THEORIES OF COLOUR- VISION. 1113 



violet is obviously a mixture of red and blue, while spectral red, if 

 isolated so as to exclude contrast from the strong yellow of the spectrum, 

 manifestly contains a yellow component. 1 



Hering advanced novel views about the sensation of blackness. He 

 objected to the generally accepted view, that this sensation could be 

 denned as that accompanying absence of external stimulation. He 

 pointed out that with total exclusion of light from the eye, i.e. in the 

 dark-adapted eye, the sensation was one of a considerable degree 

 of brightness, and was named by him the sensation of mean grey. 

 Black proper only occurs, according to Hering, under the influence 

 of external stimulation, i.e. under the influence of simultaneous or 

 successive contrast. The after-image of a white patch, or a black 

 patch seen on a white ground, give us a special quality of sensation, 

 which we recognise as present in the mean grey, but in a less degree ; 

 and it is this special quality of sensation which Hering regards as black 

 proper. Since black is a special quality of sensation, as well-defined and 

 undecomposable as white or red, it is in accordance with psychophysical 

 parallelism that it should have a separate physiological basis. 



Hering, then, regards the whole series of our visual sensations as 

 made up of six primary sensations ; but the most novel and fundamental 

 feature of his theory is the arrangement of these six sensations in three 

 pairs, the two members of each pair being antagonistic in nature to each 

 other, and depending on physiological processes of an opposite kind. 

 Hering usually speaks of his theory as the theory of antagonistic colours. 



Each of the three pairs of sensations depends upon the action of a 

 special substance, the three substances being called the black-white, the 

 red-green, and the blue-yellow respectively. The processes of an opposed 

 nature taking place in these substances are the two kinds of metabolism. 

 In most kinds of stimulation of the retina, all three substances are set in 

 action, and the character of the sensation is determined by the relative 

 amount of action of each. This relative amount determines the " weight " 

 of each primary sensation in the complex sensation. Both the general 

 character of the sensation and the " weight " of each element depend on 

 the condition of excitability of the substances, as well as on the value of 

 the stimulating light. The term " value " ( Valenz), which has been used 

 in the earlier part of this article to express a fact, has, when used by 

 Hering, a special meaning. The white value of a light expresses its 

 action on the black- white substance, and its colour value on one or 

 both of the chromatic substances. All coloured lights, except those 

 corresponding to the four primary colours, have three values, correspond- 

 ing to their action on the three substances. 



Hering 2 always distinguishes carefully between the optical value of 

 a light and its physiological value (Moment), which depends on the 

 optical value and the condition of excitability of the visual organ. 



The exact colours which were originally chosen by Hering were those 

 which appeared simple to psychological analysis, namely, a red in which 

 110 trace of either blue or yellow could be perceived, a yellow in which 

 no trace of red or green could be perceived, and similarly pure green and 



1 Wundt has objected that the choice of red, yellow, green, and blue is purely arbitrary ; 

 that all sensations are equally simple ; and that one has as much right to regard red as a 

 mixture of purple and orange, or blue as a mixture of violet and blue-green, as to regard 

 orange or violet as a mixture of red with yellow or blue. The new fundamentals of 

 Helmholtz and Konig are not open to Hering's objections. 



2 Arch. f. d. ges. Physiol., Bonn, 1887, Bd. xli. S. 29. 



