1892-93.] Dr William Pole on Colour-Blindness. 
129 
determining the former of these by experiment ; and he has made 
approximate estimates of some of the latter. 
It is curious that Helmholtz, in the new edition of his great work, 
although he does not allude to Hillebrand’s researches, notices the 
duplex nature of the luminosity. He says, p. 440 : — “ For my part 
I have, throughout, the ideal impression that in heterochromic 
comparisons of luminosity, we have to do, not with comparison of 
one quantity, but with the joint action of two, luminosity and 
colour-glow, for which I am unable to form any simple sum, 
and which I am also unable scientifically to define.” 
AE. 
Professor Clerk- Maxivell on the Palliation of Colour-Blindness. 
There is a general impression that as red-green blindness is 
congenital, and as there is no means known of changing it to normal 
vision, the sufferers have no remedy. This is a mistake. As early 
as 1855 Professor Clerk-Maxwell pointed out, in his paper to the 
Royal Society of Edinburgh (page 287) that much relief in regard 
to the confusion of red with green could be given by coloured 
spectacles, one eye being furnished with a red and the other with a 
green glass. The effect of using these would be that, looking through 
the red glass, the reds would appear brighter and the greens darker, 
while the green glass would reverse these effects. 
He was kind enough, at a later time, to send me such a pair ; and 
on my asking “what was his charge for them,” he wrote me the 
following admirable letter : — 
Glenlaie, Dalbeattie, IST.B., 
lOtli November 1869. 
My Dear Sir, — With respect to the spectacles my charge to you (and it is 
a strict one) is that you should put them on and look at some variegated 
object, such as a bright-coloured carpet or needlework, comparing the appear- 
ance of doubtful colours as seen by each eye. I mean colours which are doubtful 
to you, whether they are called by others red, green, or drab. 
Of course it will be possible for you to find out by a process of reasoning 
whether any particular colour is red, green, or drab, by observing whether it 
appears much brighter, much darker, or nearly the same through the two 
glasses. You will then act like a chemist testing bodies apparently similar by 
different reagents. 
VOL. XX. 26/7/9 
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