54 HAROLD C. BINGHAM 



The upper end is tightly closed. The lower end is fitted with a 

 removable cover in which is centered an Aubert diaphragm for 

 regulating the amount of light falling upon the experiment box. 

 To eliminate shadows in the experiment box and to increase the 

 amount of upper illumination, the diaphragm cover was removed 

 throughout the greater part of the experiment. With the lower 

 end of the cylinder open, the amount of light added to the ex- 

 periment box was that coming from a 2 c.p. lamp, passing 

 through a circular opening 10 cm. in diameter, and falling a 

 distance of 120 cm. Further variations in the upper illumina- 

 tion were made by substituting for the 2 c.p. lamp, after the 

 chicks had acquired a positive reaction tendency, larger lamps 

 with a maximum of 10 c.p. 



The upper illumination thus serves as a control of the in- 

 equality of general illumination in the two electric compart- 

 ments. It can be so arranged as to furnish enough illumination, 

 equally divided between the two compartments, to reduce the 

 proportional difference in illumination to a point below the 

 chick's threshold. The amount of upper illumination should 

 therefore depend upon the degree of difference between the sizes 

 and brightnesses of the two stimulus areas. In a quantitative 

 study, where the difference in size is being continually reduced, 

 and where variations and reversals of brightness are essential 

 features of the vital tests, the possibility of consistent reactions 

 on the basis of difference in general illumination is removable 

 by means of the upper illumination. 



Variations and reversals of brightness, as essential parts of 

 the system of control, are easily effected by changing the re- 

 spective distances of the source lamps from the display surfaces. 

 If the brightness of the respective stimuli be varied by reducing 

 the illumination of the larger, increasing that of the smaller, 

 or vice versa, the luminous intensity of the stimuli is at the same 

 time varied. Thus, a proper combination of controls, including 

 the upper illumination and relative changes in distance of the 

 sources, provides a means of determining whether the basis of 

 the chicks' choices lies in size difference or any of the three 

 possibilities (1) luminous intensity, (2) brightness, and (3) 

 general illumination. 



The method of adapting these controls to my experiment is 

 concisely stated in table 5. The location of the source lamps 



