STATEMENT OF THE THEORY 253 



and blue excite assimilation, white, red and yellow dissimilation in the 

 respective substances. Autonomous equilibrium in the black-white 

 substance corresponds to " mean grey " ; a descending change, or 

 excess of dissimilation, causes a whiter sensation, an ascending change 

 a blacker sensation. It is clearly a difficulty of the theory that 

 autonomous equilibrium of the black-white substance causes a definite 

 sensation. 



In most kinds of stimulation of the retina all three substances 

 are excited and the character of the sensation depends upon the relative 

 amounts of action of each and their direction. All coloured lights, 

 except the four primary colours, have three values or valencies, corre- 

 sponding to their action on the three substances. The physiological 

 value or " moment " of a light depends upon its physical value and also 

 upon the condition of excitability of the visual mechanism. 



Hering's explanation of the facts of adaptation and induction is 

 lucidly summarised by Rivers^ thus : 



" The conditions of allonomous equilibrium are the basis of adapta- 

 tion. When the black-white substance is completely adapted to the 

 dark, it is in the condition of mean potential, corresponding to autono- 

 mous equilibrium. During the day the eye is always adapted to the 

 surrounding illumination. The exact condition of adaptation must vary 

 to some extent with the continual changes of external illumination to 

 which one is normally exposed, but for practical purposes the condition 

 of adaptation may be regarded as changing very slowly, and in any 

 definite interval of time as corresponding to the mean illumination 

 during that period. The white-black substance is then in a condition 

 of allonomous equilibrium at a low potential. The brighter the 

 illumination to which the eye is adapted, the lower is the potential at 

 which equilibrium occurs. During the daytime the white-black sub- 

 stance is always to be regarded as in a condition of relatively low potential. 

 The condition of allonomous equilibrium at a high potential is exceptional 

 in the case of the black-white substance, owing to the fact that there is 

 no external stimulus to anabolism. Black proper, according to Hering, 

 only occurs under the influence of simultaneous or successive contrast ; 

 and the best example of allonomous equilibrium at relatively high 

 potential, in the case of the black-white substance, occurs in Hering's 

 simultaneous induction. If a black square on a white ground is fixed, 

 the whole surface after a time is seen as a uniform grey. The black- 

 white substance corresponding to the black square will have undergone 



^ Loc. cit. p. 1114. 



