116 Proceedings of Roy cd Society of Edmhurgh. [sess. 
and matching external colours, which is manifested by those having 
different varieties of the affection. 
In a letter to me, dated 28th March 1881, he says : — “ We seldom 
find two colour-blind persons of the same class who agree in all 
particulars. But that is the case in ail Nature. There are no two 
plants or animals perfectly alike, but they may, nevertheless, have 
certain common characters which may justify us in a systematic 
separation of them. I was the first to succeed in showing that the 
“red-blind” cannot subjectively see either red or green, and so may 
it well be with the green-blind. Thus the names do not refer to 
the subjective sensations of these persons.” 
“ I hold that neither of the theories now popular is perfectly 
right. But I adhere to the practical objective distinctions between 
colour-blind persons, who, in actual fact, treat certain kinds of light 
in different ways. And I am of opinion that these two classes of 
persons, who may be distinguished by my wool-test, may well 
be described as red-blind and green-blind. For so long as we 
possess no means of looking into the subjective sensations of other 
people, we shall do well to base our classification on objective signs. 
I believe further, that I have already found that the exact hues of 
the colours of the colour-blind are not always the same.” 
In a later letter, dated February 1882, he says: — “When 
I have asserted that the results of observation are consistent 
with Young’s theory, I only mean with the principles of the 
theory, but not with the form in which the theory is usually 
applied.” 
A year or two after this, in Professor Holmgren’s lecture, given 
at the Congress at Copenhagen in 1884, he gave full particulars 
of the remarkable cases he had examined of dichromic vision in one 
eye, and he exhibited diagrams of the appearance of the spectrum 
to them. It happened that these cases included patients of both 
varieties, and he described the colours that they saw. He said : — 
“ The two principal colours seen by the ‘ red-blind ’ patient were 
yellow, inclining towards green; blue, inclining to violet. While 
for the green-blind patient they w^ere orange-red and cyanide-blue ” 
(^.c., blue, inclining to green). 
The positions of the neutral points were for the “ red-blind ” near 
F, and for the “ green-blind ” nearer h. 
