KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 451 



he would call the most brilliant greens, and I should say that b and E were my warm 

 colour, but so pale that I could see hardly any colour there at all, and between b and F it 

 vanished altogether. He might point to some ladies' dresses which he would say were dark, 

 powerful green, but I should say they appeared quite black to me. He might then ask me 

 whether I could detect any idea of a colour-sensation which was recognised as running 

 through the whole of the specimens, and which he called greenness ? I should reply that 

 I certainly could not see anything of the kind ; they had no feature of connexion or 

 similarity, nothing whatever in common ; and • that consequently his greenness was an 

 idea to which I was an utter stranger. 



He might then probably show me samples of red ; and I should tell him that a 

 soldier's coat, and vermilion, belonged to my warm colour, more strongly coloured than 

 the green, but still darkened. Crimson-lake was darker still, and with less colour, and 

 in a light tint became grey. Pink ribbons appeared pale varieties of my cold colour. I 

 might add that I sometimes wrote accidentally with red ink, thinking it was ordinary ink 

 a little pale. He would soon infer that the idea of redness, to him pervading all the 

 samples, was, like greenness, quite unknown to me. 



I might then ask him if he could produce me a sample of either of these colours, green 

 or red, which would give me the maximum sensation of my warm colour. He might try, 

 but I know he could produce nothing which, compared with my brilliant colour, was not 

 defective, to my vision, either in saturation or in luminosity. 



It might then probably occur to him that there was another warm colour which he 

 had not yet tried, namely, yellow, and he would produce some samples of it. The case 

 then would be altogether changed. The moment he produced a buttercup or calceolaria, 

 or the pigment chrome-yellow, or a portion of the spectrum near D, I should instantly 

 identify them with the maximum sensation of my warm colour. I would then 

 ask him to show me a number of different samples, either saturated or pale, or darkened 

 into brown, in any variety, provided only that he himself would call them yellow. I 

 could then tell him that I myself now saw the consistent feature of one colour-sensation 

 running through all the varieties ; and I think we should both agree in the probability 

 that my warm sensation was identical with his sensation of yellow. 



By a similar process, my cold colour would easily be proved to correspond with his 

 blue. 



This is the kind of evidence procurable from the dichromic patient ; indirect and 

 inferential, it is true ; but of precisely the same practical character and weight as that which 

 determines the general similarity of vision of normal-eyed people. And it must be 

 observed that, in spite of the individual differences found in dichromic patients, whether 

 they be called red-blind or green-blind, their testimony as to the yellow and blue is always 

 the same. 



I cannot help thinking that, in spite of his defect, the testimony of a colour-blind 

 person, if he has taken due pains to investigate his impressions, and has embraced the 

 opportunities open to him for learning the ordinary facts and opinions about colour 



