SELIG HECHT 133 



the "inner stimulus" for the nerve endings. It is not necessary at 

 present to be specific on this point. It is enough to accept the general 

 idea that the decomposition product is the active agent in the early 

 stage of the formation of the visual impulse. Its effects, catalytic or 

 otherwise, will undoubtedly be a function of its concentration in the 

 sense organ. An increase in this concentration will cause an increase 

 in its effect. It would, therefore, seem a reasonable assumption to 

 make that the increase in concentration necessary to produce a 

 perceptible effect in the secondary reaction is directly proportional 

 to the concentration already present. This is the "compound inter- 

 est law" which is at the bottom of so many physical and chemical 

 phenomena. 



The assumption then is that in order to produce a photosensory 

 effect, the light must cause an increase in photolytic products propor- 

 tional in extent to the concentration already present in the sense 

 organ. Such an assumption has served successfully in the analysis of 

 the photosensory process in Mya and Ciona and in the periphery of 

 the human eye. Moreover, in the case of Ciona it was possible to 

 put the matter to a critical experimental test, the results of which 

 completely vindicated the assumption (Hecht, 1918-19 a, p. 162). 



vni. 



In the case of the present data as given in Fig. 6, this assumption 

 plus the one relating intensity and photochemical effect lead to this. 

 The ordinates, representing the intensity of the threshold, are directly 

 proportional to the amount of photolytic products formed by the light. 

 These values are in turn directly proportional to the concentration of 

 photolytic products already present in the sense organ. The curve of 

 dark adaptation of the fovea therefore represents the changes which 

 take place in the concentration of the photochemical decomposition 

 products in the sensory mechanism. 



It is apparent that this concentration is decreasing steadily. This 

 decrease can represent one of two possible processes: either the 

 products are diffusing out of the sensory cells, or they are reacting 

 chemically in the formation of some other substance. The nature of 

 the curve in Fig. 6 will be of some aid in deciding between these 

 alternatives. 



