138 EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 



bright yellow from orange to yellow, bright but not coloured from yellow- 

 green to blue, and then strongly coloured hi 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 deprive the green 

 rays, which have no optical connection with them, of their distinctive 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 shewed 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 

 similarity 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 obser- 

 vations gives 



19 G + -05 B+76 Bk=100R (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, ultra- 

 marine, and emerald-green is given in diagram (l). 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. 



