THE SENSE OF SIGHT: COLOR VISION (Continued)

There are three well-known ways in which colors may be used as stimuli in experiments on animals: by the use of colored papers (reflected light); by the use of a prism (the spectrum which is obtained may be used as directly transmitted or as reflected light); and by the use of light filters (transmitted light). In the experiments on the color vision of the dancer which have thus far been described only the first of these three methods has been employed. Its advantages are that it enables the experimenter to work in a sunlit room, with relatively simple, cheap, and easily manipulated apparatus. Its chief disadvantages are that the brightness of the light can neither be regulated nor measured with ease and accuracy. The use of the second method, which in many respects is the most desirable of the three, is impracticable for experiments which require as large an illuminated region as do those with the mouse; I was therefore limited to the employment of light filters in my further tests of color discrimination.

The form of filter which is most conveniently handled is the colored glass, but unfortunately few glasses which are monochromatic are manufactured. Almost all of our so-called colored glasses transmit the light of two or more regions of the spectrum. After making spectroscopic examinations of all the colored glasses which were available, I decided that only the ruby glass could be satisfactorily used in my experiments. With this it was possible to get a pure red. Each of the other colors was obtained by means of a filter, which consisted of a glass box filled with a chemical solution which transmitted light of a certain wave length.

For the tests with transmitted light the apparatus of Figures 20 and 21 was constructed. It consisted of a reaction-box essentially the same as that used in the brightness vision tests, except that holes were cut in the ends of the electric-boxes, at the positions G and R of Figure 20, to permit the light to enter the boxes. Beyond the reaction-box was a long light-box which was divided lengthwise into two compartments by a partition in the middle. A slit in the cover of each of these compartments carried an incandescent lamp L (Figure 20). Between the two lamps, L, L, and directly over the partition in the light-box was fastened a millimeter scale, S, by means of which the experimenter could determine the position of the lights with reference to the reaction-box. The light- box was separated from the reaction-box by a space 6 cm. wide in which moved a narrow wooden carrier for the filter boxes. This carrier, as shown in Figure 20, could be moved readily from side to side through a distance of 20 cm. The filter boxes, which are represented in place in Figures 20 and 21, consisted of three parallel-sided glass boxes 15 cm. long, 5 cm. wide, and 15 cm. deep. Each box contained a substance which acted as a ray filter. Tightly fitted glass covers prevented the entrance of dust and the evaporation of the solutions in the boxes. Figures 20 and 21 represent the two end boxes, R, R, as red light filters and the middle one, G, as a green light filter. Three filters were used thus side by side in order that the position of a given color with reference to the electric-boxes might be changed readily. As the apparatus was arranged, all the experimenter had to do when he wished to change from green-left, red-right to green-right, red-left was to push the carrier towards the right until the green filter covered the hole on the right at the end of the electric- box. When this had been accomplished the red filter at the left end of the carrier covered the hole on the left at the end of the electric-box. Thus quickly, noiselessly, easily, and without introducing any other change in conditions than that of the interchange of lights, the experimenter was able to shift the positions of his colored lights at will.

[Illustration: FIGURE 20.—Color discrimination apparatus. A, nest-box; B, entrance chamber; R, R, red filters; G, green filter; L, L, incandescent lamps in light-box; S, millimeter scale on light-box; I, door between A and B; O, O, doors between alleys and A.]

[Illustration: FIGURE 21—Ground plan of color discrimination apparatus. E, E, exits from electric-boxes. LB, light-box; R, G, R, filter boxes on carrier; L, left electric-box; R, right electric-box; IC induction apparatus; C, electric cell; K, key; S, millimeter scale.]

In the tests which are now to be reported, three portions of the spectrum were used: the red end, the blue-violet end, and a middle region, chiefly green. The red light was obtained by the use of a filter which was made by placing two plates of ruby glass in one of the glass boxes, filling the box with filtered water and then sealing it to prevent evaporation. The blue-violet was obtained by the use of a filter box which contained a 5 per cent solution of copper ammonium sulphate. The green, which, however, was not monochromatic, was obtained by the use of a filter box which contained a saturated solution of nickel nitrate. These three sets of filters were examined spectroscopically both before the experiments had been made and after their completion.[1] The red filters, of which I had two for shifting the lights, transmitted only red light. The blue-violet filters, two also, at first appeared to transmit only portions of the blue and violet of the spectrum, but my later examination revealed a trace of green. It is important to note, however, that the red and the blue-violet filters were mutually exclusive in the portions of the spectrum which they transmitted. Of all the filters used the green finally proved the least satisfactory. I detected some yellow and blue in addition to green in my first examination, and later I discovered a trace of red. Apparently the transmitting power of the solutions changed slightly during the course of the experiments. On this account certain solutions are undesirable for experiments on color vision, for one must be certain of the constancy of the condition of stimulation. It is to be understood, of course, that each of the three filters transmitted, so far as the eye is concerned, only the color named. I consider the red filter perfectly satisfactory, the blue- violet very good, and the green poor. Henceforth, in testing color vision in animals, I shall make use of colored glasses as filters, if it is in any way possible to obtain or have manufactured blue, green, and yellow glasses which are as satisfactory as the ruby.

