552 SENSORY EQUILIBRIUM AND DARK ADAPTATION 



3. The dynamics of this reversible reaction must explain the pe- 

 culiarities of photosensory adaptation. The course of dark adapta- 

 tion is, of course, clearly a function of the unopposed "dark" reaction. 

 In addition, however, it must be shown how the hypothesis accounts 

 for sensory equilibrium, and for the difference in sensitivity of light- 

 and dark-adapted sense organs. 



In the presence of any intensity of Kght the two opposing reac- 

 tions will presently reach a stationary condition, at which the con- 

 centration of the three components will remain constant. This is 

 equivalent to sensory equilibrium., and involves two characteristics. 

 One is that this intensity no longer acts as a stimulus. This follows 

 from the fact that no increase in the amount of precursor substances 

 takes place. The second is that from this level the reaction time 

 to a higher intensity is always the same. This is because a response 

 always involves the formation of the same fraction of the amount of 

 precursor substances present in the system. Since the latter is 

 constant, the reaction time will also be constant. 



The reaction system may approach a stationary condition from 

 either side and at different intensities (Sheppard, 1914). Similarly, 

 sensory equilibrium may be attained either at low intensities through 

 dark adaptation, or at high intensities through light adaptation. 

 In the reaction system a low intensity means a small quantity of pre- 

 cursor substances present at the stationary condition, whereas at 

 high intensities a large quantity will be present. This is true of re- 

 versible photochemical reactions, as well as of those that are only 

 pseudoreversible (Sheppard, 1914, p. 225). In order to produce a 

 response the amount of precursor formed must be a definite fraction 

 of the amount then present in the system. Hence at a high intensity 

 sensory equihbrium more light energy will be required to produce a 

 response than at a low intensity equilibrium. This is equivalent to 

 saying that a dark-adapted animal is more sensitive to light than a 

 light-adapted one, — which is a self-evident proposition of sensory 

 physiology (von Helmholtz, 1911, Hess, 1910). 



The photochemical system here proposed is therefore adequate to 

 account for the phenomena associated with the sensory adaptation 

 of Mya in its responses to an increase of illumination. 



