1822.[1 of Vision with Regard to Colours. 13S 



If they are insensible to the green ray, they may, as has been 

 shown, have the sensations seeing red, seeing yellow, and seeing 

 orange; or seeing redy seeing bluey dind seeing violet j produced by 

 the action of the prismatic rays. 



These individuals confound pink with blue. Pink is a faint 

 shade of red. Now as 1 have argued that the red and the 

 green rays cannot each affect the retinae of these persons so as to 

 produce seeing^ but that their retinse must be insensible to one of 

 these two prismatic rays, so may I adduce the same arguments 

 to prove that they cannot be sensible both to pink and to blue 

 rays ; but that they must be insensible either to pink or to blue 

 rays. If they are sensible to pink rays, we must conclude that 

 they are also sensible to red rays. But if they are sensible to 

 pink, we must suppose them to be insensible to blue ; conse- 

 quently, if they are sensible to red and pink rays, they are insen- 

 sible to blue rays. 



It appears then that the retinae of these persons are sensible 

 either to yellow, blue, and green rays, or to red, yellowy and 

 orange rays. In either case, their retinae will be sensible to 

 yellow rays. 



We accordingly find that these individuals never confound 

 the sensation which is excited in the presence of the yellow ray, 

 with any of those which are produced in the presence of either 

 of the other prismatic rays. Their retinae are fully sensible to the 

 yellow ray. 



If then the retinae of these persons be sensible to green and to 

 blue rays, they are insensible to red and to orange rays. 



As the green ray is compounded of the yellow and of the blue 

 rays ; as the orange ray is compounded of the red and of the 

 yellow rays ; as yellow enters into the composition both of the 

 green and of the orange rays ; and as the retinae of these persons 

 are sensible to yellow; it appears that they are insensible either 

 to the red or to the blue rays of hght. 



If then these persons can have the sensation seeing red, they 

 cannot have the sensations seeing green, seeing blue, or seeing 

 violet ; and if they can have the sensation seeing blue, they 

 cannot have the sensations seeing red, seeing orange, and seeing 

 violet. In either case then they must be insensible to the violet 

 rays. 



It sometimes happens when the retinae of ordinary individuals 

 have been for a long time acted upon by strong light, that if the 

 blended rays of light be then thrown upon the retina, the sensa- 

 tion which results from their action is that of seeing red, we 

 explain this occurrence by supposing, that in consequence of 

 long exposure to strong light, the retina has become insensible 

 to all the rays, excepting the red rays. The red ray then appears 

 to exert a more powerful influence on the retina than any of the 

 prismatic rays. 



Of all the prismatic rays, the red ray is the least refrangible, 



