AS PERCEIVED BY THE EYE. 285 



appears faintly luminous in the red ; bright yellow from orange to yellow, bright 

 but not coloured from yellow-green to blue, and then strongly coloured in the 

 extreme blue and violet, after which it seems to approach the neutral obscure 

 tint of the red. It is not easy to see why an insensibility to red rays should de- 

 prive the green rays, which have no optical connection with them, of their dis- 

 tinctive appearance. The phenomena seem rather to lead to the conclusion that 

 it is the red sensation which is wanting, that is, that supposed system of nerves 

 which is affected in various degrees by all light, but chiefly by red. We have 

 fortunately the means of testing this hypothesis by numerical results. 



Of the subjects of my experiments at Cambridge, four were decided cases of 

 colour-blindness. Of these two, namely, Mr R. and Mr S., were not sufficiently 

 critical in their observations to afford any results consistent within 10 divisions of 

 the colour-top. The remaining two, Mr N. and Mr X., were as consistent in their 

 observations as any persons of ordinary vision can be, while the results showed 

 all the more clearly how completely their sensations must differ from ours. 



The method of experimenting was the same as that adopted with ordinary 

 eyes, except that in these cases the operator can hardly influence the result by 

 yielding to his own impressions, as he has no perception whatever of the simi- 

 larity of the two tints as seen by the observer. The questions which he must 

 ask are two, Which circle appears most blue or yellow ? Which appears lightest 

 and which darkest ? By means of the answers to these questions he must adjust 

 the resulting tints to equality in these respects as it appears to the observer, and 

 then ascertain that these tints now present no difference of colour whatever to 

 his eyes. The equations thus obtained do not require five colours including 

 black, but four only. For instance, the mean of several observations gives — 



•19 G + -05B + -76Bk = 100 R (4). 



[In these experiments R, B, G, Y, stand for red, blue, green, and yellow papers 

 prepared by Mr D. R. Hay. I am not certain that they are identical with his 

 standard colours, but I believe so. Their relation to vermilion, ultramarine, and 

 emerald-green is given in diagram (1). Their relations to each other are very 

 accurately given in diagram (2).] 



It appears, then, that the dark blue-green of the left side of the equation is 

 equivalent to the full red of the right side. 



Hence, if we divide the line BG in the proportion of 19 to 5 at the point (3, 

 and join R/9, the tint at /? will differ from that at R (to the colour-blind) only in 

 being more brilliant in the proportion of 100 to 24, and all intermediate tints on 

 the line R (3 will appear to them of the same hue, but of intermediate intensities. 



Now, if we take a point D, so that RD is to R {3 in the proportion of 24 to 

 100-24, or 76, the tint of D, if producible, should be invisible to the colour-blind. 

 D, therefore, represents the pure sensation which is unknown to the colour-blind, 



