1 20 Proceedings of Royal Society of Edinburgh. [sess. 
green ray will appear less yellow. If, therefore, he has to find 
a green colour which will match to him a given red, he must 
obviously choose a much “ yellower green than would match for 
the right-hand curve patient. And this is exactly what Helmholtz 
gives as the test for “green-blindness.” 
We arrive, then, at the conclusion, as shown by the comparison 
of the experiments and the curves, that the whole of the phenomena 
characterising the variations of dichromic vision may be attributed 
to the simple fact of a slight variation of the position of the yellow 
sensation curve along the line of the wave-lengths, so that this 
sensation may be excited to a given extent, in two different persons, 
by waves slightly differing in length : the maximum difference, 
equal to some 30 or 35 millionths of a millimetre, will then 
constitute the difference between the extremes, formerly called 
“red-blind” and “green-blind.” 
There seems nothing in this to suggest different “groups” in 
classification, or any important fundamental difference in the cause. 
V. 
Lord Rayleigh^ s Experiments on Variations in Colour Vision, 
1881. 
The author says ; — “ The trials revealed an interesting peculiarity 
of colour vision quite distinct from colour-blindness. The red and 
green mixture, which to my eyes and to those of most people 
matches perfectly the homogeneous yellow of the line D, appeared 
to my three brothers-in-law hopelessly too red, ‘almost as red as 
red sealing-wax.’ In order to suit their eyes, the proportion of red 
had to be greatly diminished, until to normal sight the colour was 
a fair green with scarcely any approach to yellow at all. These 
peculiarities were quite unexpected 
“ I have obtained matches between simple and compound yellow 
from 23 male observers; of these 16 agree with myself within the 
limits of observation. The remaining seven include my three 
brothers-in-law and two others. The vision of the other two 
observers differs from mine in the opposite direction Among 
seven female observers whom I have tried, there is not one whose 
vision differs sensibly from my own.” 
