77 8 THE EYE AND VISION. [CH. LV. 



different degrees, the sensation of white being produced when the 

 three elements are equally excited. Thus if the retina is stimu- 

 lated by rays of certain wave length, at the red end of the 

 spectrum, the terminals of the other colours, green and violet, 

 are hardly stimulated at all, but the red terminals are strongly 

 stimulated, the resulting sensation being red. The orange rays 

 excite the red terminals considerably, the green rather more, and 

 the violet slightly, the resulting sensation being that of orange, 

 and so on (fig. 592). 



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

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

 or complemental colours, 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, 

 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 producing one or other colour. 

 For instance, in the white-black substance, when disintegration 

 is in excess of construction or assimilation, 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 substances ; blue rays cause constructive 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 or Daltonism, a by no 

 means uncommon visual defect. One of the commonest forms 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. 



