4G6 DR WILLIAM POLE ON THE PRESENT STATE OF 



Fick, 1874 and 1879. 



This new explanation was taken up by Professor A. Fick of Wiirzburg, who 

 brought further arguments to show the inapplicability of the ordinary theory, and the 

 preferable nature of the new one. See " Data," AA. 



Hering, 1880. 



But this solution of the difficulty was not universally approved ; and in 1880 Professor 

 Ewald Hering of Prague brought out a new explanation of colour-blindness in an article 

 entitled " Zur Erklarung der Farbenblindheit aus der Theorie der Gegenfarben." He had, 

 in 1878, published a new theory of colour- vision generally, and he now showed how this 

 might be applied to dichromic vision.* His general colour- theory involved physiological 

 points of considerable novelty, but the part applicable to the present matter is exceed- 

 ingly simple. 



Hering retains certain hues of red, green, and blue as fundamental colour-sensations, 

 but he adds a fourth — yellow. In adopting this combination, he lays the chief stress 

 on the fact that, in the hues he chooses, the four colours mentioned form two comple- 

 mentary pairs, yellow with blue, and red with green ; and he interprets this feature as 

 giving the two colours of each pair a kind of antagonism to each other. His yellow and 

 blue, for example, cannot both be seen in the same mixture, one destroying the other ; 

 and the same with his red and green. For this reason he names the two colours of 

 either pair " Gegenfarben," or antagonistic colours. 



The application of this to the explanation of dichromic vision is obvious at once, — 

 in normal vision both pairs are in action ; in dichromic vision there is only one pair, gene- 

 rally the yellow and blue. 



Hering adds another pair of special sensations, namely, white and black, which he 

 considers play a necessary part in both normal and dichromic vision. He has believed 

 from the first that all the spectral colours contain a large mixture of white, an opinion 

 now supported by recent investigations. 



The hues of Hering's four fundamental colours have been fixed since he wrote his 

 Paper, the yellow about w. 1. 575, and the blue its complement, about 483. The red and 

 green are the hues invisible to the colour-blind, i.e., the green between b and F, and a 

 non-spectral crimson or purple complementary to it. 



Krenchel, 1880. 



Dr W. Krenchel of Copenhagen wrote an able article arguing that the theories 

 of colour-blindness were unnecessary : they were no help to the understanding of the 



* I have published an account of Hering's General Colour-Theory in Nature for 1879 ; and an abstract of his 

 subsequent Colour-Blindness Essay in the Phil. Mag. for August 1893. See also Note on page 477 of this article. 



