SELIG HECHT 127 



data showing much the slower rate. For example, after 3 minutes 

 adaptation the eye requires 22.5 per cent of the illumination of the 

 Hght-adapted eye in order that both see the same brightness. After 

 30 minutes adaptation the ratio of light to dark-adapted eye is as 100 

 to 8. This represents total dark adaptation. 



The explanation for this leisurely adaptation lies in the extreme 

 rapidity with which adaptation actually proceeds during the first few 

 seconds. Consider Dittler and Koike's method. One eye is light- 

 adapted while the other is dark-adapted, and the standard of 

 measurement is the Hght-adapted one. To get into the dark room 

 takes a few seconds; to get to the apparatus takes a few more seconds; 

 to remove the bandage, to accommodate, and to make a judgment also 

 take a few seconds. During part of this time the light-adapted eye 

 is in the dark, and as Fig. 4 shows, it is very rapidly becoming dark- 

 adapted. When the comparison is being made, the Hght-adapted 

 eye is no longer Hght-adapted. Its threshold is much lower than it 

 was a few seconds ago. As a standard of comparison it is entirely 

 too low, the situation being the same as if the measurement of dark 

 adaptation were begun after the first few seconds have elapsed. This, 

 together with the fact that the degree of light adaptation is low to 

 begin with, could make the course of adaptation correspond with that 

 portion of the curve in Fig. 4 which Hes below, say, the middle of the 

 scale of ordinates. This condition accounts for the apparent slowness 

 of the process and for its small extent, both of which are characteristic 

 of these and of the older investigations. 



2. Considered from this point of view, these older results constitute 

 a significant check on the validity of the experiments presented in 

 the present paper. My measurements were made with red light, and 

 it might perhaps be thought that the course of dark adaptation would 

 be different if the experiments had been conducted with white Hght. 



To meet this criticism I made a few experiments on my own eyes, 

 using red Hght at one time and white Hght at another. Although the 

 actual threshold illumination is not the same in the two cases, the 

 rate of adaptation is the same. This matter has already been 

 considered by Nagel and Schaefer (1904, p. 283), who came to the 

 same conclusion. They used red, blue, and green Hghts and found that 

 the same order of change is secured with all three colors. In other 

 words, the Purkinje phenomenon docs not exist in the fovea. 



