SELIG HECHT 



125 



2. One thing stands out prominently in all these figures. It is 

 that the dark adaptation of the eye, as measured by foveal vision, 

 begins immediately, and proceeds at a precipitous rate for the first 

 30 seconds. The data in Figs. 2, 3, and 4 begin to record the events 

 which happen after the first 7 seconds in the dark. This is because 

 things happen too fast for accurate measurement before the first 7 

 seconds. As noted in a previous section, however, I have made some 

 observations of the threshold after 2 seconds of dark adaptation for 



o zoo ■■'f^o 600 eoo looo /zoo 



Oc/raf/oo of dark adapiation — seconds 



Fig. 4. Data for all fifteen subjects. Each point is the average of thirty measure- 

 ments. 



the purpose of securing an idea of the order of its magnitude. An aver- 

 age of twenty measurements gives this threshold a value of 140 X 10"^ 

 millilamberts. From Fig. 4 it can be seen that the threshold after 

 7 seconds of dark adaptation is about 40 X lO^-iml. After 30 seconds 

 in the dark the threshold drops to about 12 X lO'^ml. Because of the 

 nature of the curve of dark adaptation, as described in a later section, 

 it is possible to extrapolate backwards and find what the value of the 

 threshold is at zero seconds dark adaptation. This turns out to be 



