SELIG HECHT 5l3 



thus a direct measure of the amount of decomposition products at 

 that moment in the retina. The points in Figs. 3 to 8 then give the 

 concentration of the residual decomposition products. 



The data now assume a dynamic aspect. Each figure represents 

 the changes in the concentration of the residual decomposition 

 products during adaptation. It is apparent that the concentration 

 steadily decreases. The decrease is regular and follows the course 

 of a chemical reaction. In order to show this the curves in all the 

 figures are the isotherms of a bimolecular reaction. The equation for 

 the curves is 



a-t a—x 



the values of k for each experiment being given in the respective 

 figures. It must be emphasized that each of the points in these 

 figures represents only a single measurement. The agreement be- 

 tween the individual experimental values and the theoretically 

 calculated curves is truly striking. 



The fact that the curves are reactions of the second order is sig- 

 nificant. Two products of decomposition are diminishing in con- 

 centration in a manner which shows that they are combining to form 

 a chemical compound. These two products are originally the results 

 of the decomposition of a photosensitive substance. A simple expla- 

 nation is that the compound formed by their chemical union is iden- 

 tical with the photosensitive substance from which they were formed. 

 Such an interrelation is quite common in photochemical reactions. 

 Using S to denote the photosensitive substance, P the principal product 

 of decomposition, and A its accessory, we may write 



Hght 



S^P -{■ A 



"dark" 



as the equation of the photochemical reaction. This reaction forms 

 the initial step in the visual reception of dim light by the human 

 retina. The substances P and A are thus the precursors as well as 

 the decomposition products of the photosensitive substance S, the 

 three forming the components of a completely reversible reaction 

 (c/. Hecht, 1918-19, a, p. 161). 



