairs of complementary colours are indefinitely numerous. If the simple lights corresponding to colours which are not too far removed from each other in the spectrum are mingled, the result is a colour corresponding to an intermediate light. The wider the interval separating the mingled colours the whiter is the resulting colour. When the interval becomes sufficiently wide, mixture in proper proportion yields pure white. For instance, by mingling the simple lights which severally produce blue and green, we can obtain all the bluegreens. A larger proportion of the blue light yields a bluer green: a larger proportion of the green light yields a greener blue. If we mix blue with yellowishgreen, we obtain a green mingled with the white due to the combination of blue and yellow. This green may be relatively pure or it may be bluish or yellowish according to the proportion of blue or yellow light in the mixture. The combination of of pure blue with pure yellow yields white. If, proceeding further, we mix blue with red, we obtain a new colour not contained in the spectrum,—purple. By mixing the red light of the spectrum with the green in certain proportions we produce yellow : by increasing the quantity of red light, the yellow is made redder; by increasing the quantity of green light, the yellow is made greener. The laws of combination which hold good of simple lights apply also to those mixtures which produce the same colours as the simple lights.

If we select three colours so related that by combining any two of them we can obtain a colour which is complementary to the third, it is possible, by varying combinations of the three, to produce all the colours of the spectrum. But there is only one triplet of colours by which the rest can be produced in a high degree of saturation. This triplet is red, green, and a bluish violet. For this reason red, green, and violet, have been called primary colours.

The best method of mixing lights of different wavelengths, so as to ascertain the resulting sensation, is to allow two different parts of the spectrum to fall on the same part of the retina at the same time. Another way is by using the colourwheel or colourtop. Sectors of the colours to be investigated are placed on a disk. The pigments used in colouring must be as pure as possible; in other words, they must as nearly as possible reflect simple and not compound lights.* The disk is set rapidly spinning so that one kind of light is brought to bear on the retina before the effect of the other has ceased. Thus the different modes of stimulation are superposed. If one sector of the disk is blue, and another yellow, and if the colours are present in due proportion, the rapidly rotating disk will appear grey.

* The mixture of the pigments themselves, in the way that artists mix them, is by no means equivalent to a mixture of the lights which they reflect.