152 The Dancing Mouse 



experiments. With this it was possible to get a pure red. 

 Each of the other colors was obtained by means of a filter, 

 which consisted of a glass box filled with a chemical solution 

 which transmitted light of a certain wave length. 



For the tests with transmitted light the apparatus of 

 Figures 20 and 21 was constructed. It consisted of a re- 

 action-box essentially the same as that used in the brightness 

 vision tests, except that holes were cut in the ends of the 

 electric-boxes, at the positions G and R of Figure 20, to per- 

 mit the light to enter the boxes. Beyond the reaction-box 

 was a long light-box which was divided lengthwise into two 

 compartments by a partition in the middle. A slit in the 

 cover of each of these compartments carried an incandescent 

 lamp L (Figure 20). Between the two lamps, L, L, and 

 directly over the partition in the light-box was fastened a 

 millimeter scale, S, by means of which the experimenter 

 could determine the position of the lights with reference to 

 the reaction-box. The light-box was separated from the 

 reaction-box by a space 6 cm. wide in which moved a narrow 

 wooden carrier for the filter boxes. This carrier, as shown in 

 Figure 20, could be moved readily from side to side through 

 a distance of 20 cm. The filter boxes, which are represented 

 in place in Figures 20 and 21, consisted of three parallel- 

 sided glass boxes 15 cm. long, 5 cm. wide, and 15 cm. deep. 

 Each box contained a substance which acted as a ray filter. 

 Tightly fitted glass covers prevented the entrance of dust and 

 the evaporation of the solutions in the boxes. Figures 20 

 and 21 represent the two end boxes, R, R, as red light filters 

 and the middle one, G, as a green light filter. Three filters 

 were used thus side by side in order that the position of a 

 given color with reference to the electric-boxes might be 

 changed readily. As the apparatus was arranged, all the 

 experimenter had to do when he wished to change from 



