1 1 1 4 VISION. 



blue. The colours so adopted on purely psychological grounds were found 

 later to correspond with the colours which were found not to change in 

 colour-tone in different regions of the retina, namely, a red more purple 

 than the extreme red of the spectrum (and obtained by mixing blue light 

 with the latter), a yellow at 574 - 5 X, a green at 495 a, and a blue at 471 X. 



Hering's theory of metabolism. — In order to consider the appli- 

 cation of Hering's theory to the conditions of excitability of the visual 

 substances, it will be necessary to enter more fully into his general theory 

 of metabolism. 1 When a substance is protected from external stimulation, 

 it undergoes spontaneous or autonomous katabolism and anabolism, which 

 are equal to each other ; the substance is in a condition of autonomous 

 equilibrium. If the substance is affected by a stimulus to katabolism, it 

 undergoes a descending change. Hering now calls the katabolism allono- 

 mous ; the substance becomes less capable of performing work, and is 

 regarded as at a low potential. With this there is a spontaneous tendency 

 to return to a condition of autonomous equilibrium, or of mean potential, 

 i.e. its autonomous anabolism becomes stronger. Allonomous katabolism 

 is therefore accompanied by autonomous anabolism, and the latter be- 

 comes the greater, the longer the katabolic stimulation lasts. Sooner or 

 later, with constant stimulation, the autonomous anabolism becomes equal 

 to the allonomous katabolism, and a new condition of equilibrium is set 

 up, which Hering speaks of as allonomous equilibrium at a low potential. 



If, on the other hand, the substance is affected by a stimulus to 

 anabolism, it undergoes an ascending change. The substance becomes 

 more capable of work, and is at a high potential. There is a spontaneous 

 tendency to return to a condition of mean potential, and the allonomous 

 anabolism is therefore accompanied by autonomous katabolism, which 

 sooner or later, with constant stimulation, produces allonomous equili- 

 brium at a high potential. 



It is the direction of the change, i.e. whether ascending or descending, 

 which determines the nature of the sensation ; the ascending change in 

 the three substances being the basis of black, blue, and green respectively ; 

 the descending, of white, yellow, and red. These general conceptions may 

 now be applied in detail to the hypothetical visual substances of 

 Hering. In the case of the white-black substance, the sensation, corre- 

 sponding to equilibrium, between katabolism and anabolism, is the neutral 

 grey, i.e. a grey of the brightness of that seen after prolonged stay in 

 the dark. A descending change from a condition of higher to one of 

 lower potential determines a sensation brighter than this neutral grey ; 

 the degree of brightness depends on the rapidity of the change, or, in 

 other words, on the degree in which katabolism preponderates over 

 anabolism. An ascending change from lower to higher potential deter- 

 mines a sensation darker than the neutral grey, the depth of darkness 

 again depending on the rapidity of the ascending change. 



Adaptation and induction.— The conditions of allonomous equi- 

 librium are the basis of adaptation. When the black-white substance 

 is completely adapted to the dark, it is in the condition of mean 

 potential, corresponding to autonomous 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 



1 Cf. vol. i. pp. 868-870. 



