338 



H. M. JOHNSON 



the lamp, which rests on a wooden photometer track. Each 

 lamp, it may be seen, illuminates an equal area on the cube to 



D' 



® 



L' 



fez 



5c' ► 



m 





HWS?////ff/UiV»lM 



Lib 



(food) 



urn 







/\ l 



(£»,r) 



H 



Hjb 



(food) 



© 



© 





+ 



% 



FIG. 6. Floor plan of Yerkes box, arranged for contrast-discrimination 



which it belongs. The sectors D 1 and D 2 each have an angular 

 opening of 180°. Thus the face of the cube which they illuminate 



