514 DARK ADAPTATION OF EYE 



VI. 



In terms of such a reversible photochemical process the phe- 

 nomena of dark adaptation are fundamentally simple. During the 

 stay in the light, a large amount of photosensitive substance is de- 

 composed according to the reaction S —^ P + A. The rate of this 

 decomposition will depend entirely on the intensity of the light. 

 The reverse, "dark" reaction P -}- A —^ S, being an ordinary chemical 

 reaction, will proceed whenever any products of decomposition are 

 formed. According to the mass law the rate of synthesis of 5 from 

 P and A will depend on the concentration of the latter two sub- 

 stances. Between the two opposing reactions a stationary state will 

 be reached (cf. Weigert, 1911, p. 15, for the difference between a 

 stationary state and a condition of true equilibrium) . This stationary 

 state will represent a definite concentration of the three components, 

 and will depend entirely on the light intensity. 



Removal into the dark at once causes the light reaction, 

 S —^ P + A, to stop, leaving the "dark" reaction, P -\- A -^ S, to 

 go on unopposed. The continuous action of this "dark" reaction 

 then determines the course of dark adaptation. In order to meas- 

 ure the irritability during dark adaptation a visual test is used. The 

 necessary light decomposes S into fresh P and A. The ratio be- 

 tween fresh and residual P and A is constant. Therefore as the 

 residual products of decomposition disappear, less and less fresh P 

 and A are required to initiate a visual effect. In other words, the 

 retina becomes more and more irritable, as we already know. 



The fact that the eye may become adapted to any intensity of light 

 finds its explanation in the stationary state of the opposing chemical 

 reactions. This is entirely a function of the intensity of the light, 

 provided the temperature remains constant. At high intensities the 

 concentration of P and A during the stationary state will be much 

 greater than at lower intensities. This means that the light required 

 to produce a minimum visual effect will vary similarly. Thus the 

 sensory threshold will be higher at higher intensities than at lower, 

 which again is a truism of retinal physiology {cf. Hecht, 1918-19, b, 

 p. 553). 



