136 ESSENTIALS OF PHYSIOLOGY. 



an inability to distinguish between red and green. This defect is 

 associated in some cases with an absence of sensation from the rays of 

 the red end of the spectrum, while in others there is no inability to 

 distinguish the spectral red. Cases of the former type are red-blind, 

 of the latter green-blind, and on the Young-Helm holtz theory they are 

 accounted for by the absence of the red and green elements respectively 

 from the retina. 



According to ffering's theory there are four instead of three primary 

 colour sensations red, green, blue, and yellow. These are arranged in 

 pairs, the two colours in each pair being complementary or antagonistic. 

 If a circular disc, coloured one half red and the other half green, is 

 rotated rapidly, the sensations of red and green will be fused and the 

 resulting sensation will be grey, or absence of colour. The same fact 

 holds for blue and yellow. Red and green on the one hand and blue 

 and yellow on the other are therefore antagonistic colours. On 

 Hering's theory there are three photo-chemical substances in the 

 retina, each of which arouses a different sensation according as it is 

 undergoing assimilation or dissimilation. One of these substances is 

 broken down when stimulated by red rays and built up when green 

 rays fall upon it ; the second undergoes dissimilation or katabolism 

 under the influence of yellow, and assimilation or anabolism under the 

 influence of blue rays ; the third is broken down by white light and 

 built up again in the absence of light. 



Hering's theory does not account for the shortening of the red end 

 of the spectrum in those colour-blind persons who cannot distinguish 

 red from green, but it is supported by the effects of fatigue of the 

 visual apparatus and by the facts of simultaneous and successive 

 contrast. 



Fatigue of the mechanism connected with visual sensations can be 

 demonstrated by looking fixedly at a bright object for a short time and 

 then transferring the gaze to a white surface, when a dark spot or 

 negative after-image will be seen, corresponding with the position 

 on the retina of the image of the bright object. Owing to the fatigue 

 of the part of the apparatus already stimulated, the white paper does 

 not excite the same sensation in it as in the remainder of the retina. 

 It may be supposed that the white-black substance undergoes active 

 katabolic change as the result of the excessive stimulus, and that 

 this is succeeded by anabolism when the intensity of the stimulus is 

 reduced. 



The effect of simultaneous contrast may be shown by placing a disc 

 or cross of grey paper on a coloured sheet, and covering the whole with 

 tissue paper. The grey will appear green on a red background, red on 



