Practically the same kind of color tests as those described for Nos. 2 and 3 were given to Nos. 1000 and 5. The results appear in Table 19. These tests followed upon the formation of a habit to choose white instead of black (that is, the greater brightness). From the first both No. 1000 and No. 5 chose the light blue in preference to the orange or the red. It therefore seems probable that the former was considerably brighter than the latter. Number 1000, to be sure, was led into three erroneous choices by the brightness check series (series 7), but, on the other hand, No. 5 was not at all disturbed in her choices by similar check tests. It seems natural to conclude from these facts that both of these mice chose the blue at first because of its relatively greater brightness, and that they continued to do so for the same reason. In other words, their behavior indicates that the brightness check tests were valueless because not enough allowance had been made for the possible differences between the vision of mouse and man. TABLE 19 LIGHT BLUE-ORANGE AND DARK BLUE-RED TESTS No. 1000 No. 5 SERIES DATE Condition Right Wrong Right Wrong (Light (Orange (Light (Orange Blue or or Blue or Dark Red) Dark Red) Blue) Blue) 1 Jan. 25 Blue-red 8 2 10 0 2 26 Blue-red or Light blue-orange 10 0 10 0 3 27 Light blue-orange 10 0 5 5 4 29 Light blue-orange 9 1 8 2 5 30 Light blue-orange 10 0 8 2 6 31 Light blue-orange 10 0 10 0 7 Feb. 1 Light blue-orange or Dark blue-red 7 3 10 0 If only the final results of my experiments with the dancer and the conclusions to which they lead were of interest, all of this description of experiments which served merely to clear the ground and thus make possible crucial tests might be omitted. It has seemed to me, however, that the history of the investigation is valuable, and I am therefore presenting the evolution of my methods step by step. To be sure, not every detail of this process can be mentioned, and only a few of the individual results can be stated, but my purpose will have been fulfilled if I succeed in showing how one method of experimentation pointed the way to another, and how one set of results made possible the interpretation of others.