apart, fitted into shallow grooves in the edges of the board, and thus, as well as by being drawn taut, they were held firmly in position. The coils of the two pieces of wire alternated, forming an interrupted circuit which, when the key _K_ was closed, was completed if the feet of a mouse rested on points of both pieces of wire. Since copper wire stretches easily and becomes loose on the wooden base, it is better to use phosphor bronze wire of about the same size, if the surface covered by the interrupted circuit is more than three or four inches in width. The phosphor bronze wire is more difficult to wind satisfactorily, for it is harder to bend than the copper wire, and it has the further disadvantage of being more brittle. But when once placed properly, it forms a far more lasting and satisfactory interrupted circuit for such experiments as those to be described than does copper wire. In the case of the electric-boxes under consideration, the oak boards which carried the interrupted circuits were separate, and the two circuits were joined by the union of the wires between the boxes. The free ends of the two pieces of wire which constituted the interrupted circuit were connected with the secondary coil of a Porter inductorium whose primary coil was in circuit with a No. 6 Columbia dry battery. In the light of preliminary experiments, made in preparation for the tests of vision, the strength of the induced current received by the mouse was so regulated, by changing the position of the secondary coil with reference to the primary, that it was disagreeable but not injurious to the animal. What part the disagreeable shock played in the test of brightness vision will now be explained. [Illustration: FIGURE 15.--Ground plan of discrimination box. _A_, nest- box; _B_, entrance chamber; _W,W_, electric-boxes; _L_, doorway of left electric-box; _R_, doorway of right electric-box; _E_, exit from electric- box to alley; _I_, swinging door between _A_ and _B_; _O_, swinging door between alley and _A_; _IC_, induction apparatus; _C_, electric cell; _K_, key in circuit.]