THE RELATION BETWEEN THE WAVE-LENGTH OF LIGHT 

 AND ITS EFFECT ON THE PHOTOSENSORY PROCESS.* 



By SELIG HECHT. 



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



Omaha.) 



(Received for publication, November 11, 1920.) 



1 . In a series of publications I have described several phases of the 

 mechanism which underlies the sensitivity of certain animals {Mya 

 and Ciond) to light (Hecht, a, b, c, d). The basis of this mechanism is 

 essentially a process involving two intimately connected chemical 

 reactions. The first of these is a reversible photochemical reaction 



light 



S^P^ A 



"dark" 



in which S is the sensitive substance, P the principal product of de- 

 composition, and A its accessory. The second reaction is an ordinary 

 chemical reaction 



which is catalyzed by the products of reaction of the photochemical 

 reaction. The thermolabile material T is the end-product of the 

 sensory process, and a definite quantity of it is required for the initia- 

 tion of an impulse which results in the response of the animal to illumi- 

 nation. 



The properties of these different substances and reactions are 

 derived from quantitative studies of the responses of animals under 

 conditions of experimentation calculated to bring them out. As the 

 characteristics of the reactions and their components become known 



* These experiments were carried out with the aid of a grant from the Elizabeth 

 Thompson Science Fund, to the trustees of which I wish here to express my thanks. 



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