So many of the results of my color experiments have indicated the all- important role of brightness vision that I have hesitated to interpret any of them as indicative of true color discrimination. But after I had made all the variations in brightness by which it seemed reasonable to suppose that the mouse would be influenced under ordinary conditions, and after I had introduced all the check tests which seemed worth while, there still remained so large a proportion of correct choices that I was forced to admit the influence of the quality as well as of the intensity of the visual stimulus.

The first of the facts mentioned above, that brightness discrimination is more important in the life of the mouse than color discrimination, is attested by almost all of the experiments whose results have been reported. The second fact, namely, that the dancer possesses something which for the present we may call red-green vision, also has been proved in a fairly satisfactory manner by both the reflected and the transmitted light experiments. I wish now to present, in Table 26, results which strikingly prove the truth of the statement that red appears darker to the dancer than to us.

The brightness conditions which appeared to make the discrimination between green and red most difficult were, so far as my experiments permit the measurement thereof, green from 1 to 4 candle meters with red from 1200 to 1600. Under these conditions the red appeared extremely bright, the green very dark, to the human subject.

According to the description of conditions in Table 26, Nos. 2 and 5 were required to distinguish green from red with the former about 3 candle meters in brightness and the latter about 1800 candle meters. In the eighth series of 20 tests, each of these animals made a perfect record. As it seemed possible that they had learned to go to the darker of the two boxes instead of to the green box, I arranged the following check test.

The filters were removed, the illumination of one electric-box was made 74 candle meters, that of the other 3, and the changes of the lighter box from left to right were made at irregular intervals. In February, No. 2 had been trained to go to the black in black-white tests, and at the same time No. 5 had been trained to go to the white in white-black tests. The results of these brightness check tests, as they appear in the table, series 8 _a_, are indeed striking. Number 2 chose the darker box each time; No. 5 chose it eight times out of ten. Were it not for the fact that memory tests four weeks after his black-white training had proved that No.

2 had entirely lost the influence of his previous experience (he chose white nine times out of ten in the memory series), it might reasonably be urged that this individual chose the darker box because of his experience in the black-white experiment. And what can be said in explanation of the choices of No. 5? I can think of no more reasonable way of accounting for this most unexpected result of the brightness tests than the a.s.sumption that both of these animals had learned to discriminate by brightness difference instead of by color.

TABLE 26

GREEN-RED TESTS

Brightnesses Different for Human Eye

No. 2 No. 5

SERIES DATE BRIGHTNESS RIGHT WRONG RIGHT WRONG VALUES (GREEN) (RED) (GREEN) (RED)

1 May 7 Green Red 1800 c.m. 10 10 12 8 2 8 Same 12 8 11 9 3 9 Same 15 5 14 6 4 10 Same 18 2 12 8 5 11 Same 18 2 14 6 6 12 Same 19 1 16 4 7 13 Same 19 1 18 2 8 14 Same 20 0 20 0

Brightness tests without colors were now given to determine whether the mice had been choosing the brighter or the darker instead of the green.

TABLE 26--CONTINUED

NO. 2 NO. 5

SERIES DATE BRIGHTNESS VALUES RIGHT WRONG RIGHT WRONG (GREEN) (RED) (GREEN) (RED)

8a 14 Brighter 74 c.m. 0[1] 10[2] 2[1] 8[2]

Darker 3 c.m.

9 15 3 c.m. on left 1800 c.m. on right 8 12 16 4 10 16 4 c.m. on left 36 c.m. on right 5 5 7 3 11 16 Green 4 c.m.

Red 36 c.m. 9 1 8 2 12 17 11 c.m. on left 1800 c.m. on right 7 3 6 4 13 17 Green 11 c.m.

Red 1800 c.m. 9 1 8 2 14 18 Mixed values 3 to 1800 c.m. 7 3 8 2 15 19 Same 7 3 7 3 16 20 Same 7 3 7 3 17 21 Same 7 3 9 1 18 22 Same 9 1 8 2 19 23 Same 7 3 9 1 20 24 Same 10 0 8 2 21 25 Same 10 0 9 1 22 26 Same 9 1 10 0

[Footnote 1: Brighter]

[Footnote 2: Darker]

Immediately after the brightness series, the influence of making first one color, then the other, the brighter was studied. Throughout series 9 the brightness value of the left box remained 3 candle meters, that of the right side 1800 candle meters. Number 2 was so badly confused by this change that his mistakes in this series numbered 12; No. 5 made only 4 incorrect choices. Then series after series was given under widely differing conditions of illumination. The expression "mixed values," which occurs in Table 26 in connection with series 14 to 22 inclusive, means that the brightnesses of the green and the red boxes were changed from test to test in much the way indicated by the sample series of Table 25.

In view of the results of these 22 series, 320 tests for each of two mice, it is evident that the dancer is able to discriminate visually by some other factor than brightness. What this factor is I am not prepared to say. It may be something akin to our color experience, it may be distance effect. No other possibilities occur to me.

Table 26 shows that discrimination was relatively easy for Nos. 2 and 5 with green at 3 candle meters and red at 1800. That their discrimination was made on the basis of the greater brightness of the red, instead of on the basis of color, is indicated by the results of the brightness check series 8a. Increase in the brightness of the green rendered discrimination difficult for a time, but it soon improved, and by no changes in the relative brightness of the two colors was it possible to prevent correct choice.

In addition to giving point to the statement that red appears darker to the dancer than to us, the above experiment shows that the animals depend upon brightness when they can, and that their ability to discriminate color differences is extremely poor, so poor indeed that it is doubtful whether their records are better than those of a totally color blind person would be under similar conditions. Surely in view of such results it is unsafe to claim that the dancer possesses color vision similar to ours.

Perfectly trained as they were, by their prolonged green-red tests, to choose the green, or what in mouse experience corresponds to our green, Nos. 2 and 5 offered an excellent opportunity for further tests of blue- green discrimination. For in view of their previous training there should be no question of preference for the blue or of a tendency to depend upon brightness in the series whose results const.i.tute Table 27.

TABLE 27 BLUE-GREEN TESTS

NO. 2 NO. 5

SERIES DATE BRIGHTNESS VALUES RIGHT WRONG RIGHT WRONG (BLUE) (GREEN) (BLUE) (GREEN)

1 June 1 Blue 74 c.m.

Green 36 c.m. 3 7 3 7 2 2 Same 5 5 4 6 3 3 Same 5 5 6 4 4 4 Same 6 4 3 7 5 5 Same 6 4 5 5 6 6 Blue 21 c.m.

Green 21 c.m. 6 4 7 3 7 7 Same 2 8 3 7 8 8 Same 5 5 4 6 9 9 Same 3 7 6 4 10 10 Same 2 8 4 6 11 12 Same 6 4 3 7 12 13 Blue 36 c.m.

Green 21 c.m. 3 7 4 6 13 14 Same 5 5 14 15 Blue 62 c.m.

Green 21 c.m. 4 6 15 16 Same 5 5 16 17 Same 5 5 17 18 Same 6 4

Now, as a final test, blue and green gla.s.ses were placed over the electric-boxes, the brightness of the two was equalized for the human eye, and the tests of series 18 and 19 were given to No. 2:--

TABLE 27--CONTINUED

NO. 2 SERIES DATE BRIGHTNESS VALUES RIGHT WRONG (Blue) (Green)

18 18 Blue 62 c.m.

Green 21 c.m 4 6 19 19 Same 6 4 20 20 Blue 21 c.m.

Green 88 c.m. 2 8

The green was now made much the brighter.

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 gla.s.s were placed over one electric-box, two plates of green gla.s.s 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.