[Footnote 1: A Janssen-Hoffman spectroscope was used.]

The apparatus needs no further description, as its other important features were identical with those of the reflected light experiment box. The use of artificial light for the illumination of the electric-boxes made it necessary to conduct all of the following tests in a dark-room. The method of experimentation was practically the same as that already described. A mouse which had been placed in A by the experimenter was permitted to enter B and thence to return to A by entering one of the electric-boxes, the red or blue or green one, as the case might be. Mistakes in choice were punished by an electric shock. One further point in the method demands description and discussion before the results of the tests are considered, namely, the manner of regulating and measuring the brightness of the lights.

Regulating brightness with this apparatus was easy enough; measuring it accurately was extremely difficult. The experimenter was able to control the brightness of each of the two colored lights which he was using by changing the position or the power of the incandescent lamps in the light- box. The position of a lamp could be changed easily between tests simply by moving it along toward or away from the electric-box in the slit which served as a lamp carrier. As the distance from the entrances of the electric-boxes to the further end of the light-box was 120 cm., a considerable range or variation in brightness was possible without change of lamps. Ordinarily it was not necessary to change the power of the lamps, by replacing one of a given candle power by a higher or lower, during a series of tests. Both the candle power of the lamps and their distance from the filters were recorded in the case of each test, but for the convenience of the reader I have reduced these measurements to candle meters[1] and report them thus in the descriptions of the experiments.

[Footnote 1: The illuminating power of a standard candle at a distance of one meter.]

But measuring the actual brightness of the red light or the green light which was used for a particular series of tests, and the variations in their brightnesses, was not so simple a matter as might appear from the statements which have just been made. The influence of the light filters themselves upon the brightness must be taken into account. The two red filters were alike in their influence upon the light which entered them, for they were precisely alike in construction, and the same was true of the two blue-violet filters. The same kind of ruby glass was placed in each of the former, and a portion of the same solution of copper ammonium sulphate was put into each of the filter boxes for the latter. But it is difficult to say what relation the diminution in brightness caused by a red filter bore to that caused by a blue-violet or a green filter. My only means of comparison was my eye, and as subjective measurement was unsatisfactory for the purposes of the experiment, no attempt was made to equalize the amounts of brightness reduction caused by the several filters. So far as the value of the tests themselves, as indications of the condition of color vision in the dancer is concerned, I have no apology for this lack of measurement, but I do regret my inability to give that accurate objective statement of brightness values which would enable another experimenter with ease and certainty to repeat my tests. The nearest approach that it is possible for me to make to such an objective measurement is a statement of the composition and thickness of the filters and of the candle-meter value of the light when it entered the filter. The distance from this point to the entrance to the electric-box was 20 cm.

To sum up and state clearly the method of defining the brightness of the light in the following experiments: the candle-meter value of each light by which an electric-box was illuminated, as determined by the use of a Lummer-Brodhun photometer and measurements of the distance of the source of light from the filter, is given in connection with each of the experiments. This brightness value less the diminution caused by the passage of the light through a filter, which has been defined as to composition and thickness of the layer of solution, gives that degree of brightness by which the electric-box was illuminated.

Tests of the dancer's ability to discriminate green and blue[1] in the transmitted light apparatus were made with four animals. An incandescent lamp marked 16-candle-power was set in each of the light-boxes. These lamps were then so placed that the green and the blue seemed to be of equal brightness to three persons who were asked to compare them carefully. Their candle-meter values in the positions selected were respectively 18 and 64, as appears from the statement of conditions at the top of Table 22.

[Footnote 1: Hereafter the light transmitted by the blue-violet filter will be referred to for convenience as blue.]

