368 TIME AND INTENSITY IN STIMULATION 



If equation (3) is true experimentally, it should furnish a corrobo- 

 ration of the previous findings as expressed in equations (1) and (2). 

 Moreover, it would then give substantial support from a new angle 

 to the hypothesis which I have proposed to account for the photo- 

 sensory behavior of Mya (Hecht, 1918-19, a). I have therefore 

 undertaken a number of simple measurements which will give an 

 unequivocal answer to the requirements of equation (3). 



II. 



An outstanding characteristic of the response of Mya to light is its 



reaction time. It has already been repeatedly shown in these studies 



on the photosensory process that this reaction time is composed of 



two parts, the sensitization period and the latent period (Hecht, 



1918-19, a; 1919-20, b). The sensitization period represents the 



actual time of action (/) of the light in its relation to the reversible 



photochemical reaction 



S^P + A 



in the sense organ. The latent period is the portion of the reaction 

 time during which the light is not required. It is occupied by the 

 time taken for the secondary reaction 



to produce the amount of T necessary to initiate the nervous impulse 

 which begins the response of the animals. 



The velocity of the latent period reaction is directly proportional 

 to the photochemical effect produced by the initial photochemical 

 reaction. A constant photochemical effect gives a constant latent 

 period. During ordinary stimulation of Mya by light, the latent 

 period is constant and of minimal duration because the photochemical 

 effect produced during the sensitization period is constant and maxi- 

 mal. The sensitization period, and consequently the reaction time, 

 simply prolongs itself until the required accumulation of P and A 

 is produced by the light. 



The duration of the latent period cannot be decreased beyond this 

 point because the velocity of the reaction 



