For the tests with transmitted light the apparatus of Figures 20 and 21 was constructed. It consisted of a reaction-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 permit 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 green-left, red-right to green-right, red-left was to push the carrier towards the right until the green filter covered the hole on the right at the end of the electric- box. When this had been accomplished the red filter at the left end of the carrier covered the hole on the left at the end of the electric-box. Thus quickly, noiselessly, easily, and without introducing any other change in conditions than that of the interchange of lights, the experimenter was able to shift the positions of his colored lights at will. [Illustration: FIGURE 20.--Color discrimination apparatus. _A,_ nest-box; _B,_ entrance chamber; _R, R,_ red filters; _G,_ green filter; _L, L,_ incandescent lamps in light-box; _S,_ millimeter scale on light-box; _I,_ door between _A_ and _B; O, O,_ doors between alleys and _A_.] [Illustration: FIGURE 21--Ground plan of color discrimination apparatus. _E, E_, exits from electric-boxes. _LB_, light-box; _R, G, R_, filter boxes on carrier; _L_, left electric-box; _R_, right electric-box; _IC_ induction apparatus; _C_, electric cell; _K_, key; _S_, millimeter scale.]