Part 15 (1/2)

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)

1 July 31 1800 c.m. on left 24 c.m. on right 5 5 6 4 2 Aug. 1 21 c.m. on left 1800 c.m. on right 6 4 6 4 3 2 1800 c.m. on left 21 c.m. on right 8 2 6 4 4 3 19 c.m. on left 1800 c.m. on right 9 1 6 4 5 4 1800 c.m. on left 7 c.m. on right 7 3 5 5 6 5 6 c.m. on left 1800 c.m. on right 10 0 7 3 7 6 18 c.m. on left 74 c.m. on right 10 0 9 1 8 7 1800 c.m. on left 7 c.m. on right 8 2 8 2 9 8 7 c.m. on left 1800 c.m. on right 7 3 8 2 10 9 Mixed values 6 to 1800 c.m. 8 2 9 1 11 10 Blue 3 c.m.

Red 1800 c.m. 7 3 6 4

Brightness tests were now made, without the use of colors.

11a 10 4 6 5 5

12 10 Blue 3 c.m.

Red 8 c.m. 4 6 6 4 13 11 Blue 3 c.m.

Red 7200 c.m. 8 2 5 5 14 13 Mixed values 3 to 7200 c.m. 7 3 7 3 15 13 Same 7 3 9 1 16 14 Blue 3 to 6 c.m.

Red 112 to 3650 c.m. 10 0 10 0

Series were now given to test the a.s.sumption that red appears dark to the dancer.

17 14 Darkness on one side Red 3 c.m. 5 5 7 3 18 14 Blue 3 to 3650 c.m.

Red 3 to 3650 c.m. 10 0 10 0 19 15 Darkness on one side Red 3 c.m. 5 5 4 6 20 15 Blue 3 to 3650 c.m.

Red 3 to 3650 c.m. 10 0 9 1 21 16 Darkess on one side Red 72 c.m. 5 5 7 3 22 16 Darkness on one side Red 1800 c.m. 6 4 10 0

As is shown by the results in Table 28, no combination of brightnesses rendered correct choice impossible in the case of the blue-red tests which are now to be described. Choice was extremely difficult at times, even more so perhaps than the table would lead one to suppose, and it is quite possible that color played no part in the discrimination. But that brightness difference in the colors was not responsible for whatever success these mice attained in selecting the right box is proved by the brightness-without-color series which follows series II of the table.

Neither No. 2 nor No. 205 showed preference for the lighter or the darker box. At the end of the sixteenth blue-red series, I was convinced that one of two conclusions must be drawn from the experiment: either the dancers possess a kind of blue-red vision, or red is of such a value for them that no brightness of visible green or blue precisely matches it.

The latter possibility was further tested by an experiment whose results appear in series 17 to 22 inclusive, of Table 28. The conditions of series 17 were a brightness value of 0 in one box (darkness) and in the other red of a brightness of 3 candle meters. Despite the fact that they had been perfectly trained in _blue-red tests_ to avoid the red, neither of the mice seemed able to discriminate the red from the darkness and to avoid it. This was followed by a series in which the brightness of both the blue and the red was varied between 3 and 3650 candle meters, with the striking result that neither mouse made any mistakes. In series 19 red was used with darkness as in series 17, and again there was a total lack of discrimination. Series 20 was a repet.i.tion of series 18, with practically the same result. I then attempted to find out, by increasing the brightness of the red, how great must be its value in order that the dancers should distinguish it readily from darkness. For the tests of series 21 it was made 72 candle meters, but discrimination did not clearly appear. At 1800 candle meters, as is shown in series 22, the red was sufficiently different in appearance from total darkness to enable No. 205 to discriminate perfectly between the two electric-boxes. For No. 2 discrimination was more difficult, but there was no doubt about his ability. It would appear from these tests that the dancers had not learned to avoid red. Therefore we are still confronted with the question, can they see colors?

TABLE 29

VISUAL CHECK TESTS With the Electric-boxes Precisely Alike Visually