118 THE NATURE OF FOVEAL DARK ADAPTATION 



experiments the lamp was always farther than 30 cm. from the opal 

 glass; in this way a uniform illumination of the cross was assured. 



The wires for the lamp run through a brass tube. This is rigidly 

 set in the lamp carriage, and projects through a slit in the upper 

 wall of the blackened box. The brass tube then turns a sharp right 

 angle, and lies about a milHmeter above the top of the box perpen- 

 dicular to the long axis of the box, as shown in the top view of Fig. 1 . 

 Because of the rigidity of the construction, the lamp carriage can 

 be easily moved by pushing on the outside arm of the brass tube. 

 Moreover, the location of the lamp is in this way indicated by the 

 position of the horizontal brass arm. An exact record of this position 

 is made in the dark by running a pencil along the edge of the arm, and 

 making a mark on the long strip of paper R which is stretched along 

 the top of the box beneath the brass arm. The position of the brass 

 arm indicative of a given location of the lamp is carefully measured. 

 Its subsequent locations may then be read off with a meter stick from 

 the estabhshed zero point. 



The lamp is controlled by the experimenter by means of a noiseless 

 switch W attached to the edge of the protruding brass arm. 



III. 



The actual manner of making a series of measurements deserves 

 some attention. With each subject I spent a preliminary period of 

 about 30 minutes explaining the nature of the experiments. and the 

 construction of the apparatus. Aside from putting the subject into 

 an interested frame of mind, this preHminary period serves as an 

 experimental precaution. It allows time for the effects of the outside 

 light, which is variable, to disappear. The starting point is therefore 

 the same for all the experiments. 



The subject is seated in a comfortable chair, the height of which 

 is so adjusted that the eyes of the particular subject can look com- 

 fortably into the opening of the viewing box B. He then lowers 

 his head, rests his forehead against the edge P of the box, and looks 

 at the light-adapting screen. The subject keeps himself in a relaxed 

 condition. To faciHtate this, the chair is so placed as to give full 

 support to the subject's back. Also, the edges of the viewing box 

 are padded with black felt. 5 minutes are allowed for light 



