118 R + 146 G + 96 U = 75 W + 285 B.
(Here R, G, U, W, and B are used to indicate Red, Green, Blue, White, and Black.)
This match was exact also for all the colour blind, for the deficiency in one grey is also a deficiency in the other. With a red-blind, however, very different matches can be made, as the red pigment is a complex colour. There is in it, besides red, a certain amount of yellow, whilst in the green there is, besides green, a small amount of a red and a larger amount of yellow. The yellow will not only stimulate the green sensation, but also the red where it is present. Although in complete red-blindness the red sensation is totally absent, we may expect that a mixture of red and blue, as well as of green and blue, will enable a match to be made of the grey produced by the mixture of white and black.
This was the case. We have the following proportions—
295 R + 65 U = 45 W + 315 B.
When the green disc is substituted for the red, the red-blind made the following mixture—
229 G + 131 U = 120 W + 240 B.
It is worth noticing that the amount of blue in the first mixture is about half that required for the second. This tells us that the amount of green sensation stimulated in the first case is much less than in the second. As red can be substituted for green, it should follow that green, when rendered darker, should match the red. To try this a red disc replaced the black disc, and a black disc replaced the blue. The following match was then made—
131 G + 229 B = 340 R + 20 W.
It seems impossible to believe that these mixtures, so dissimilar in colour, could ever form a satisfactory match. This last equation might have been derived from the two first, in which case it would have stood—