Photoreception 



407 



that something which is sensitive to light must have accumulated durino the 

 stay in the dark. This sensitivity to light, argued Hecht, resides in a substance 

 which we shall call S. 



The shape of the reaction-time curve of the dark-adapting clam is signific- 

 ant, for it can be described adequately by the equation for a biomolecular re- 

 action 



1 X 



k = — X 



at a - X 



where k is the velocity constant, t is the time, a is the initial concentration 

 of some reactants, and x is the concentration of a reaction product. Hecht 

 concluded that a synthesis of photosensitive substance, S, from two precurs- 



O 



TIME-MINUTES 



Fig. 116. Curves showing the recovery of sensitivity (dark adaptation) after exposure 

 to white light in the clam,'* in Litmdus,'^''' and in the frog.'^" Ordinate for two upper 

 curves is the logarithm of the threshold intensity; ordinate for lowest curve is the reaction 

 time in seconds; abscissa is time in minutes. 



ors (P and A) took place in the dark-adapting photoreceptor. These precurs- 

 ors, he suggested, originated from the photochemical decomposition of S dur- 

 ing the period of illumination preceding the sojourn in the dark. This gave 

 rise to the three component reversible schemes of Hecht 



Light 

 Dark 



P + A 



where S is the photolabile pigment, P, a precursor, and A, an accessory. 



RELATION BETWEEN ILLUMINATING INTENSITY AND PHOTOCHEMICAL EF- 

 FECT. Realizing that the reaction time was occupied by an event of great im- 

 portance in the visual process, Hecht believed that here a reaction occurred 



