114 THE NATURE OF FOVEAL DARK ADAPTATION 



proved to be of advantage, because it opened the possibility of con- 

 necting the findings with what is known of the properties of visual 

 purple. Moreover, the conclusions which were secured pointed to 

 certain crucial experiments to be made with visual purple, thus sug- 

 gesting a new mode of attack in this line of work (Hecht, 1920-21, J). 

 However, as far as the eye as a whole is concerned, the analysis 

 was incomplete, because it did not include the properties of the fovea. 

 Although the fovea is essentially an instrument for bright vision, 

 and general dark adaptation, a phenomenon of dim vision, it still seems 

 desirable to find out what occurs in the fovea during a stay in the 

 dark. A study of the published data^ shows that they are to a large 

 extent insufficient and inconsistent, and that they involve some 

 fundamental sources of error. It has therefore become necessary to 

 investigate the matter from the beginning from a new point of view 

 and with a new type of apparatus. I have done this, and the results 

 show in a gratifying fashion that it was worth while. Not only has 

 it been possible to find the facts and regularities, but it has been 

 possible to account for the discrepancies of the results of other 

 workers. 



II. 



1. There are three major and several minor sources of error which 

 enter into a study of dark adaptation. With but few exceptions 

 previous work on foveal adaptation has been done without much 

 consideration of the major sources, and with but scant attention to 

 the minor ones. It is necessary to consider these possibihties of 

 error carefully. 



Measurements of the course of dark adaptation presume an original 

 condition of light adaptation. It would seem obvious that this must 

 be kept constant if the measurements are to have any significance. 

 Still, this necessary starting point has been consistently disregarded, 

 even when its significance has been recognized. We find that light 

 adaptation is secured by such irregular means as reading in a room 

 {cf. Dittler and Koike, 1912) or walking in the street {cf. Nagel and 

 Schaefer, 1904). In some cases it is noted whether the day is cloudy 

 or bright, but no account is taken of the circumstance. 



^ A review of the literature will be found in the papers by Tschermak (1902), 

 Wolffiin (1910), and Dittler and Koike (1912). 



