SELIG HECHT 119 



adaptation. Measurements made with regard to this point show 

 that the process is accompHshed in much less time (c/. Lohmann, 1906), 

 but this interval is chosen to cover all contingencies. Near the end 

 of the time the subject is given notice, and at the proper moment, 

 the experimenter turns out the bright lights. The subject, raises 

 his head at once, and looks into the viewing box. 



A few preliminary experiments showed at once that the rate of dark 

 adaptation of the fovea is exceedingly rapid during the first minute 

 of darkness. To make frequent and rapid measurements during this 

 short interval is out of the question. Aside from the technical diih- 

 culties, there is the fact that repeated tests of the visual threshold must 

 disturb the process of adaptation. The failure to appreciate this 

 source of error has vitiated many of the experiments of Piper (1903) 

 on peripheral adaptation, and nearly all of the results of Wolfflin (1910) 

 on foveal adaptation. An ideal method would be to make but one 

 reading following each light adaptation. This however would weary 

 the subject, even if it did not fatigue the eye, which is not at all 

 certain. The method finally adopted is a compromise between these 

 two extremes. 



Only one eye is used at a time in making the measurements. The 

 eye to be used is under the control of the experimenter, who determines 

 it by moving a slide D near the ocular end of the viewing box (Fig. 1). 

 The subject acts as if he were looking with both eyes. Indeed, many 

 of the subjects were unaware of which eye they were using, or that 

 they were using only one eye; a few were able to distinguish them 

 easily. 



Before the bright lights are turned off, the small lamp is set at a 

 given distance from the opal glass. When the lights are turned off, 

 the subject looks in and announces the moment he sees the red cross. 

 The time is taken by the experimenter with a stop-watch. The 

 bright Kghts are turned on, the subject becomes light-adapted again, 

 and the process is repeated with the other eye; this time the hght has 

 been placed nearer or farther depending upon the time it took the 

 subject to see the cross. In this way after three or four trials the 

 distance is determined at which the subject can see the red cross almost 

 as soon as he looks into the box, immediately after the room has been 

 darkened. The rate of dark adaptation at the beginning is so rapid 



