I.—PHYSIOLOGY 173 
stimulated by the ‘ yellow ’ of the spectrum. ‘The red and green sensa- 
tions are lost, but their disappearance cannot be due to processes in the 
layers of the retina. As Macdougall points out, the alternative suggested 
by Hering that his four-dimensional system is cerebral rather than retinal 
deprives his hypothesis of its special value as a theory of colour vision. 
Hering’s theory then becomes part of a general problem of how afferent 
stimuli are combined to produce perceptions, which is too complex a 
matter to be discussed here.® 
As the unitary sensations yellow, white, and black can be built up from 
stimuli associated with other sensations, itis possible to reduce the number 
of data for colour perception to three. 
The object of the above discussion is to show that there is no real 
objection to the trichromatic explanation of colour vision proposed by 
Thomas Young. 
CoLour CONTRAST. 
In many observations on colour, contrast phenomena occur. ‘These 
effects are always related to complementary colours. When, for example, 
one looks at a grey surface surrounded by a colour, the grey is tinted with 
the colour complementary to that used. This phenomenon is called 
simultaneous contrast or spatial induction. There does not seem to be 
any reasonable explanation of this effect except on a psychological basis. 
To say that the inducing colour lowers the threshold of surrounding 
areas is purely hypothetical, and the evidence is in favour of the threshold 
being raised in surrounding areas. This effect of one area of the retina 
on another is part of the problem of adaptation to light.® 
When a grey surface is viewed alongside a coloured surface, the light 
coming from the grey surface contains less of the dominant wave-length 
characteristic of the coloured surface than that coming from the coloured 
surface itself. 
The grey surface is less coloured with the inducing colour, and as there 
is no fixed standard for white (or grey), it appears tinged with the com- 
plementary colour. Objectively it has a greater proportion of the com- 
plementary colour than the other surface.1° 
Successive contrast or temporal induction is produced by looking at a 
coloured surface and then at a grey one. The grey surface under appro- 
priate conditions appears tinged with the colour complementary to the 
inducing one. This is easily explained by the process of adaptation 
whereby the frequency of impulses falls off rapidly during stimulation of 
the receptors. On looking at a neutral surface the impulses initiated in 
those end-organs which were stimulated by the inducing colour will be 
( 8 J. H. Parsons, Introduction to the Theory of Perception, Camb. Univ. Press 
1927). 
® H. E. Roaf, Proc. Roy. Soc., B, 110, p. 448 (1932). W. D. Stiles and B. H. 
Crawford, Proc. Roy. Soc., B, 118, p. 496 (1933). W.D. Wright, Proc. Roy. Soc., 
B, 115, p. 49 (1934). a 
0 F. W. Edridge-Green, Physiology of Vision (G. Bell & Son), pp. 232-234 
(1920). 
