CII. LVI.] 



THEORIES OF COLOUR VISION 



799 



siderably, the green rather more, and the violet slightly, the result- 

 ing sensation being that of orange, and so on (fig. 594). 



Another theory of colour vision (Hering's) supposes that there 

 are six primary colour sensations, viz. : three pairs of antagonistic 

 colours, black and white, red and green, and yellow and blue ; and 

 that these are produced by thp changes either of disintegration or of 

 assimilation taking place in certain substances, somewhat, it may be 

 supposed, of the nature of the visual purple, which (the theory 

 supposes) exist in the retina. Each of the substances corresponding 

 to a pair of colours is capable of undergoing two changes, one of 

 construction and the other of disintegration, with the result of pro- 

 ducing one or other colour. For instance, in the white-black 

 substance, when disintegration is in excess of construction or assimi- 

 lation, the sensation is white, and when assimilation is in excess of 

 disintegration the reverse is the 

 case ; and similarly with the red- 

 green substance, and with the 

 yellow-blue substance. When the 

 repair and disintegration are equal 

 with the first substance, the visual 

 sensation is grey ; but in the other 

 pairs, when this is the case, no 

 sensation occurs. The rays of 

 the spectrum to the left produce 

 changes in the red-green substance 

 only, with a resulting sensation 

 of red, whilst the (orange) rays 

 further to the right affect both the 

 red-green and the yellow-blue sub- 

 stances ; blue rays cause construc- 

 tive changes in the yellow-blue substances, but none in the red-green, 

 and so on. These changes produced in the visual substances in the 

 retina are perceived by the brain as sensations of colour. 



Neither theory satisfactorily accounts for all the numerous 

 complicated problems presented in the physiology of colour vision. 

 One of these problems is colour blindness, a by no means uncommon 

 visual defect. Some people are completely colour blind, but the 

 commonest form is the inability to distinguish between red and 

 green. Helmholtz's explanation of such a condition is, that the 

 elements of the retina which receive the impression of red, etc., are 

 absent, or very imperfectly developed, and Hering's would be that 

 the red-green substance is absent from the retina. Other varieties of 

 colour-blindness in which the other colour-perceiving elements are 

 absent, have been shown to exist occasionally. 



Hering's theory appears to meet the difficulty best, for if the red 



FIG. 594. Diagram of the three primary colour 

 sensations. (Young-Helmholtz theory.) 1 is 

 the red ; 2, green, and 3, violet, primary 

 colour sensation. The lettering indicates the 

 colours of the spectrum. The diagram indi- 

 cates by the height of the curve to what 

 extent the several primary sensations of 

 colour are excited by vibrations of different 

 wave lengths. 



