RETINAL PHOTOPIGMENTS 123 



istics of a photochemical reaction, while the subsequent 

 one is an ordinary thermal reaction which is catalyzed by 

 the substance formed in the first one. In the first reaction 

 the photosensitive substance S is decomposed by light into 

 the products P, A, and B. This reaction is pseudo-reversible; 

 in the dark or light, P and A recombine in the presence of 

 a new material C, present in excess, to form the original 

 photosensitive substance. The process in its simplest form, 

 therefore, was written by Hecht as follows : 



Ught 



S — ^P + A + B 



S^ — P + A + C 



"dark" 



In this scheme the substance B acts to initiate the sensory 

 impulse. The chemistry of the rhodopsin system as since 

 analyzed by Wald agrees in its general properties with this 

 scheme. This in itself is a marked tribute to the logical 

 excellence of these pioneer researches of Hecht. 



Visual purple, since its discovery, has been known to be an 

 unstable substance decomposed by light and it was originally 

 hoped that this substance would become the basis of an all- 

 inclusive photochemical theory of vision. However, its 

 reported complete absence in many animals lacking rods, 

 as well as its absence at the fovea where rods are absent, 

 soon made it clear that the existence of other substances 

 must be recognized in order to account for the physiology 

 of cone vision. Hecht and Williams (1922), however, studied 

 the visibility curves at high and at low intensities of light 

 and found their absorption spectra to be so close as to sug- 

 gest the presence of visual purple in dilute solution in the 

 cones. Logical reasons based upon the findings of Weigert 

 (1921) were given to support their contention that both 

 cone and rod vision might be accomplished by a single photo- 

 sensitive substance. This idea in so far as I am aware has 

 since been abandoned, and cone vision is accomplished 

 apparently by the presence of a different photopigment. 



