113 
1892 - 93 .] Dr William Pole on Colour-Blindness. 
with pigments, then with revolving discs, and finally with spectral 
colours. 
The results fully confirmed what was known before, and gave 
additional weight to the important fact of the permanence of the two 
complementary pairs of colours, namely, definite varieties of yellow 
and blue, and of purple-red and blue-green. Any other colour, the 
moment it passes out of the central area of vision, will begin to 
change, and will ultimately turn into one of the four colours named. 
Moreover, in the final extinction of these colours, as they 
approach the colourless peripheral zone, the red and green pair 
first disappear, the yellow and blue remaining much longer. White 
light remains unchanged over the whole retina. 
Thus the two complementary pairs stand out as naturally and 
strongly distinguished from all other hues, the yellow and blue pair 
having a special preference. And it is difficult to avoid the inference 
that these properties must have an important bearing on the explana- 
tion of colour-blindness. 
O. 
Cohn^s Examination in the Government Schools, 1877. 
At the beginning of his work he did not know what theory 
would best apply, and chose tests which would throw light on the 
matter. Out of 3490 young persons examined, 100 were found 
Avith defective vision, and were then examined very fully with 
spectral and other tests of various kinds. Eighty showed unmistake- 
ably they had no correct normal sensations of either red or green, 
for the most part placing the brightest part of their spectrum in or 
near the yellow. The following are opinions extracted from his 
summary : — 
No system can be deduced from the comparisons which these 
persons made with the wool-tests, they were too varied. If really 
the red and the green blindness were distinct from each other, the 
patients would have made certain matches which they did not 
make. 
Every red-blind person is also green-blind ; every green-blind 
person is also red-blind. If mistakes with green are ever so small, 
mistakes with red are also discovered. 
It is only by the careful investigation of every case with all the 
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