CH. LVITI.] 



COLOUR VISION 



843 



FIG. 536. 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. 



tary) pairs, black and white, red and green, and yellow and blue-; 

 and that these are produced by the changes either of disintegration 

 or of assimilation taking place in certain substances, which (the 

 theory supposes) exist in the 

 cerebro-retinal apparatus. Each 

 of the substances corresponding 

 to a pair of colours is capable of 

 undergoing two changes, one of 

 disintegration, and the other of 

 construction, with the result of 

 producing one or other colour. 

 For instance, in the white-black 

 substance, when disintegration is 

 in excess of construction or as- 

 similation, 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 colour-sensation occurs. 

 The rays of the spectrum to the red end produce changes in the 

 red-green substance, with a resulting sensation of red, whilst the 

 (orange) rays further to the right affect both the red-green and the 

 yellow-blue substances ; blue rays cause constructive changes in the 

 yellow-blue substances, but none in the red-green, and so on. All 

 colours act on the white-black substance as well as on the red-green 

 or yellow-blue substance. 



Neither theory satisfactorily accounts for all the numerous com- 

 plicated 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 (see further p. 848), 

 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 or green 

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

 the red-green substance is absent from the cerebro-retinal apparatus. 

 Other varieties of colour-blindness, in which the other colour-perceiv- 

 ing elements are absent, have been shown to exist occasionally. 



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

 element of Helmholtz were absent, the patient ought not to be able 

 to perceive white sensations, of which red is a constituent part; 

 whereas, according to Bering's theory, the white-black visual sub- 

 stance remains intact. These two theories have been for a long time 



