SELIG HECHT 661 



the reaction time and the exposure period is the latent period. There- 

 fore the latent period also varies inversely with the duration of the 

 exposure. The suggestion that the latent period is in some way 

 related to the formation of precursor materials is consequently 

 justified. 



III. 



The latent period, being the interval during which Mya may re- 

 main in the dark, is certainly not a time during which "nothing 

 happens." The fact that it varies under certain conditions is reason 

 enough against such an idea. We may, therefore, rest assured that a 

 process takes place during the latent period, which in some way is a 

 vital Hnk in the chain of events between the incidence of the Hght 

 and the appearance of the response. There seem to be three pos- 

 sibihties for the nature of this process. The first is that the pre- 

 cursor substances formed by the Hght exert their stimulating action 

 at a place other than where they are produced. The latent period 

 would thus represent the time for the diffusion of these substances. 

 A second possibihty is that the latent period represents the duration 

 of a subsidiary chemical reaction. This results from the interaction 

 of one or both of the precursor substances with some other materials 

 present in the sense organ. The time for the formation of the product 

 of reaction would account for the duration of the latent period. The 

 third possibility is that the latent period is determined by an inde- 

 pendent chemical reaction which is catalyzed by the presence of the 

 freshly formed precursor substances. 



In order to decide among these possibilities it is necessary to 

 inquire quantitatively into the relation between the exposure period 

 and the latent period. Fig. 3 shows this for the experiment given 

 in Fig. 1. Each point is the reciprocal of the average latent period 

 plotted against the exposure time. That the points form a straight 

 line is undoubted. Fig. 4 gives the reciprocals of the latent periods 

 of the experiments presented in Fig. 2. In spite of the rather large 

 observational error, it seems clear that the points may best be 

 arranged in straight lines. 



The exposure time measures the amount of photochemical activity. 

 For these extremely short intervals, the amount of precursor ma- 



