21 21 Blue 21 c.m.
Green 18 c.m. 7 3
22 23 Same 8 2
To begin with, the blue and the green were made quite bright for the human subject, blue 74 candle meters, green 36. Later the brightness of both was first decreased, then increased, in order to ascertain whether discrimination was conditioned by the absolute strength of illumination. No evidence of discrimination was obtained with any of the several conditions of illumination in seventeen series of ten tests each.
On the supposition that the animals were blinded by the brightness of the light which had been used in some of the tests, similar tests were made with weaker light. The results were the same. I am therefore convinced that the animals did justice to their visual ability in these experiments.
Finally, it seemed possible that looking directly at the source of light might be an unfavorable condition for color discrimination, and that a chamber flooded with colored light from above and from one end would prove more satisfactory. To test this conjecture two thicknesses of blue glass were placed over one electric-box, two plates of green glass over the other; the incandescent lamps were then fixed in such positions that the blue and the green within the two boxes appeared to the experimenter, as he viewed them from the position at which the mouse made its choice, of the same brightness.
Mouse No. 2 was given two series of tests, series 18 and 19, under these conditions, with the result that he showed absolutely no ability to tell the blue box from the green box. The opportunity was now taken to determine how quickly No. 2 would avail himself of any possibility of discriminating by means of brightness. With the blue at 21 candle meters, the green was increased to about 1800. Immediately discrimination appeared, and in the second series (22 of Table 27) there were only two mistakes.
The results of the blue-green experiments with light transmitted from in front of the animal and from above it are in entire agreement with those of the experiments in which reflected light was used. Since the range of intensities of illumination was sufficiently great to exclude the possibility of blinding and of under illumination, it is necessary to conclude that the dancer does not possess blue-green vision.
Again I must call attention to the fact that the behavior of the mice in these experiments is even more significant of their lack of discriminating ability than are the numerical results of the tables. After almost every series of tests, whether or not it came out numerically in favor of discrimination, I was forced to add the comment, "No satisfactory evidence of discrimination."
We have now examined the results of green-red, green-blue, and blue-green tests. One other important combination of the colors which were used in these experiments is possible, namely, blue-red. This is the most important of all the combinations in view of the results already described, for these colors represent the extremes of the visible spectrum, and might therefore be discriminable, even though those which are nearer together in the spectral series were not.
TABLE 28 BLUE-RED TESTS
No. 2 No. 205
SERIES DATE BRIGHTNESS VALUES
RIGHT WRONG RIGHT WRONG
(BLUE) (RED) (BLUE) (RED)