TIME AND INTENSITY IN PHOTOSENSORY STIMULATION. 



By SELIG HECHT. 



{From the Physiological Laboratory , College of Medicine, Creighton University, 



Omaha.) 



(Received for publication, November 11, 1920.) 



The photochemical effect of light depends upon its intensity and the 

 time of its action. In previous studies I have investigated the sepa- 

 rate effects of these two variables in their action on the photic 

 responsesof M3'aarewana(Hecht, 1918-19, a; 1919-20, c). Ihavealso 

 considered the interaction of these two factors under certain specially 

 circumscribed conditions of particular interest (Hecht, 1919-20, h, c). 

 In its more general bearings, however, the relation between the two 

 variables has not been examined. This I propose to do now. 



If the intensity of the stimulating light is kept constant and its 

 time of action varied, it has been found (Hecht, 1918-19, a) that the 

 photochemical effect (£) is a linear function of the time (/). This 

 may be written 



E = kit (1) 



where ki is a constant of proportionality. On the other hand, if the 

 time of action is kept constant and the intensity varied, the photo- 

 chemical activity of the light is found (Hecht, 1919-20, c) to be 

 directly proportional to the logarithm of the intensity (/). This may 

 be expressed as 



E = h log I (2) 

 ki being a constant. 



It follows from equations (1) and (2) that if both the intensity 

 and the time are permitted to vary, the photochemical effect should be 

 proportional to the product of the time of action into the logarithm 

 of the intensity. In other words 



E = kt log I (3) 



the terms having their previous significance. 



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