Yerkes and Dodson, Habit Formation. 465 



was 1 1.5 cm. in height and 5.4 cm. in width, with a hole 3-5 by 

 3.5 cm. in the middle of its lower edge as is shown in fig. i. These 

 entrance cards were held in place by small metal carriers at the 

 edges of the electric boxes. The area of white surface exposed 

 to the view of a mouse as it approached the entrances to the elec- 

 tric boxes was 49.85 sq. cm. and the same amount of black sur- 

 face was exposed. The white cardboard reflected 10.5 times as 

 much Hght as the black cardboard. 



Special conditions of set I . The inside length of each electric 

 box was 28.5 cm. the width 7 cm. and the depth 11.5 cm. The 

 inside cards extended from the inner edge of the front of each 

 box a distance of 13.5 cm. toward the back of the box. Conse- 

 quently there was exposed to the view of the mouse a surface 

 13.5 cm. by 1 1.5 cm. (the depth of the box and of the cardboard 

 as well) on each side of the box. The section of cardboard at the 

 top measured 13.5 cm. in length by 6.5 cm. in width. The total 

 area of the white (or black) cardboard exposed on the inside of an 

 electric box was therefore 13.5 X 11. 5 X 2 (the sides) + 13.5 

 X 6.5 (the top) = 398.25 sq. cm. If to this we add the area of 

 the entrance card we obtain 448.10 sq. cm. as the amount of 

 surface of cardboard carried by each electric box. 



But another condition, in connection with the amount of card- 

 board present, determined the diflPerence in the brightness of the 

 boxes, namely, the amount of open space between the end of the 

 inner cardboards and the end of the experiment box. The larger 

 this opening the more light entered each box. In the case of the 

 experiments of set I this uncovered portion of each electric box 

 was 15 cm. long by 7 cm. wide; its area, therefore, was 105 sq. cm. 



Special conditions of set II. Both the outer and the inner card- 

 boards were precisely the same in form and arrangement as in the 

 case of set I, but in order that discrimination might be rendered 

 easier, and the time required for the acquisition of the habit thus 

 shortened, a hole 8.7 cm. long by 3.9 cm. wide was cut in the mid- 

 dle or top section of the white cardboard. This greatly increased 

 the amount of light in the white electric box. The difference in 

 the brightness of the boxes was still further increased by a reduc- 

 tion of the space between the end of the cardboard and the end 

 of the box from 15 cm. to 2 cm. or, in terms of area, from 105 sq. 

 cm. to 14 sq. cm. This was accomplished by cutting 13 cm. from 

 the rear end of the experiment box. For the experiments of set 



