KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 443 



The first scientific notice of the existence of an abnormal vision of colours in certain 

 individuals appears to have been Huddart's letter to Dr Priestley, printed in the Philo- 

 sophical Transactions for 1777, after which several accounts of such cases were published 

 from time to time, including Dalton's description of his own vision in 1794, and See- 

 beck's detailed analyses of several cases in 1837. But the first step of real scientific 

 importance was the discovery by Sir John Herschel of the dichromic explanation of the 

 phenomenon. This was conveyed by him in a letter to Dalton, dated 30th May 1833 ;* 

 but as Dalton did not agree with the view (having a pet theory of his own), he made no 

 use of the letter. In 1845 Sir John wrote his well-known article on "Light," for the 

 Encyclopedia Metropolitano:, in which (Art. 507-8) he spoke of " the curious affection of 

 vision occasionally met with in certain individuals who distinguish only two colours, which 

 are generally found to be yellow and blue," and he illustrated this by a set of very in- 

 teresting and instructive observations on " a celebrated optician" (Mr Troughton), which 

 explained the theory. This was the first publication of the idea of dichromic vision ; 

 but it seems to have attracted no notice till the celebrated letter to Dalton was found, 

 and published after his death in 1854. 



In 1855 appeared the first important book on the subject, namely, Researches on 

 Colour- Blindness, by Professor George Wilson, M.D., of Edinburgh, an excellent and 

 elaborate work, treating of the subject very fully in all its bearings, so far as it was then 

 known. Among other things the author described a practical system of testing colour- 

 vision by samples of coloured wool — a mode which, as subsequently extended by Holm- 

 gren, has become most popular and useful. Dr Wilson had taken much interest in 

 Dalton's case, and had published a special essay upon it. He, however, doubted the 

 application to it of Herschel's dichromic principle ; he believed that Dalton was not, 

 under favourable circumstances, insensible to red ; and he inclined strongly to the opinion 

 that the number of instances where the vision was perfectly dichromic, i.e., where the 

 true sensation of red was altogether wanting, were very few. 



In the same year Mr Clerk Maxwell communicated to the Royal Society of Edin- 

 burgh his valuable paper (now become classical), "Experiments on Colour as perceived by 

 the Eye, with Remarks on Colour-Blindness," in which he made known his novel mode 

 of experimenting on colour mixtures by the elegant device of revolving circular discs. 

 In this paper he appeared to concur with Dr Wilson that the dichromic theory was not 

 strictly applicable to the more definite cases of colour-blindness, for he said (page 284), — 

 " In experiments made with the pure spectrum, it appears that though the red appears 

 much more obscure than other colours, it is not wholly invisible." 



My own work immediately followed. I had long known that I was one of the unfor- 

 tunate sufferers from the abnormal vision of colours, and it occurred to me that it would 



* Sir John Herschel, being interested in Dalton's description of his vision of colours, had sent him some 

 " Optical Queries," accompanied by test-glasses and by samples of coloured silks, to which Dalton seDt replies, as 

 mentioned in the letter, which was in fact Herschel's judgment upon them. At a later time Sir John put these 

 data in my possession, and I was enabled thereby to discover the exact particulars of the nature of Dalton's vision. — 

 See Phil. Mag., July 1892. 



