Physiological colours admit, in like manner, of being mixed with others. If, for example, we produce the blue shadow ([65]) on a light yellow paper, the surface will appear green. The same happens with regard to the other colours if the necessary preparations are attended to.
If, when the eye is impressed with visionary images that last for a while, we look on coloured surfaces, an intermixture also takes place; the spectrum is determined to a new colour which is composed of the two.
Physical colours also admit of combination. Here might be adduced the experiments in which many-coloured images are seen through the prism, as we have before shown in detail ([258], [284]).
Those who have prosecuted these inquiries have, however, paid most attention to the appearances which take place when the prismatic colours are thrown on coloured surfaces.
What is seen under these circumstances is quite simple. In the first place it must be remembered that the prismatic colours are much more vivid than the colours of the surface on which they are thrown. Secondly, we have to consider that the prismatic colours may be either homogeneous or heterogeneous, with the recipient surface. In the former case the surface deepens and enhances them, and is itself enhanced in return, as a coloured stone is displayed by a similarly coloured foil. In the opposite case each vitiates, disturbs, and destroys the other.