468 DR WILLIAM POLE ON THE PRESENT STATE OF 



Donders, though a strong adherent of the Young-Helmholtz three-sensation theory, 

 did not admit that the vision of the colour-blind was formed by the simple absence of one 

 of the three colours. He held their white to be the same as the normal white, and their 

 two visible hues to be yellow, inclining to red or green, and the complementary modifica- 

 tions of blue. 



He considered dichromic vision an independent phenomenon, being a step in the 

 evolution of the colour-sense, antecedent to the present system. He was led to this by 

 the remarkable variations in the colour-perception of the normal retina. He suggested 

 that, formerly, the whole retina had colourless vision as its anterior zone has now ; that 

 afterwards an improved state was developed, with two colour-sensations, which gradually 

 extended throughout its greater part ; then came a still further improvement by the 

 addition of other colour-sensations, extending to a smaller area in the pole or centre, 

 and forming normal vision. Then, according to the well-known phenomena of atavism, 

 we find some individuals reverting to the second stage, — the dichromic patients ; and a still 

 smaller number to the first stage, — the totally colour-blind. 



Donders considered he could trace, in the retina, vestiges of several evolutionary 

 steps, somewhat as follows : — 



1 . Sensations of light and shade only. 



2. Dichromic imperfect vision (called red-blindness, with short spectrum and low 

 sensitiveness to the long- waved rays). 



3. Dichromic perfect vision (called green-blindness, with full length of spectrum and 

 full sensitiveness to the long- waved rays). 



4. Imperfect normal vision (as mentioned by Lord Rayleigh), with low sensitiveness 

 to certain colours. 



5. Perfect normal vision. 



Whether there may be any arguments from history, or philology, in support of this 

 evolution-idea, is a curious question. Many years earlier Professor Clerk Maxwell 

 (Letter to Monro, 15th March 1871) made the following significant remark — " I am not 

 up in ancient colours, but my recollection of the interpretations of the lexicographers is 

 of considerable confusion of hues between red and yellow. Q. If this is true, has the 

 red sensation become better developed since those days ? " 



Mr Gladstone, some seven years afterwards, called attention to the peculiar colour- 

 terms used by the Greeks, which I found gave a distinct idea of colour-blindness.* The 

 matter is worthy of further attention. 



Konig and Dieterici. 



A brief account of some important investigations undertaken about 1886 by these 

 physicists, under the patronage of Helmholtz, is given in "Data," AC. Dr 



* See my remarks on the subject in Nature, 1878. 



