COLOR-BLINDNESS AND COLOR-PERCEPTION. 93 



and, when we consider the infinite degrees of incapacity that may exist 

 for all the different colors, we can readily understand the infinite varia- 

 tion 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 dif- 

 ferentiate the various impressions it receives. This opinion will seem 

 the more plausible when we remember that the sense of sight is a de- 

 veloped or educated one. Though we have received from our ances- 

 tors the potentiality of vision, every child that is born must learn to 

 see for itself. Without here entering into a discussion of the ques- 

 tion 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 differen- 

 tiation 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 devel- 

 oped where the faculty was most used. And so we do find it. Wom- 

 en, who are much more concerned than men in the selection and com- 

 parison of colors, are rarely affected with color-blindness ; and we all 

 know how much quicker the feminine eye is in detecting slight differ- 

 ences 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 discrim- 

 inating between the impressions corresponding to the different colors. 

 The retina may be capable of properly responding to these various im- 

 pressions, 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 gov- 

 erning the other senses at least, I do not believe it will be found nec- 

 essary 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 phys- 

 ical reaction of certain simple substances to the undulations of the lu- 

 miniferous ether. This reaction may be in its restricted sense chem- 

 ical, 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 e^her. In other words, the variation in the sensation 



