256 COLOUR VISION 



after-image, is attributed by Hering to the effect of simultaneous 

 contrast. Thus, when a black square has been fixated upon a white 

 background, the bright after-image of the former is produced by a 

 subsequent process of assimilation which evokes a simultaneous dis- 

 similation process in the adjoining retinal region. 



Hering's theory thus gives a satisfactory explanation for the most 

 important facts of temporal and spatial induction, and a less compre- 

 hensive explanation for the facts of adaptation. Hitherto the question 

 of luminosity has been left out of account. In the earliest exposition 

 of his theory (Zur Lehre vom Lichtsinne) Hering considered the luminosity 

 of a visual sensation as wholly due to the black-white component. 

 If this were true the four primary colours seen in a state of purity would 

 be of equal brightness. This explanation of luminosity is untenable, 

 and the " specific brightness of colours " has been substituted for it 1 . 

 According to it the warm colours, red and yellow, which are due to 

 dissimilation, increase the total brightness, whilst the cold colours, 

 green and blue, which are due to assimilation, diminish it. Red and 

 yellow therefore possess an inherent brightness (Eigenhelligkeit), green 

 and blue an inherent darkness (EigendunJcelheit). ' A toned colour 

 may generally be regarded as made up of four fundamental components, 

 two toned and two tone-free (white and black). It is only in colours 

 of the tone of a primary that a single toned component is present. 

 In any red-yellow colour, e.g. orange, we have therefore to distinguish 

 three bright fundamental components (red, yellow, white) and one dark 

 (black) ; in any green-blue on the other hand three dark (green, blue, 

 black) and one bright (white). The red-blue and green-yellow colours 

 would contain two bright and two dark fundamental components 2 ." 

 Hering deduces the following rules : 



" If two colours of equal tone and equal purity differ in brightness 

 it is due to the difference in their black-white components." 



" Two colours of different tone may differ in brightness in spite of 

 equal purity and equality of their black- white components." 



" When the black- white components are equal, a yellow, red, or 

 yellowish red colour is so much brighter, and a blue, green, or bluish 

 green so much darker the more distinct the tone of the colour in compari- 

 son with the black-white components 3 ." 



The specific brightness of colours is held to explain the achromatic 



1 Hillebrand, Sitz. d. Wiener Akad. xcvm. 3, 70, 1889. 



2 Hering, in Graefe-Saemisch Handb. d. yes. Augenhlkde, I. iii, xii, p. 61, l'J05. 



3 Loc. cit. 



