1892 - 93 .] Dr William Pole on Colour-Blindness. 
ri7 
T. 
Explanations and Remarks on my own Ohservations of the 
Variations in Dichromic Vision, 1859 and 1892. 
The observations are described in the Philosophical Magazine 
for July 1892, and they show considerable variations of the precise 
kind met with in practice. 
It will be seen that the more important variations lie in two 
particulars only, viz., in the colour intensity (or, as I call it, the 
“ chromic strength ”) of the impressions produced by the rays 
corresponding to the normal red and green. There is little doubt 
that the two extreme cases fairly represent the two classes into 
which dichromic vision has been erroneously divided. I myself 
form one extreme, having been pronounced “green-blind.” Mr 
Parry forms the other extreme, and would certainly have been 
called “ red-blind.” The others are intermediate between these. 
I had no means of testing the cases with spectral colours ; but, 
thanks to the elegance and the accuracy of Clerk Maxwell’s 
contrivance, the facts come out clearly and consistently. 
Let us first look at the impressions of the red rays. Carmine, 
the strongest specimen of the normal red colour, required to match 
it for me 10 parts of yellow mixed with 90 of black ; for Mr Parry 
it required only 3 parts of yellow with 97 of black ; showing that 
the colour impression made by the carmine on me was 3J- times as 
strong as on Mr Parry. Probably this colour may have corresponded 
to somewhere in the spectrum between B and C, so that, as it was 
scarcely visible to Mr Parry, he must have had a shortened 
spectrum. 
Vermilion required 23| parts of yellow for me ; 8 for Mr Parry, 
or about 3 to 1. Orange gave a proportion of 5 to 3J. 
I7ow let us turn to the green element, of which there are two 
specimens, a very dark one, brunswick-green, and a lighter one, 
emerald-green. With these colours the comparative strengths are 
reversed. For me brunswick-green required only 9J parts of 
yellow, for Mr Parry it required 15. Emerald-green required for 
me 23 parts, for Mr Parry 43 parts. Hence, the “ chromic strength ” 
of the green colour may be said in round numbers to be about twice 
as great on Mr Parry as on me. 
