SELIG HECHT 121 



with the right eye. It will be seen that observations with the same 

 eye are made never less than 6 minutes apart. In the other three 

 instances the interval is greater than that, being 8, 9, and 10 minutes, 

 respectively. 



After the last observation the bright Hghts are turned on, and the 

 subject again becomes light-adapted for 5 minutes. The entire 

 process described in the last three paragraphs is then repeated, except 

 that the first observation is made with the right eye, the others follow- 

 ing in alternating order. In this way an independent set of deter- 

 minations is secured for each eye, reaching from the beginning to the 

 end of dark adaptation. These six pairs of dupHcate points are, 

 however, not sufficient to map out accurately the entire course of 

 dark adaptation. Therefore intermediate points are secured in the 

 following way. 



The subject is again light-adapted. The lights are turned off, and 

 a determination is made with the left eye after about 30 seconds dark 

 adaptation. The lamp is brought closer and closer in the usual 

 manner until the subject announces the appearance of the red cross. 

 The time is taken and the distance recorded. The subject rests in the 

 dark, and after 3 minutes in the dark a measurement is made, this 

 time with the slide set for the right eye. The bright Hghts are then 

 turned on, the subject becomes Hght-adapted, and the 2 measurements 

 are repeated, this time first with right eye for 30 seconds adaptation, • 

 and then with the left eye after 3 minutes dark adaptation. 



It is not necessary to detail the method any further. Suffice it to 

 say that two sets of ten determinations are secured, a set for each eye. 

 The whole procedure may be shown in Table I, which gives the details 

 of a single experiment. The table shows first, the order in which the 

 observations are made; second, the period extending from the time 

 when the bright lights were turned off, to the time when adaptation 

 was announced; and third, the distance which the measuring lamp 

 had to be brought before the subject announced that she saw a red 

 cross. Each horizontal line in the body of the table indicates 5 

 minutes light adaptation. Thus, between two horizontal fines are 

 the readings made during a single period of dark adaptation. An 

 entire experiment takes about an hour and a half. 



