1892-93.] Dr William Pole on Colour-Blindness. 
119 
but becomes strongly marked in the red and green. This, com- 
bined with the absence of grouping, may be said to be the essence 
of the results obtained. 
It may now be shown how well these results correspond with 
the careful determinations of the intensity curves obtained a 
quarter of a century later. The following diagram represents the 
intensity curves of the yellow colour-sensation as determined in 
1886 by Konig and Dieterici for two dichromic patients; the left- 
hand curve for one called “ green-blind,” the right-hand curve for 
one called “red-blind.” For facility of comparison the ordinates 
have been so arranged as to make the maximum intensity the 
same ^ in both curves. 
Suppose we now take a point at 620 between C and D, 
where red rays fall, we find that the intensity of the colour im- 
iwession, measured by the ordinate of the curve, will be about 
four times as great for the “ green-blind ” as for the “ red-blind.” 
But if we take a point, say at h, where green rays fall, we find 
that the colour impression on the “ red-blind ” is nearly 50 per 
cent, greater than on the “green-blind.” At 500 it is about 
double. 
It is curious, too, how these curves confirm the peculiar feature in 
my experiments, that the difference in the impressions of the two 
classes is so much greater with the red rays than with the green 
ones. 
We can also now explain the tests applied to distinguish the 
two classes by their matches (see Transactions^ page 459). To the 
person with the left-hand curve, a given red ray will appear 
yellower than to him with the right-hand curve, while a given 
^ In the original diagrams (copied in Helmholtz, p. 367) the ordinates for 
the maximum intensities vary, but this is done for a special object not wanted 
here. The ordinates do not in any case represent real intensities, but only 
proportional ones for each curve separately. 
