258 COLOUR VISION 



complementary colours is due to the combined action on the black- 

 white substance. With the correct intensities the anabolic effect of 

 one colour neutralises the katabolic effect of the complementary on the 

 chromatic substances, whilst their combined black and white valencies 

 manifest themselves as white or grey. 



The regional effects of stimulation of the retina are attributed to 

 absence of response of one or of two of the substances. In the outermost 

 zone only the black- white substance is stimulated ; in the middle zone 

 only the black- white and the blue-yellow substances are stimulated. 

 Hess's experiments on the whole confirm this hypothesis (v. p. 70). 



The fact, however, that the colour values diminish unequally with 

 increasing excentricity, those red- wards of green (495 JU/A) most rapidly, 

 those violet-wards most slowly, necessitates that the sensibility of the 

 red-green substance diminishes more quickly than that of the yellow-blue. 

 The total colour blindness of the extreme periphery affords another 

 method of determining white valencies, but it gives totally different 

 results from the values obtained with low intensities and dark adaptation. 

 The latter can be explained as due to a different mechanism, the rods. 

 If the normal peripheral values give the true white valencies then they 

 must be different in the protanopic eye. But Hering considers both 

 protanopia and deuteranopia as due to absence of the red-green substance 

 (vide infra). This would be insufficient alone to account for the alteration 

 in white valencies, which must depend upon change in the white-black 

 substance. 



As shown by Hess's experiments, confirmed by Baird and others, 

 Hering's fundamental colours are those which, when passed from the 

 periphery to the centre of the retina, develop sensations which undergo 

 no change in hue. The red is a spectral red mixed with a small amount 

 of blue. According to Hering spectral red has a not inconsiderable 

 action on the yellow-blue substance. A mixture of spectral red and green 

 produces the sensation of yellow because, although the red and green 

 processes neutralise each other, the effect of the spectral red stimulus 

 on the yellow-blue substance remains. It is noteworthy that the 

 fundamental red and blue agree with those adopted by adherents of the 

 Young-Helmholtz theory (p. 220). 



Colour blindness is explained on the same principle. Thus, in total 

 colour blindness only the black- white substance responds to stimulation 

 by all lights. Hence the luminosity curve of the totally colour-blind 

 should be the same as that of the achromatic scotopic eye, which is 

 the case. On the other hand, the luminogjty values of coloured lights 