TABLE 22

GREEN-BLUE TESTS

Brightnesses Equal for Human Eye

Green 18 candle meters Blue 64 candle meters

SERIES DATE NO. 10 NO. 11
1906 RIGHT WRONG RIGHT WRONG
(GREEN) (BLUE) (GREEN) (BLUE)
A and B[1] April 2 10 10 12 8
1 3 6 4 5 5
2 4 5 5 6 4
3 5 5 5 5 5
4 6 5 5 5 5
5 7 7 3 5 5
6 8 7 3 3 7
7 9 7 3 5 5
8 10 3 7 7 3
9 11 5 5 4 6
10 12 5 5 6 4
[Footnote: A single preference series of twenty tests.]

Numbers 10 and 11 exhibited no preference for either of these colors in the series of 20 tests which preceded the training tests, and neither of them gave evidence of ability to discriminate as the result of ten series of training tests. In this case, again, the behavior of the animals was as strongly against the inference that they can tell green from blue as are the records of choices which appear in the table. Granted, that they are unable to discriminate green from blue when these colors are of about the same brightness for the human eye, what results when they differ markedly in brightness? Table 23 furnishes a definite answer to this question. Numbers 5 and 12 were given eight series of green-blue tests with each light at 18 candle meters. Little, if any, evidence of discrimination appeared. Then, on the supposition that the difference was not great enough for easy discrimination, the blue light was reduced almost to 0, the green being left at 18. The tests (series 9) immediately indicated discrimination. For series 10 the green was made 64 candle meters, the blue 18, and again there was discrimination. These results were so conclusively indicative of the lack of color vision and the presence of brightness vision, that there appeared to be no need of continuing the experiment further.

Accepting provisionally the conclusion that the dancers cannot tell green from blue except by brightness differences, we may proceed to inquire whether they can discriminate other colors. Are green and red distinguishable?

Green-red discrimination now was tested by a method which it was hoped might from the first prevent dependence upon brightness. The light in the light-box on the left was so placed that it had a value of 18 candle meters, that in the light-box on the right so that it had a value of 1800 candle meters. Neither light was moved during the first four series of the green-red tests which were given to Nos. 151 and 152.

TABLE 23

GREEN-BLUE TESTS

Brightnesses Different for Human Eye

Green 18 candle meters Blue 18 candle meters

No. 5 No. 12
DATE
SERIES 1906 RIGHT WRONG RIGHT WRONG
(GREEN) (BLUE) (GREEN) (BLUE)

1 April 10 6 4 5 5 2 11 5 5 7 3 3 12 6 4 7 3 4 13 4 6 7 3 5 14 7 3 5 5 6 15 4 6 6 4 7 16 6 4 8 2 8 17 5 5 4 6

As it was now evident that the intensity difference was not sufficient to render discrimination easy, the blue was reduced to 0 and the green left at 18.

9 17 7 3 8 2

Now the brightnesses were made, green 64, blue 18, just the reverse of those of series of Table 22.

10 17 8 2 8 2

Each of these series consisted of 20 tests instead of 10. As a result of the arrangement of the lights just mentioned, the green appeared to me very much brighter than the red when it was on the right and very much darker when it was on the left. If this were true for the mouse also, it is difficult to see how it could successfully depend upon brightness for guidance in its choices. Such dependence would cause it to choose now the green, now the red.

The first four series of green-red tests so clearly demonstrated discrimination, of some sort, that it was at once necessary to alter the conditions of the experiment. The only criticism of the above method of excluding brightness discrimination, of which I could think, was that the red at no time had been brighter than the green. In other words, that despite a value of 1800 candle meters for the red and only 18 candle meters for the green, the latter still appeared the brighter to the mouse. To meet this objection, I made the extreme brightness values 1 and 1800 candle meters in some of the later series, of which the results appear in Table 24. From day to day different degrees of brightness were used, as is indicated in the second column of the table. Instead of having first one color and then the other the brighter, after the fourth series I changed the position of the lights each time the position of the filters was changed; hence, the table states a certain brightness value for each color instead of for each electric-box.

