[Footnote 1: These were the Milton Bradley blue and orange papers.]
[Illustration: FIGURE 18.--Food-box apparatus for color discrimination experiments. _O_, orange food-box; _B_, blue food-box; 1, 2, 3, 4, different positions of the food-boxes, _O_ and _B_; I, II, III, IV, figures in which the arrows indicate the direction in which the food-boxes were moved.]
[Illustration: FIGURE 19.--Food-box apparatus with movable partitions. _O_, orange food-box; _B_, blue food-box; _X_, starting point for mouse; _A_, point at which both food-boxes become visible to the mouse as it approaches them; 1, 2, two different positions of the food-boxes; _T_, _T_, movable partitions. (After Doctor Waugh.)]
An improvement on this method, which was suggested by Doctor Karl Waugh, has been used by him in a study of the sense of vision in the common mouse. It consisted in the introduction, at the middle of the experiment box, of two wooden partitions which were pivoted on their mid-vertical axes so that they could be placed in either of the positions indicated in Figure 19. Let us suppose that a mouse to be tested for color vision in this apparatus has been placed at _X_. In order to obtain food it must pass through _A_ and choose either the orange or the blue box. If it chooses the former, the test is recorded as correct; if it goes to the blue box first, and then to the orange, it is counted an error. While the animal is eating, the experimenter shifts the boxes to position 1 of Figure 19, and at the same time moves the partitions so that they occupy the position indicated by the dotted lines. The chief advantage of this improvement in method is that the animal is forced to approach the color boxes from a point midway between them, instead of following the sides of the experiment box, as it is inclined to do, until it happens to come to the food-box. This renders the test fairer, for presumably the animal has an opportunity to see both boxes from _A_ and can make its choice at that point of vantage.
Two males, A and B, of whose age I am ignorant, were each given seventeen series of tests in the apparatus of Figure 18. A single series, consisting of twenty choices, was given daily. Whether the animal chose correctly or not, it was allowed to get food; that is, if it went first to the blue box, thus furnishing the condition for a record of error, it was permitted to pass on to the orange box and take a piece of "force." No attempt was made to increase the animal's desire for food by starving it. Usually it sought the food-box eagerly; when it would not do so, the series was abandoned and work postponed. "Force" proved a very convenient form of food in these tests. The mice are fond of it, and they quickly learned to take a flake out of the box instead of trying to get into the box and sit there eating, for when they attempted the latter they were promptly pushed to one side by the experimenter and the box, as well as the food, was removed to a new position.