26 ROBERT M. YERKES AND JOHN B. WATSON 



2. Points of method 



The use of the " brightness " apparatus involves a number 

 of important points of method. 



The sources of Hght should be well seasoned tungsten lamps 

 burned on a carefully regulated circuit, and every precaution 

 should be taken to avoid sources of error from reflection within 

 the light box. 



The stimulus area (i.e., the illuminated window of opal glass 

 which is framed by an accurately cut standard plate) should 

 consist of a carefully selected piece of " flashed opal glass." ^ 

 As this glass varies in thickness and contains imperfections, it 

 is absolutely essential that the three windows of the stimulus 

 adapter be fitted with pieces which are as closely matched as 

 possible. Unless great care be exercised in selecting and plac- 

 ing the diffusing glasses the value of the apparatus is certain 

 to be impaired. The lights must be centered with the stimulus 

 windows, and these, in turn, with the openings into the experi- 

 ment box. It is desirable that the experiment box be so arranged 

 that the photic stimulus may be measured photometrically and 

 radiometrically just as it appears to the animal. Since the 

 value of the method which we are proposing depends upon the 

 comparison of two photic stimuli which differ from one another, 

 in a certain definite amount, with respect to some one char- 

 acter, it is important that every precaution be taken to keep 

 the stimuli separate. This means that the apparatus must in 

 effect be double, the one half being wholly beyond the photic 

 influence of the other. 



Discrimination by difference in the degree of illumination of 

 the two passage-ways of the experiment box should be prevented 

 by covering the portions of the passage-ways near the stimulus 

 windows with black velvet. In some cases it is likely to prove 

 necessary to arrange a truncated cone of dead black velvet 

 about each stimulus area. Thus the animal can be forced, in 

 its attempts to discriminate in favor of the right passage-way, 

 to depend solely upon a visual difference in the two illuminated 

 windows. 



In its perfect form the " brightness " apparatus should enable 

 the experimenter to present two visual stimuli which differ from 



' This glass is obtainable through Semon Bache and Company, West and Hubert 

 Streets, New York, at 40 cents per square foot, in cut sizes. 



