132 Dr» Nichoil on a peculiar Imperfection [Feb. 



the latter case, seeing redy seeing blue, and seeing violet, may 

 arise from the action of light. In either case, seeing blue may- 

 be produced. 



If a retina be insensible to the green ray, it may be so from 

 beino' insensible, either to the yellow, or to the blue rays, of 

 whicli two rays the green ray is compounded. In the former 

 case, seeing red, seeing blue, and seeing violet, may arise ; in the 

 latter case, seeing red, seeing yellow, and seeing orange, may 

 arise from the action of light. In either case, seeing red may 

 arise. 



If a retina be insensible to the violet ray, it may be so from 

 being insensible either to the red or the blue rays, of which two 

 rays the violet ray is compounded. In the former case, seeing 

 yellow, seeing blue, and seeing green, may arise ; in the latter 

 case, seeing red, seeing yellow, and seeing orange, may arise from 

 the action of light. In either case, seeing yellow may arise. 



It appears then that if a retina be insensible to a primary or to 

 a compound ray of light, it can only be affected by light so as to 

 give rise to the sensations seeing three prismatic colours, two of 

 which colours vvill be primary colours, and the third, a colour 

 compounded of these two. 



Let us apply the preceding observations to the retinae of the 

 individuals who are the subjects of the cases alluded to at the 

 commencement of this paper. 



By these individuals red is confounded with green, and pink 

 with blue. 



We have seen that, in ordinary retinae, the sensual state A, or 

 that which produces seeing red, is different from, and indeed 

 opposite to, that state, D, which gives rise to seeing green. It is 

 highly improbable then that in these individuals the red ray and 

 the green ray should excite the same kind of sensual state of 

 retina, or that two different sensual states (as A and D) should 

 be produced by the action either of the red or of the green ray. 

 If then the red ray and the green ray cannot produce the same 

 sensual state, and if two different sensual states (as A and D) 

 be not produced by those two rays, it follows, that in these indi- 

 viduals, one of those rays fails to produce a sensual state of the 

 retina; that, in other words, their retinae are insensible, either to 

 the red or to the green ray ; consequently, that if they have 

 sensation produced by the red ray, they have none excited by 

 the green ray, and vice versa. 



We arrive then at this conclusion, that the retinae of these 

 individuals are insensible, either to the red ray, or to the green 

 ray. 



If they are insensible to the red ray, those individuals can, as 

 we have seen, only have the sensations seeing yellow, seeing blue, 

 'dnd seeing green, produced, by the prismatic rays. 



