SELIG HECHT 



557 



the order of magnitude associated with photographic processes. The 

 second is a latent period of about 1.3 seconds, during which Mya 

 need not remain exposed to the stimulating light. 



2. The process of dark adaptation in Mya is orderly. Its prog- 

 ress may be represented by the formation of a photosensitive substance 

 according to the dynamics of a bimolecular reaction. 



/GO 



I 

 I 



60 



^O 





/o 



zo 



JO 



^4^ 



SO 



60 



T/noe- 



r)/nufe^ 



Fig. 3. The progress of the hypothetical reaction, P + ^4 — > 5, at the tempera- 

 tures indicated. The points are calculated from the averages given in Table 

 IV, and represent the amount of photosensitive material formed at different 

 stages of dark adaptation. The solid lines give the theoretical values. 



'3. Photosensory equilibrium as represented by the light- and 

 dark-adapted conditions finds a rational explanation in terms of the 

 "stationary state" of a reversible photochemical reaction involving a 

 photosensitive substance and its two precursors. 



4. There are two corollaries to this hypothesis. The first re- 

 quires that the reaction time at sensory equilibrium for a given in- 

 tensity should vary inversely with the temperature; the second, that 

 the rate of dark adaptation should vary directly wi'th the tempera- 

 ture. Experiments verified both of these requirements. 