Series 5 to 14 so clearly indicated discrimination, that it seemed necessary to devise some other means than that of changing the brightnesses of the colored lights themselves to test the assumption that the animals were choosing the brighter light. I therefore removed the light filters so that the colors which had been present as conditions of discrimination were lacking, and arranged the apparatus so that first one box, then the other, was illuminated the more brightly. The purpose of this was to discover whether as the result of their green-red training the mice had acquired the habit of choosing uniformly either the lighter or the darker box. One series was given under the conditions of illumination specified in Table 24 with the result that the brighter box was chosen eight times in ten by No. 151 and every time by No. 152. Since neither of these individuals had previously been trained by white-black tests to go to the white, and since, furthermore, the dancers usually manifest a slight preference for the lower instead of the higher illumination, this result may be interpreted as indicative of dependence upon brightness in the previous color tests. It looks very much indeed as if the green had been chosen, not because of its greenness, but on account of its relatively greater brightness.

This test of brightness preference was followed by two series, 16 and 17, under conditions similar to those of the first four series of the table. For series 16 the value of the light in the left box was 1 candle meter, that of the light in the right box 1800 candle meters. Discrimination was perfect. For series 17 the value for the left remained at 1 candle meter, but that of the right box was decreased to 0. In this series No. 152 was entirely at a loss to know which box to choose. Of course this was an entirely new set of conditions for choice, namely, a colored box, the green or the red as the case might be, beside a dark box, the one which was not illuminated. If the mice really had been choosing correctly because of a habit of avoiding the red or of seeking the green, this method should bring out the fact, for the red box, since with it the disagreeable electric shock had always been associated, should be a box to be avoided. For No. 151 this seemed to be the case.

Series 23 to 27 of Table 24 were given as final and crucial tests of the relation of brightness discrimination to color discrimination. As it is not possible to express in a simple formula the conditions of the tests, a sample series which indicates the brightness of the colors in each of the twenty tests of a series, and in addition the results given by No. 151 in the first of these final series, is reproduced in Table 25. For an animal which had presumably learned perfectly to choose green in preference to red, the record of 8 mistakes in 20 choices as a result of changes in relative brightness is rather bad, and it renders doubtful the existence of color discrimination in any of these experiments. No. 152 showed no ability whatever to choose the green in the first of the series (series 23 of Table 24) of which that of Table 25 is a sample. His record, 10 mistakes in 20 choices, was even poorer than that of No. 151. That both of these mice learned to choose fairly accurately in these final tests is shown by the results of series 24, 25, 26, and 27. I must admit, however, that these records indicate little ability on the part of the animals to discriminate colors.

TABLE 24

GREEN-RED TESTS

Brightnesses Extremely Different for Human Eye
Intensities are given in candle meters (c.m.)

NO. 151 NO. 152 SERIES DATE CONDITIONS RIGHT WRONG RIGHT WRONG (GREEN) (RED) (GREEN) (RED)

1 April 26 18 c.m. on left
1800 c.m. on right 11 9 7 13
2 27 Same 16 4 16 4
3 28 Same 20 0 17 3
4 29 Same 19 1 19 1
5 30 Green 18 c.m.
Red 18 c.m. 9 1 10 0
6 30 Green 64 c.m.
Red 18 c.m. 9 1 8 2
7 May 1 Green 6 c.m.
Red 1500 c.m. 7 3 9 1
8 1 Green 4 c.m.
Red 1500 c.m. 8 2 7 3
9 2 Both varied from
4 to 1500 c.m. 18 2 18 2
10 3 Green 2 c.m.
Red 1800 c.m. 6 4 7 3
11 3 Same 10 0 10 0
12 4 Same 7 3 8 2
13 4 Same 8 2 6 4
14 5 Green 1 c.m.
Red 1800 c.m. 19 1 19 1

Filters were now removed. An illumination of 15 c.m. was established on one side and an illumination of 0 on the other side, in order to ascertain whether the mice would choose the brighter box. This was done to test the assumption that the green in the previous tests had always appeared brighter to the mice than did the red, and that in consequence they had chosen the brighter box instead of the green box.

TABLE 24—CONTINUED

No. 151 No. 152

SERIES DATE CONDITIONS RIGHT WRONG RIGHT WRONG (GREEN) (RED) (GREEN) (RED)

15 May 5 Brighter 15 c.m. 8[1] 2[2] 10[1] 0[2]
Darker 0 c.m.
16 5 1 c.m. on left
1800 c.m. on right 10 0 10 0
17 5 1 c.m. on left
0 c.m. on right 9 1 4 6
18 5 Green 18 c.m.
Red 18 c.m. 19 1 17 3
19 9 Same 9 1 9 1
20 9 Same 10 0 10 0
21 10 Same 10 0 10 0
22 11 Same 10 0 10 0
23 June 1 Both varied from
1 to 1800 c.m. 12 8 10 10
24 2 Same 18 2 14 6
25 June 3 Both varied from
2 to 1800 c.m. 19 1 17 3
26 4 Same 17 3 17 3
27 5 Same 18 2 18 2

[Footnote 1: Brighter.]
[Footnote 2: Darker.]

