434 ELEMENTARY PHYSIOLOGY 



sensations of colour of which we are at anj- time 

 conscious. 



Another theory has been started in whose support very 

 much may be said.' It resembles the previous one 

 inasmuch as it also assumes that all our sensations 

 of colour result from a limited number of primary sen- 

 sations. But it differs in the number and selection of 

 the primary colours. Thus according to thi.s theory there 

 are three pairs of primary sensations arising from 

 three pairs of primary colours : these are red-green, 

 yellow-blue, white-black. Further, this theory 

 assumes the existence in the visual apparatus of three 

 kinds of visual substance of which each corre- 

 sponds to one of the above pairs of colours, and 

 it assumes that these substances may be made, some- 

 what in the same way as the sensitive salts on a photo- 

 graphic plate, to undergo a destnictive change by the action 

 of light in such a way as to give rise to the sensations of 

 red, yellow and white. Finally it supposes that the other 

 three antagonistic sensations arise from the subsequent 

 constructive building up again of their visual substances 

 which must take place as soon as the luminous cause of 

 their destructive change has been removed. Thus the 

 breaking down of the red-green visual substance gives rise 

 to the sensation of red, and its building up again to the 

 sensation of green, and similarly for the other two pairs. 



Both these theories attempt to account for observed 

 facts, and each goes a long way in doing so ; but neither 

 of them accounts completely for all the facts of colour 

 vision. Neither may we enter into any discussion 

 of their respective merits, for such discussion would be 

 useless if it were not lengthy and detailed, and its 

 intricacy Avould be still further out of place in an elemen- 

 tary work which excludes as far as possible all debatable 

 matter. 



The excitability of the retina is readily exhausted. 

 1 This is the theory of Bering. 



