PHOTOCHEMISTRY OF VISUAL PURPLE. 



I. The Kinetics of the Decomposition of Visual Purple by 



Light. 



By SELIG HECHT. 



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



Omaha.) 



(Received for publication, July 24, 1920.) 

 I. 



An analysis of the progress of retinal dark adaptation in man has 

 shown that the process follows the course of a bimolecular reaction 

 (Hecht, 1919-20). The interpretation to be placed on these findings 

 is that dark adaptation depends on the disappearance in the retina of 

 two products of decomposition resulting from the photolysis of a sen- 

 sitive material. This rests on the assumption that the sensitivity of 

 the peripheral retina measured in terms of the threshold stimulus is 

 directly proportional to the quantity of residual decomposition prod- 

 ucts then present in the retina. 



A photosensitive substance undoubtedly exists in the irritable ele- 

 ments, and its decomposition by hght must form the first step in the 

 receptor process. It may be supposed that this sensitive substance 

 breaks down into two products of decomposition; and that in the 

 dark these two products reunite to form the sensitive material again. 

 This reversible reaction may be expressed as 



light 



"dark" 



in which 5 is the sensitive substance, P is the principal precursor de- 

 composition product, and A its accessory. Such a scheme has been 

 shown to fit the facts of retinal adaptation, and to offer a starting 

 point for quantitative investigations into the mxcchanism of visual 

 reception (Hecht, 1919-20). 

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