INTENSITY AND THE PROCESS OF PHOTORECEPTION. 



By SELIG HECHT. 



{From the Physiological Laboratory, College of Medicine, Creighton 

 University, Omaha.) 



(Received for publication, January 22, 1920.) 



I. 



Photoreception in animals like Mya and Ciona is essentially a two- 

 fold process. This is evidenced by the duality of the reaction time in 

 its division into an initial exposure period and a subsequent latent 

 period. The underlying mechanism of photoreception follows this com- 

 position of the reaction time. During the exposure or sensitization 

 period a photochemical reaction is initiated in which a photosensitive 

 substance 6* is decomposed into its precursors P and A. In the 

 subsequent latent period these freshly formed precursors serve to 

 catalyze an independent reaction involving the transformation of an 

 indifferent material L into an active substance T. This active mate- 

 rial T sets off the nervous impulse for a response (Hecht, 1918-19, a, 

 h). 



The exact interrelation between the two reactions of photoreception 

 and the external energy is of importance in a complete understanding 

 of the nature of the receptor mechanism. Of the two processes only 

 the initial photochemical reaction is amenable to direct experimental 

 modification. However, the duration of the subsequent latent period 

 reaction is dependent on the products of the photochemical reaction. 

 In addition, variations in the velocity of the latent period reaction 

 are easily measureable with a stop-watch. These circumstances en- 

 able one to study quantitatively the effect of changes in the photo- 

 chemical reaction on the properties of the latent period reaction. 



The amount of fresh precursors produced by the initial photochemi- 

 cal reaction is a function of the available energy. Both components 

 of this energy can be controlled independently, and their actions in- 

 vestigated. The effect of changes in the duration of exposure at con- 



337 



