126 THE NATUliE OF FOVEAL DARK ADAPTATION 



454 X 10~^ ml. The threshold thus drops in the first 30 seconds to 

 about 3 per cent of its initial value. 



During the next 20 minutes the threshold further decreases, but 

 only to about 3 X 10~^ml., and remains there. It is therefore clear 

 that the change which takes place after the first 30 seconds is compara- 

 tively insignificant in the light of the first rush of events. There is, 

 however, no sudden point at which the process of adaptation begins 

 to slow down. It slows down from the moment it begins, and as can 

 be seen from the figures presented so far, its course is continuous and 

 smooth. 



V. 



1. The precipitous course of foveal adaptation during the first few 

 seconds is at the bottom of the curious results obtained by previous 

 workers. It will not be irrelevant to consider in some detail the 

 better and more recent of these investigations. Inouye and Oinuma 

 (1911) studied foveal adaptation by the following method. A double 

 tube was constructed through which the two eyes could look simultane- 

 ously at different fields. The brightness of the two fields could be 

 separately controlled by a rotating sector of black and white card- 

 board. The idea was to keep one eye light-adapted, and the other 

 dark-adapted, and then to vary the illumination in one tube until 

 both eyes saw equally bright fields. 



The actual procedure calls for a preliminary light adaptation of 

 both eyes by reading in the laboratory near the dark room. One eye 

 is then dark-adapted with a black bandage. After the proper interval 

 the subject rushes into the dark room, tears the bandage from his eye, 

 and looks through the two tubes. The sector wheel has been set, and 

 the subject compares the brightness of the two fields. After repeated 

 trials for the same amount of dark adaptation, the position of the 

 sector is found which results in apparent equality of the two fields. 

 The entire process is then repeated for different amounts of dark 

 adaptation. These experiments were repeated by Dittler and Koike 

 (1912), who used smoked glasses instead of a sector wheel with which 

 to reduce the illumination of the dark-adapted eye. 



The results secured by these two sets of investigators show a 

 leisurely kind of dark adaptation for the fovea, Dittler and Koike 's 



