These contrast colours are usually said to be _complementary_ to the spectrum colours employed, though it must be recollected that what a complementary colour should be is determined by the quality of the white light which the two, when mixed, are made to match. But recent measures of my own show that they are not truly complementary in most instances, whatever the white light may be. But whether they are or not does not much matter when the explanation offered by the followers of the Young theory is considered, for it is asserted that such contrast colours have no real existence, but are psychological, or--what this comes to be--simply delusions. If they are not real colours felt by the retina, they have a very good resemblance to them, and the same series of delusions are so persistent and so constant for all normal vision that they can always be measured as having a constant value. I bear in mind the experiment in which the contrast colour, after being produced, is isolated in the eye from the colour producing it and the background, and the continuance of the hue produced by the contrast. This _retention_ may be psychological, but there are no grounds to my mind for saying that its _production_ is due to the same cause, more especially as experiments have been arranged to show that one eye may see a contrast colour, whilst the other may see it of its uncontrasted hue. In this last experiment it can scarcely be conceived that one eye should be subject to delusion, whilst the other was free from it. If, then, we may presume that they are real colours, the Young theory fails to explain them, and the explanation offered by the Hering theory is much more acceptable, as it propounds the idea that the retina has to be considered as a whole, and that if (say) red light is at work at one part its complementary colour (blue-green) must be felt at another. It would be still more acceptable had it happened that the contrast colours were truly complementary, and if the same action was noticeable when the adjacent part of the retina was not also stimulated.