The brightness vision tests described in this chapter were made in a room which is lighted from the south only, with the experiment box directed away from the windows. The light from the windows shone upon the cardboards at the entrances to the electric-boxes, not into the eyes of the mouse as it approached them. Each mouse used in the experiments was given a series of ten tests in succession daily. The experiment was conducted as follows. A dancer was placed in _A_, where it usually ran about restlessly until it happened to find its way into _B_. Having discovered that the swing door at _I_ could be pushed open, the animal seemed to take satisfaction in passing through into _B_ as soon as it had been placed in or had returned to _A_. In _B_, choice of two entrances, one of which was brighter than the other, was forced by the animal's need of space for free movement. If the right box happened to be chosen, the mouse returned to _A_ and was ready for another test; if it entered the wrong box, the electric shock was given, and it was compelled to retreat from the box and enter the other one instead. In the early tests with an individual, a series sometimes covered from twenty to thirty minutes; in later tests, provided the condition of discrimination was favorable, it did not occupy more than ten minutes. To exhibit the methods of keeping the records of these experiments and certain features of the results, two sample record sheets are reproduced below. The first of these sheets, Table 6, represents the results given by No. 5, a female,[1] in her first series of white-black tests. Table 7 presents the results of the eleventh series of tests given to the same individual. [Footnote 1: It is to be remembered that the even numbers always designate males; the odd numbers, females.]