insensibility to the impressions of certain colours. 137 



ray from the red object is efficacious, and the only effect is the 

 substitution of a sensation of yellow or green in place of red, 

 then we might expect that the object would appear brighter, 

 in so far as a i/ellow sensation produced by a given number of 

 red rays should be brighter than a red sensation produced by 

 the same number. • 



After some observations on the foregoing quotation, Prof. 

 Wartmann proceeds thus; " Plus tard Sir D. Brewster parait 

 avoir change d'opinion*. En partant de la supposition que la 

 choroide est essentielle a la vision, il conjecture que I'invisibi- 

 lite de la couleur rouge chez les Daltoniens est due a ce, que 

 la retine elle-meme a une teinte bleu, de telle sorte que la lu- 

 miere etant privee de ses rayons rouges par le pouvoir absorb- 

 ant de cette membrane, I'impression coloree sur la choroide 

 sera denuee de rouge.** 



Prof. Wartmann is quite mistaken in thinking that I have 

 changed my opinion ; the passage which he quotes as mine is 

 from an anonymous notice on the subject, and he was not, 

 therefore, entitled to attach my name to an opinion which I 

 did not avow. It is quite true that I wrote the anonymous 

 notice, but the notice itself, had it been Jiilly quoted, would 

 have shown what was my real opinion, as distinguished from 

 a conjecture founded upon a possible and expressed supposi- 

 tion. The passage in the anonymous notice quoted by Prof. 

 Wartmann is as follows: — "Dr. Brewster conceives that the 

 eye is in those cases (of colour blindness) insensible to the co- 

 lour at one end of the spectrum, just as the ear of certain per- 

 sons has been proved by Dr. Wollaston to be insensible to 

 sounds at one extremity of the scale of musical notes, while it 

 is perfectly sensible to all other sounds." Now this is my opi- 

 nion to which, though writing anonymously, I attach my name; 

 and in the very next sentence 1 throw out the conjecture, 

 quoted by Prof. Wartmann, from which I withheld my name ; 

 and yet he supposes that the conjecture is mine, and attaches 

 my name to it, while he overlooks my real opinion, bearing 

 my name, in the very sentence which precedes the conjecture. 



But, after all, the conjecture was not an idle one. During 

 the dissection of many hundred eyes, I observed in several 

 cases that the vitreous humour was of a decided greenish-blue 

 colour, and in other cases that the retina had a marked French 

 gray or pale blue tint, which decidedly absorbed red light. I 

 knew that in cases of colour blindness the vitreous humour 

 was not blue, or even greenish-blue, as Dr. Dalton conjectured ; 

 but 1 could not assert that in the same cases the retina might 

 not be blue, and hence I was led to hazard the idea of a blue 



* Edinburgh Journal of Science, No. 7. or vol. iv. p. 86, Jan. 1826. 



