Too frequently investigators, in their work on vision in animals, have assumed that brightness vision and color vision are inseparable; or, if not making this assumption, they have failed to realize that the same wave-length probably has markedly different effects upon the retinal elements of the eyes of unlike organisms. In a study of the sense of sight it is extremely important to discover whether difference in the quality, as well as in the intensity, of a visual stimulus influences the organism; in other words, whether color sensitiveness, as well as brightness sensitiveness, is present. If the dancer perceives only brightness or luminosity, and not color, it is evident that its visual world is strikingly different from that of the normal human being. The experiments now to be described were planned to show what the facts really are. [Illustration: Figure 14.--Discrimination box. _W_, electric-box with white cardboards; B, electric-box with black cardboards. Drawn by Mr. C.H. Toll.] As a means of testing the ability of the dancer to distinguish differences in brightness, the experiment box represented by Figures 14 and 15 was devised. Figure 14 is the box as seen from the position of the experimenter during the tests. Figure 15 is its ground plan. This box, which was made of wood, was 98 cm. long, 38 cm. wide, and 17 cm. deep, as measured on the outside. The plan of construction and its significance in connection with these experiments on vision will be clear from the following account of the experimental procedure. A mouse whose brightness vision was to be tested was placed in the nest-box, A (Figure 15). Thence by pushing open the swinging door at _I_, it could pass into the entrance chamber, _B_. Having entered _B_ it could return to _A_ only by passing through one of the electric-boxes, marked _W_, and following the alley to _O_, where by pushing open the swing door it could enter the nest-box. The door at _I_ swung inward, toward _B_, only; those at _O_, right and left, swung outward, toward _A_, only. It was therefore impossible for the mouse to follow any other course than _A-I-B-L-W-E-O_ or _A-I-B-R-W-E-O_. The doors at _I_ and _O_ were pieces of wire netting of 1/2 cm. mesh, hinged at the top so that a mouse could readily open them, in one direction, by pushing with its nose at any point along the bottom. On the floor of each of the electric-boxes, _W_, was an oak board 1 cm. in thickness, which carried electric wires by means of which the mouse could be shocked in _W_ when the tests demanded it. The interrupted circuit constituted by the wires in the two electric-boxes, in connection with the induction apparatus, _IC_, the dry battery, _C_, and the hand key, _K_, was made by taking two pieces of No. 20 American standard gauge copper wire and winding them around the oak board which was to be placed on the floor of each electric-box. The wires, which ran parallel with one another, 1/2 cm.