and, when we consider the infinite degrees of incapacity that may exist for all the different colors, we can readily understand the infinite variation in the mistakes of the color-blind, and the impossibility of laying down exact rules for diagnosis.
It is my belief that a large number, perhaps a majority, of the cases of congenital color-blindness have not their seat in the retina at all, but are cerebral in their character. In other words, I believe that in these cases the brain-center of vision has not the power to differentiate the various impressions it receives. This opinion will seem the more plausible when we remember that the sense of sight is a developed or educated one. Though we have received from our ancestors the potentiality of vision, every child that is born must learn to see for itself. Without here entering into a discussion of the question of the development of the color-sense, which has received much attention at the hands of Mr. Gladstone, Magnus, and others, it is safe to assume, with our knowledge of analogous matters, that the differentiation of colors is a power partly inherited and partly developed in the individual; and, moreover, we should expect to find this power, which is undoubtedly cerebral in its character, most strongly developed where the faculty was most used. And so we do find it. Women, who are much more concerned than men in the selection and comparison of colors, are rarely affected with color-blindness; and we all know how much quicker the feminine eye is in detecting slight differences in shades of color than is that of men who are not color-blind. In those cases of color-blindness which, for the sake of distinction, we shall call central, we believe that the brain-center of vision has not been developed to its full or at least to its ordinary power for discriminating between the impressions corresponding to the different colors. The retina may be capable of properly responding to these various impressions, and the optic nerve may carry them as separate impressions to the brain-center; but this has not the power of converting them into individual sensations.
From what has already been said, it is evident that neither of the two at present prominent theories satisfactorily accounts for all the phenomena of color-blindness. Moreover, it seems to me, the true theory of colors when found will be simple; and the laws governing the sense of vision will be found to bear some analogy to those governing the other senses—at least, I do not believe it will be found necessary to invent new processes and new reactions of tissues to agents affecting the economy. The true theory, I believe, will be found to lie in the direction pointed out by the recent researches on the physical reaction of certain simple substances to the undulations of the luminiferous ether. This reaction may be in its restricted sense chemical, purely physical, or chemico-physical; but it will be due to the changes in the molecular structure of simple substances, caused by the action of the either. In other words, the variation in the sensation