SELIG EECHT 135 



Granting the chemical nature of the process, it is obvious that there 

 exist two products of decomposition which are disappearing from the 

 reaction mixture by combining to form something else. Concomitant 

 with this, in the same sense cells, there is being produced an increasing 

 concentration of photosensitive material. What more reasonable step 

 is there than to assume that it is being formed by the combination 

 of the decomposition products that are disappearing? 



We are therefore led to the conclusion that in the photosensory 

 mechanism of the fovea there exists the following arrangement of 

 materials. There is a photosensitive substance 5 whose rate of 

 photolysis at threshold intensities is a linear function of the intensity. 

 The photolysis of this material results in the formation of two products 

 of decomposition P and A . The concentration of these decomposition 

 products at any moment determines the sensitivity of the sensory 

 mechanism, in that the amount of photolytic action necessary for a 

 threshold, visual effect is directly proportional to the concentration of 

 photolytic products already present. Finally, these decomposition 

 products constantly recombine to form fresh sensitive material, their 

 combination proceeding according to the kinetics of a bimolecular 

 reaction. We have thus the familiar reversible reaction 



light 



"dark" 



in which the products of decomposition serve as the precursors of 

 the sensitive material. 



IX. 



It is not necessary at this time to dilate on the simplicity with 

 which such a reversible system accounts for the observed facts {cf. 

 Hecht, 1919-20, c, p. 514). It will suffice to point out that the "dark" 

 reaction obviously accounts for dark adaptation; that the stationary 

 state, in which the "dark" and light reactions are balanced, takes care 

 of the condition of sensory equilibrium in which the eye has become 

 adapted to a given light intensity; and that the process of light 

 adaptation represents merely the displacement of the stationar}- state 

 of the reaction to the right, due to the increased action of the light. 



