1 Light blue on right Orange on left Right ____ 2 Light blue on left Orange on right Right ____ 3 Light blue on right Red substituted for orange Right ____ 4 Light blur on left Red substituted for orange Right ____ 5 Dark blue on right Orange on left Right ____ 6 Dark blue on left Orange on right Right ____ 7 Dark blue on left Orange on right Right ____ 8 Dark blue on right Red substituted for orange Right ____ 9 Dark blue on left Red substituted for orange Right ____ 10 Dark blue on left Red substituted for orange Right ____ Totals 10 0 The nature of this brightness check series, as well as the results which No. 2 gave when tested by it, may be appreciated readily by reference to Table 18. Tint No. 1 of the blue, which is considerably brighter, in my judgment, than the Bradley blue, was replaced at intervals in this series by the latter. For it was thought that in case the mouse were choosing the blue of the series because it seemed brighter than the orange, this substitution might mislead it into choosing the orange. These blues are referred to in the table as light blue (tint No. 1) and dark blue (standard blue). Again a change in the opposite direction was made by substituting Bradley red for orange. As this was for the human eye the substitution of a color whose brightness was considerably less than that of the orange, it seemed possible that the mouse, if it had formed the habit of choosing the box which seemed the darker, might by this change be misled into choosing the red instead of the light blue. In a word, changes in the conditions of the experiments were made in such a way that now one color, now the other, appeared to be the brighter. But I did not attempt to exclude brightness discrimination on the part of the mouse by dependence upon the human judgment of brightness equality, for it is manifestly unsafe to assume that two colors which are of the same brightness for the human eye have a like relation for the eye of the dancer or of any other animal. My tests of color vision have been conducted without other reference to human standards of judgment or comparisons than was necessary for the description of the experimental conditions. In planning the experiments I assumed neither likeness nor difference between the human retinal processes and those of the dancer. It was my purpose to discover the nature of the mouse's visual discriminative ability.