Dr. W. Pole on Colour-Blindness. 1 89 



As the theory embodied in it is a matter of constant com- 

 ment whenever colour-blindness comes under discussion, it 

 may not be amiss to give to the English public some account 

 of the author's own manner of stating his opinions, which I 

 am enabled to do with his concurrence. 



The title of the paper is " An Essay towards the Explana- 

 tion of Colour- Blindness by the Theory of Opposite Colours/' 

 and it is divided into five sections. 



In the First Section, headed " Some passages from the 

 Theory/' the author explains generally what the theory is, 

 so far as is necessary for the purpose in view, giving par- 

 ticularly an explanation of the term " Gegenfarben/' He 

 holds that there are essentially six simple visual sensations' — 

 white, black, yellow, blue, red, aud green, by the mixtures of 

 which, in different proportions, all colour-impressions arise. 



The mixture of white and black produces various degrees 

 of grey, which he believes to be present, to some extent, in 

 all colour-impressions. But the result of mixing the other 

 coloured sensations depends on certain relations which the 

 four fundamental colours yellow, blue, red, and green have 

 to each other. It is found that more than two of these 

 can never be recognized in the same mixture, and that certain 

 pairs of them cannot both appear in the same hue. Hence 

 the two colours forming such a pair are called Gegenfarben, 

 i. e. opposed, or contrary, or antagonistic colours. There are 

 two such opposed pairs : yellow and blue, in certain definite 

 hues, form one pair; red and green, in certain definite hues, 

 form the other pair. The yellow, for example, may combine 

 with red, forming a new compound colour, in which the 

 yellow and red are both recognizable ; or with green, 

 forming a yellow-green in which both yellow and green are 

 recognizable ; but it cannot form a new compound colour 

 with its opposite blue; for, put what proportions you will 

 (except the neutralizing ones), the mixture will not give any 

 new hue, but will remain either blue or yellow. Similarly, 

 the red may combine with blue or yellow to form new hues, 

 but cannot do so with green. And so on with the others*. 



* This relation of "opposed" colours will be recognized as corre- 

 sponding to what are usually called " complementary " pairs ; but Hering 

 considers that the idea of opposition is more forcible and emphatic ; and 

 it is on this principle that, as expressed in the title, his theory chiefly 

 depends. 



The precise hues of his four Urfarben, or fundamental colours, were, in 

 this publication, left undetermined, but they have since been pretty 

 nearly settled. The yellow and blue pair agree fairly with the ordinary 

 ideas attached to the names, but the fundamental red and green do not 

 do so, both inclining more towards blue : they correspond with the two 

 neutral hues of the red-green blind. 



