134 Dr. Nicholl on a peculiar Imperfection [Feb. 



and the most heating ; next in order, in these respects, is the 

 orange ray ; and, thirdly, the yellow ray. 



From what has been stated then it appears probable that if a 

 retina be insensible either to the red, or to the blue rays, it is 

 insensible to the latter of these rather than to the former. 



If then we suppose that the retinae of these individuals are 

 insensible to the blue rays, they are sensible to the red, to the 

 orange, and to the yellow rays, which are the three most power- 

 ful, most heating, and least refrangible, of all the calorific rays. 



We arrive then at this conclusion that the retinae of these 

 individuals are sensible to red and to yellow rays, and to the 

 fays compounded of these two sets of rays, or to orange rays ; 

 but that they are insensible to green, to blue, and to violet rays. 



I proceed, lastly, to compare the conclusions at which I have 

 arrived with the facts observable in the cases of these indivi- 

 duals. But it must be recollected, that it is impossible to ascer- 

 tain the kind of the sensation seeing which any ray of light 

 produces in another person by its action -upon his retina. We 

 can only ascertain w hether two different rays do or do not excite 

 in him two sensations, which he describes as differing from each 

 other. For instance, it is impossible to ascertain whether the 

 red ray produces in another person the same kind of sensation 

 that it excites in me ; but I can ascertain whether the red ray 

 produces in him a sensation different from that which he receives 

 n\ the presence of any other ray. Both he and I agree to call 

 the sensation which the red ray excites in each of us seeing red, 

 but this term does not describe the kind of sensation ; it merely 

 denotes that such sensation is different from the sensations 

 which are produced in him by other prismatic rays. 



The action of the first set of the prismatic rays upon the retinae 

 of these individuals gives rise to the sensation seeing a colour; 

 and to describe this sensation, they employ the term which 

 others use to denote the sensation produced by the action of that 

 set, they call it seeing red.* 



The action of the second set of the prismatic rays upon the 

 retinae of these persons gives rise to the sensation seeing a colour, 

 which they describe as being of a kind different from that which 

 arises from the action of the first set of rays ; and to describe 

 this sensation, they employ the term which others use to denote 

 the sensation. produced by the action of the second set of rays; 

 they call it seeing orange. 



The action of the third set of the prismatic rays upon the 

 retinae of these persons gives rise to the sensation seeing a colour, 

 which they describe as being different from those which are 

 produced by the first and second sets of prismatic rays ; and to 

 describe this sensation they employ the term which others use to 



♦ They sometimes call it also seeing green^ since they use the terms -teeing red and 

 tc^Hg green indiseriminatcly to denote the same kind of sensation. 



