132 THE NATtJRE OF FOVEAL DARK ADAPTATION 



ing startling, it being the kind ordinarily encountered in chemical 

 processes. In treating the data further I have therefore assumed that 

 the photochemical action of the light is directly proportional to the 

 intensity. This is a common enough situation in photochemical 

 reactions, and in addition, obtains in the photic sensitivity of Ciona 

 (Hecht, 1918-19, a). 



It follows from Fig. 6 that the quantity of sensitive material de- 

 composed in order to produce a threshold effect in the fovea gradu- 

 ally but consistently decreases during dark adaptation. What is 

 the basis for this variation? In other words, with what chemical or 

 physical entity or process in the fovea can this gradual decrease be 

 associated? I have previously outlined a possible hypothesis to 

 account for this, and because of its success in the study of the responses 

 of Ciona and Mya as well as of the periphery of the human eye (Hecht, 

 1921, e), I venture to suggest its appUcation in the present circumstance. 



Let us consider what might conceivably happen in the fovea. We 

 have accepted the existence of a photosensitive substance. During 

 light adaptation this substance must be decomposed to a considerable 

 extent. Associated with this is a decided increase in the threshold 

 of stimulation. During the succeeding period of dark adaptation 

 there is produced a characteristic decrease in the threshold which 

 proceeds according to a definite pattern. During this time it seems 

 reasonable to suppose that the sensitive material is in some way being 

 formed again, and that its concentration in the sense cells is increasing. 



The continuously decreasing threshold cannot be directly associated 

 with this increase in sensitive material. One reason for this is the 

 fact that they vary in the opposite sense. A more cogent reason is 

 the nature of the sensitive material itself. In its essence it must be 

 an inert material so far as the sensory process is concerned. Otherwise 

 it would continually be initiating visual effects. It is only after it 

 has been changed into something else by the light, that it produces its 

 characteristic reaction. 



Indeed, it is to this decomposition product that we must look for 

 some objective visualization of the process of dark adaptation. This 

 decomposition product undoubtedly enters into some secondary 

 reaction. I have elsewhere suggested that in Mya and Ciona it serves 

 to catalyze a second reaction, the end-products of which constitute 



