THE SENSES 945 



Helmholtz looked upon simultaneous contrast as a result of false 

 judgment, and not of a change of excitability in parts of the retina 

 bordering on the actually excited parts. For the sake of perspective, 

 it will be worth while to apply this theory by way of illustrating it, to 

 the explanation of the case of contrast we have just been consider- 

 ing, from the other point of view, in Meyer's experiment. Helm- 

 holtz's explanation of this experiment is as follows : When a coloured 

 surface is covered with translucent paper, the latter appears as a 

 coloured covering spread over the field. The mind does not recog- 

 nise that at the grey patch there is any breach of continuity in this 

 covering ; it is therefore assumed that the greenish veil extends over 

 this spot too. Now, the grey seen through the translucent white 

 paper is objectively white i.e., sends to the eye the vibrations 

 which together would give the sensation of white light. But with a 

 green veil in front of it, this could only happen if the really grey 

 patch was the colour complementary to green that is, rose-red. 

 The mind, therefore, judges falsely that the patch is red. Hering 

 has severely criticised this theory of Helmholtz as to false judgments ; 

 and the weight of evidence certainly seems to be in favour of the 

 view that simultaneous, like successive, contrast is due to the influence 

 of one portion of the retina, or retino-cerebral apparatus, on another. 



Hering's Theory of Colour Vision. The Young- Helmholtz 

 theory of colour vision has not met with universal acceptance. 

 The best-known rival theory is that of Hering, who takes his 

 stand upon the fact that certain visual sensations (red, yellow, 

 green, blue, white, black) do appear to us to be fundamentally 

 distinct from each other, while all the rest are obviously mixtures 

 of these. Accepting these six as primary sensations, he assumes 

 the existence in the visual nervous apparatus of substances 

 of three different kinds, which may be called the black-white, 

 the green-red, and the blue-yellow. Like all other constituents 

 of the body, these substances are broken down and built up 

 again in other words, undergo disassimilation and assimilation, 

 destructive and constructive metabolism. The sensations of 

 black, of green, and of blue he supposes to be associated with 

 the constructive, and the sensations of white, of red, and of 

 yellow with the destructive, processes in the three substances. 

 The black-white substance is used up under the influence of all 

 the rays of the spectrum, but in different degrees ; the smaller 

 the quantity of light falling on the retina, the more rapidly is it 

 restored, and the more intense is the sensation of black. The 

 green-red substance is built up by green rays, broken down by 

 red. The blue-yellow substance is destroyed by yellow rays, 

 restored by blue. A prominent difference between this and the 

 Young- Helmholtz theory, and, so far as it goes, an advantage, 

 is that Hering's theory attempts to assign a direct objective 

 cause for the visual sensations of white, black, and yellow, as 

 well as for red, green, and blue, instead of making the sensations 

 depend upon the magnitude of the stimulation process. When 

 any of the visual substances are consumed at one part of the 



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