Data on Colour-Blindness. 103 



seemed a very near match to him ; while to me there was the 

 difference that the green was the darker by two numbers of 

 Chevreul's yellow gamme. This fixed Dalton's variation as 

 about intermediate between my two extremes, probably 

 corresponding pretty nearly with Mr. Lloyd's. 



The quantitative data here collected, and all comparable, 

 furnish very important materials for inference and reasoning as 

 to the still open question of the varieties of colour-blindness. It 

 is not my object to enter further on that argument now, but it 

 is necessary to show that, in order to make use of the facts by 

 proper classification, more detailed attention must be devoted 

 to the minute peculiarities of dichromic vision ; and this 

 opens another point in which my original paper was deficient. 

 I had given a general comparison between the normal and 

 the dichromic hues seen, referring briefly to various tints and 

 shades of them ; but it is now necessary to examine these 

 tints and shades more accurately than has, I think, hitherto 

 been done. For the proper understanding of what we really 

 do see depends almost entirely on this point, and it is often 

 much misunderstood. 



Although the dichromic patient, as his name implies, sees 

 only two varieties of hue, yet the number of colour-impressions 

 he derives from these admit of great diversity of character : 

 and this is the reason why his defect so often escapes notice. 

 He hears of the variety in colour presented by nature ; and he 

 knows that nature also offers great variety to him : what he 

 does not know till he is taught, is the different nature of the 

 variety in the two cases. It is this we have now to explain. 



The colour-impressions received in dichromic vision may 

 be classed in eleven species, seven of which further admit each 

 of infinite variety in degree. And in describing them I shall 

 adhere, without discussion, to my original names of yellow 

 and blue for the two hues seen. Then we have : — 



1. The impression of the full yellow colour in its most 

 powerful form. 



2. The corresponding impression of the full blue colour. 



3. Then there is the impression of what Sir John Herschel 

 called " the equilibrium of the two," or full white. And 



4. There is what he called the " negation " of the two, i. e. 

 black. 



According to the method of testing by pigments, these four 

 form what we may call the fundamental colour-impressions 

 of dichromic vision, by combinations of which all other colour- 

 impressions are produced. These combinations form by far 

 the most important part of the visual phenomena, and require 



