500 DARK ADAPTATION OF EYE 



theoretical bearing is practically nil. The changes in sensitivity 

 are remarkably constant, not only in a given individual, but in dif- 

 ferent individuals as well. Age does not change the regularity of the 

 course. The effect of drugs is practically negligible. Even persons 

 possessing deficiencies of color vision present a normal type of dark 

 adaptation. Still the meaning of this uniformly regular change in 

 sensitivity has remained obscure. The course of dark adaptation has 

 not given us a hint of the physicochemical basis of visual reception, 

 though it is apparent that the two phenomena must be fundamen- 

 tally related to each other. 



What are the causes of this failure? In order to answer this ques- 

 tion profitably we must consider first the nature of the published data, 

 and second the obstacles in the way of their interpretation inherent 

 in the data. Extensive summaries of the literature of retinal adapta- 

 tion have been made (Tschermak, 1902; Nagel, 1911). Such is not 

 my purpose. It is rather to analyze the data, and if possible to find 

 some explanation of the pronounced regularity evident in every experi- 

 ment on the dark adaptation of the human eye. 



n. 



The pioneer experiments of Aubert (1865), followed by those of 

 Charpentier (1886) and Treitel (1887), demonstrate the qualitative 

 fact that dark adaptation follows a definite course. Due to matters 

 of technique, however, they are not sufficiently accurate to stand on 

 a par with the later experiments of Piper (1903) and Nagel (1911). 

 We shall therefore confine ourselves to the work of the latter investi- 

 gators. Piper in particular has published the complete results of the 

 retinal adaptation of eighteen people. These detailed data are in- 

 valuable in the quantitative treatment of the material. 



The experiments consist in finding the intensity of a square area of 

 Hght which is just barely visible to the eye. Observations are made 

 at regular intervals during the stay in the dark. The subject fixes 

 his eyes on one corner of the square of light, so that most of the 

 light falls on the retina outside the fovea centrahs. What one finds 

 is this. At first this minimum intensity is large; as the stay in the 

 dark is prolonged it becomes less and less; and finally it reaches a 

 constant minimum. 



