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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 says 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+-05 B+-76 Bk=100 R Bidet Gist. comet: 
_ [In these experiments R, B, G, Y, stand for red, blue, green, and yellow papers 
prepared by Mr D. R. Hay. Iam 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 8, 
and join R £, 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 6 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@ 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, 
