120 THE NATURE OF FOVEAL DARK ADAPTATION 



that it is impossible to measure this point accurately. The difficulty 

 lies not so much with the determination of the intensity, as with the 

 measurement of the exact time of dark adaptation. Half a second 

 makes an enormous difference in the threshold, as will presently be 

 apparent. However, a rough idea of the order of magnitude of the 

 .threshold after about 2 seconds dark adaptation can be secured in 

 this way, and as such it is valuable. 



All this time the subject has gained practice in locating the red cross. 

 The experiment then begins in earnest. After the subject has again 

 been light-adapted, the measuring lamp is set at such a place that 

 the subject will see it after about 5 seconds in the dark. The slide 

 is set for observation with the left eye, and the subject looks in as 

 usual and announces when the red cross appears. The time is taken 

 with a stop watch, the measuring lamp is turned out, and the position 

 of the lamp marked on the strip of paper. The subject withdraws 

 his head from the viewing box, and sits comfortably in the dark. 



Readings of the stop-watch and observations of the time in the dark 

 are made by a very dim light which is carefully screened from the 

 subject. This dim light is turned on momentarily when it is needed by 

 means of a spring contact controlled by the experimenter's foot. 

 Repeated tests on this point have never revealed an instance when 

 the subject was able to notice this illimiination. 



The slide in the viewing box is now adjusted for the right eye. The 

 lamp is moved far back in the long box, and 2 minutes after the begin- 

 ning of dark adaptation, the subject is requested to look into the 

 viewing box. The measuring lamp is then turned on and brought 

 nearer and nearer the cross opening at a rate of about 3 cm. per second. 

 The subject then announces the moment he sees the red cross. The 

 time is taken with a stop-watch which was started exactly 2 minutes 

 after dark adaptation had begun. The lamp is turned off, its position 

 is marked and numbered on the recording paper, and the subject is 

 told to sit back comfortably in the dark. 6 minutes after the beginning 

 of dark adaptation, the slide is set for the left eye, and a measurement 

 made as in the last instance. The process is then repeated after 10 

 minutes of dark adaptation, this time with the slide set for the right 

 eye. 15 minutes after the beginning of dark adaptation, a reading 

 is made with the left eye again; and after 20 minutes adaptation, 