These long-continued and varied tests with Nos. 151 and 152 revealed three facts: that the mice depend chiefly upon brightness differences in visual discrimination; that they probably have something which corresponds to our red-green vision, although their color experience may be totally unlike ours; and that the red end of the spectrum seems much darker to them than to us, or, in other words, that the least refrangible rays are of lower stimulating value for them than for us.

TABLE 25

GREEN-RED TESTS

June 1, 1906 No. 151

BRIGHTNESS VALUE IN CANDLE RIGHT WRONG
TEST POSITION METERS (GREEN) (RED)
1 Green on left Green 4, Red 448 Right —
2 Green on right Green 448, Red 4 Right —
3 Green on right Green 4, Red 448 Right —
4 Green on left Green 448, Red 4 Right —
5 Green on left Green 3, Red 1800 — Wrong
6 Green on right Green 1800, Red 3 — Wrong
7 Green on right Green 3, Red 1800 — Wrong
8 Green on left Green 1800, Red 3 Right —
9 Green on right Green 5, Red 34 Right —
10 Green on left Green 34, Red 5 Right —
11 Green on right Green 6, Red 74 Right —
12 Green on left Green 74, Red 6 Right —
13 Green on left Green 4, Red 448 — Wrong
14 Green on right Green 448, Red 4 Right —
15 Green on right Green 4, Red 448 — Wrong
16 Green on left Green 448, Red 4 Right —
17 Green on right Green 3, Red 1800 — Wrong
18 Green on left Green 1800, Red 3 — Wrong
19 Green on right Green 1800, Red 3 — Wrong
20 Green on left Green 3, Red 1800 Right —

Totals 12 8

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 assumption 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 constitute 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 glasses 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 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)

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 assumption 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 repetition 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

No. 151 No. 152
SERIES DATE
RIGHT WRONG RIGHT WRONG

1 Sept. 29 6 4 4 6 2 30 5 5 6 6 3 Oct. 1 3 7 4 6 4 2 5 5 3 7 5 3 3 7 5 5 6 4 6 4 5 5 7 5 5 5 5 5 8 6 — — 3 7 9 7 — — 6 4 10 8 — — 4 6

Averages 4.7 5.3 4.5 5.5

The account of my color vision experiments is finished. If it be objected that other than visual conditions may account for whatever measure of discriminating ability, apart from brightness discrimination, appears in some of the series, the results of the series of Table 29, in which all conceivable visual means of discrimination were purposely excluded, and those of the several check tests which have been described from time to time in the foregoing account, should furnish a satisfactory and definite answer. I am satisfied that whatever discrimination occurred was due to vision; whether we are justified in calling it color vision is quite another question.

I conclude from my experimental study of vision that although the dancer does not possess a color sense like ours, it probably discriminates the colors of the red end of the spectrum from those of other regions by difference in the stimulating value of light of different wave lengths, that such specific stimulating value is radically different in nature from the value of different wave lengths for the human eye, and that the red of the spectrum has a very low stimulating value for the dancer. In the light of these experiments we may safely conclude that many, if not most, of the tests of color vision in animals which have been made heretofore by other investigators have failed to touch the real problem because the possibility of brightness discrimination was not excluded.

Under the direction of Professor G. H. Parker, Doctor Karl Waugh has examined the structure of the retina of the dancing mouse for me, with the result that only a single type of retinal element was discovered. Apparently the animals possess rod-like cells, but nothing closely similar to the cones of the typical mammalian retina. This is of peculiar interest and importance in connection with the results which I have reported in the foregoing pages, because the rods are supposed to have to do with brightness or luminosity vision and the cones with color vision. In fact, it is usually supposed that the absence of cones in the mammalian retina indicates the lack of color vision. That this inference of functional facts from structural conditions is correct I am by no means certain, but at any rate all of the experiments which I have made to determine the visual ability of the dancer go to show that color vision, if it exists at all, is extremely poor. It is gratifying indeed to learn, after such a study of behavior as has just been described, that the structural conditions, so far as we are able to judge at present, justify the conclusions which have been drawn.