SELIG HECHT 



159 



anthracene into dianthracene by light, and its reversal in the dark 

 (Sheppard, 1914, p. 214). 



Let us consider the reaction which occurs normally, and of course 

 in the dark, and which is supposed to regenerate the photosensitive 

 material from its precursor (P^^S). The curve in Fig. 3 indicates 

 the process on the basis of the ordinary velocity curves for chemical 

 reactions. The formation of the photosensitive material (ordinates 

 at the right) proceeds at first rapidly, then it loses speed gradually, 

 and finally it reaches a point of equilibrium which represents a definite 

 ratio between the concentration of the sensitive material and its pre- 

 cursor. The important thing to be noticed in the curve is the change 



I 



I 



0% 



77/ve — Hours 



Fig. 3. Hypothetical curve of reaction P —^ S based on data of Fig. 2. 



in the quantity of precursor (ordinates to the left) during the reaction. 

 At first the amount of this substance is large, then it decreases rap- 

 idly, then more slowly, and finally it reaches a constant minimum. 

 On comparing this rate of change with the one undergone by the re- 

 action time during dark adaptation it is seen that the two run quite 

 parallel. It has been assumed that the action of the light during the 

 sensitization period causes the photosensitive material to be changed 

 into its precursor. This means that the variation in the amount of 

 the precursor formed at the successive reactions during dark adapta- 

 tion is in general similar to the change in the amount of precursor 

 still unused in the chemical reaction. From the relationship between 

 these two variables we must conclude that, in order to act as a stimu- 



