SELIG HECHT 



551 



In the equation, k equals 0.245 at this temperature of 22°C.; a is 

 the amount of precursor material present at the first moment of 

 darkness (equal to 100 per cent) ; x is the per cent of photosensitive 

 substance formed in the time t; and a-x is the amount of precursor 

 substances still remaining. 



In making the calculations, the sensitization period corresponds 

 to a-x, and a is its value when t = o, in this case 1.79 seconds. In 

 addition it must be remembered that the reaction time represents 

 not only the sensitization period, but also, at this temperature, a 

 latent period of 1.31 seconds. 



TABLE III. 



Reaction Time During Dark Adaptation. 

 yfe = 0.245 



As a result of these experiments on dark adaptation it is therefore 

 possible to express quantitatively the hypothesis previously sug- 

 gested. The photosensitive system resolves itself into three compon- 

 ents concerned in the two reactions of a reversible system. The first 

 reaction is independent of light and consists in the combination of 

 two precursor substances (P and ^) to form a photosensitive material 

 (5). The second reaction involves the decomposition of 5 into its 

 two precursors. The system may therefore be written 



light 

 S^P + A 

 "dark" 



with a full consciousness of the quantitative significance of the 

 expression. 



