1892 - 93 .] Dr William Pole on Colour-Blindness. 123 
inatcli for his fmiclamental red with colour cards, one required 261° 
R 
red -1- 99° blue, the other 347° red + 13° blue; the ratio g being 
in one case = 2 ’6, in the other = 26'7. 
In another example, with five persons, combining a spectral red of 
j> 
'w.l. 660 with a spectral blue of 447, the proportion jr in the five 
cases was 
= M5, 3-00, 4-71, 5-66, 7-00. 
To make his fundamental green with a yellow-green and blue, 
G_ 
the proportion for two individuals was 5 and 16. In the re- 
lations between yellow and blue no difference in hue was observed. 
He then gives explanations and arguments at much length, tend- 
ing to show that the cause of all these variations, as well as of the 
variations in dichromic vision, is probably to be found in varying 
absorptions by colouring of the macula, and to a certain extent of 
the lens also. He also considers that the stronger absorption will 
be accompanied by greater sensitiveness to colour generally. 
This being so, his explanation Avould be as follows : — 
A dichromic patient A. has strong sensitiveness and strong absorp- 
tion. He will be powerfully impressed by the long-waved rays 
(red) ; but his strong absorption diminishes the colour of the shorter 
rays (green). This is the “green-blind.” 
Another dichromic patient has weaker sensitiveness and weaker 
absorption. The former diminishes the force of the long-waved 
rays (red), but the less absorption gives him a stronger sense of the 
short ones (green). This is the “red-blind.” 
Helmholtz objects to this: — (1) That the colouring of the lens 
does not exist to any appreciable extent in healthy eyes ; and (2) 
that the colouring of the macula only acts on a limited portion of 
the visual field and only on certain colours. But he admits that 
further investigation on this point is desirable. 
AA. 
FicBs Explanation of the Dichromic Colours^ 1874 and 1879. 
Referring to the dichromic zone of the normal retina, he re- 
marked that the ordinary application of the Young-Helmholtz theory 
