COLOR-BLINDNESS AND COLOR-PERCEPTION. 



95 



sion has passed beyond the retina, there is no special or important dif- 

 ference m the views as to the final conversion into a sensation The 

 objections to these two hypotheses we have already stated. The accept- 

 ance of such an hypothesis as we propose, however, does not involve 

 the necessity of inventing new laws, or of creating new issues, but 

 only applies known laws and analogous reactions of other substances 

 to the elucidation of the phenomena observed. We know that there 

 are membranes which respond with promptness to any number of sim- 

 ple aerial vibrations at the same time, and recent discoveries have 

 shown that there are substances which, when in proper condition thus 

 respond to wave-lengths of light. Silenium, when in a crystalline 

 condition, alters its molecular condition (as manifested by its varying 

 resistance to the passage of the electric current), not only when acted 

 on by light of varying intensity, but also by the different wave-leno-ths 

 If, then, we suppose the retina to be a substance of this nature but 

 responding more promptly, and in a more delicate manner, than' anv 

 other known substance to the wave-lengths of light, we have a basis 

 tor a theory of vision which is extremely simple in its nature, and 

 tounded on known physical laws. 



We will not here enter upon a detailed application of this theory 

 to the elucidation of all the phenomena of colored vision, but will sim- 

 ply mention a few points in connection with color-blindness. One 

 general principle may be laid down which will cover all cases of reti- 

 nal color-blindness as distinguished from cerebral or central, and that 

 is, that in these cases the molecular structure of the retina is so al- 

 tered as to allow it to respond feebly or not at all to light rays of cer- 

 tain wave-lengths. We know, for example, that silenium must be in 

 a crystalline state-that is, its molecular structure must be in a certain 

 definite condition-before it can respond in such a delicate manner to 

 variation m the intensity of the light-waves ; and we know that there 

 are certain wave-lengths of the ether-the ultra-red and the ultra- 

 vl olet-which call forth no reaction on the part of the retinal sub- 

 stance. It would, therefore, be a highly justifiable supposition that a 

 slight alteration m the molecular structure of the retina would render 

 it incapable of being affected by certain wave-lengths to which it, 

 when in a normal condition, readily responds. This incapability may 

 be partial or complete as regards any particular wave-lengths. In 

 some instances of color-blindness, for example, the spectrum is short- 

 ened at the red end even under the most intense illumination, while 

 in others there is a shortening only when the illumination is feeble- 

 becoming of normal length when the intensity of the illumination is 

 increased-showmg, in the latter case, that the reaction to the red rays 

 is still present when they are of sufficient intensitv. 



When we come to cerebral color-blindness, which is, according to 

 my view, the most common, the explanation is still simple. In this 

 instance we have only to suppose the cerebral center of vision incapa- 



