Data on Colour '-Blindness . 101 



paper left incomplete, and I was advised to add supplementary 

 explanations, but, for various reasons, this was not done. As, 

 however, there is still much uncertainty and misunderstanding 

 on the subject generally, it appears to me that, even after 

 this lapse of time, it may be useful to put on record any facts 

 or observations which may tend to throw light upon it. 



The paper contained, so far as I know, the earliest com- 

 plete quantitative demonstration of the general facts of 

 dichromic vision, made, at the suggestion of Prof. Stokes, by 

 Clerk Maxwell's elegant device of the colour-top, then newly 

 introduced. I carried out the necessary series of experiments, 

 first on my own vision, and afterwards on that of some other 

 persons reputed to be colour-blind ; and I was surprised to 

 find in the latter an anomaly which led me to add the 

 following postscript, dated October 1859 : — 



"Since the foregoing paper was written I have had the oppor- 

 tunity of examining three colour-blind persons, and have found 

 that the visiou of each is perfectly dichromic, corresponding pre- 

 cisely in general character with my own. But the remarkable 

 feature has presented itself, that the coefficients of the colour-top 

 equations vary considerably in the different cases. Thus, for 

 example, although my equation XV. will always hold in its general 

 form, 



m Vermilion + n Ult. =p Black -\-q "White, 



yet the values of m, n, p, and a will vary for different individuals." 



The observations themselves were not given, but I propose 

 to add them now, with others, as being in many respects of 

 permanent interest. 



Two of the persons examined were brothers, named Nicoll- 

 Carne, gentlemen of high position and education, residing 

 near Cowbridge in South Wales ; and they had already, at a 

 much earlier time, acquired some public celebrity in regard 

 to this matter. On the 25th of June, 1816, a paper was read 

 before the Royal Medical and Chirurgical Society of London 

 by Whitlock Nicholl, Esq., M.D., of Cowbridge, entitled 

 "Account of a curious Imperfection of Vision;" and it referred 

 to " a boy 11 years old,*clever, lively, and healthy," who made 

 the kind of mistakes which are now so familiar to us, but 

 which were then thought so strange and unaccountable. He 

 was said also to have " an infant brother," of whose vision 

 nothing could then be known. Now these two brothers were 

 the gentlemen whom, forty-three years afterwards, 1 accident- 

 ally heard of, and who kindly gave me permission to examine 

 their vision. I may give the following extracts from letters 

 received from one of them : — 



