14 17 3 7 6 4

15 19 6 4 6 4

16 20 7 3 5 5

17 21 4 6 8 2

18 22 3 7 4 6

19 23 6 4 4 6

20 24 6 4 5 5

Twenty series, 200 tests for each of the individuals in the experiment, yielded no evidence whatever of the dancer's ability to tell green from blue. As it has already been proved that they readily learn to choose the right box under discriminable conditions, it seems reasonable to conclude either that they lack green-blue vision, or that they have it in a relatively undeveloped state.

If it be objected that the number of training tests given was too small, and that the dancer probably would exhibit discrimination if it were given 1000 instead of 200 tests in such an experiment, I must reply that the behavior of the animal in the tests is even more satisfactory evidence of its inability to choose than are the results of Table 20. Had there been the least indication of improvement as the result of 200 tests, I should have continued the experiment; as a matter of fact, the mice each day hesitated more and more before choosing, and fought against being driven toward the entrance to the experiment box. That they were helpless was so evident that it would have been manifestly cruel to continue the experiment.

TABLE 21 VIOLET-RED TESTS With Odor of All Cardboards the Same

SERIES DATE NO. 7 NO. 998 RIGHT WRONG RIGHT WRONG (VIOLET) (RED) (VIOLET) (RED) A MAR. 7 8 2 5 5 B 7 3 7 2 8 1 14 3 7 6 4 2 15 4 6 4 6 3 16 5 5 5 5 4 19 4 6 4 6 5 20 5 5 6 4 6 21 4 6 8 2 7 22 8 2 4 6 8 23 4 6 6 4 9 24 6 4 4 6 10 25 4 6 6 4

Further color tests with reflected light were made with violet and red.

Two dancers, Nos. 998 and 7, neither of which had been in any experiment previously, were subjected to the ten series of tests whose results are to be found in Table 21. In this experiment the cardboards used had been coated with sh.e.l.lac to obviate discrimination by means of odor. It is therefore impossible to give a precise description of the color or brightness by referring to the Bradley papers.[1] Both the violet and the red were rendered darker, and apparently less saturated, by the coating.

[Footnote 1: The violet was darker than Bradley's shade No. 2, and the red was lighter than Bradley's red.]

These violet-red tests were preceded by two series of preference tests (_A_ and _B_), in which no shock was given and escape was possible through either electric-box. Although the results of these preference tests as they appear in Table 21 seem to indicate a preference for the red on the part of No. 998, examination of the record sheets reveals the fact that neither animal exhibited color preference, but that instead both chose by position. Number 998 chose the box on the right 15 times in 20, and No. 7 chose the box on the left 15 times in 20.

Ten series of tests with the violet-red cardboards failed to furnish the least indication of discrimination. The experiment was discontinued because the mice had ceased to try to discriminate and dashed into one or the other of the boxes on the chance of guessing correctly. When wrong they whirled about, rushed out of the red box and into the violet immediately. They had learned perfectly as much as they were able to learn of what the experiment required of them. Although we are not justified in concluding from this experiment that dancers cannot be taught to distinguish violet from red, there certainly is good ground for the statement that they do not readily discriminate between these colors.

The experiments on color vision which have been described and the records which have been presented will suffice to give the reader an accurate knowledge of the nature of the results, only a few of which could be printed, and of the methods by which they were obtained.

In brief, these results show that the dancer, under the conditions of the experiments, is not able to tell green from blue, or violet from red. The evidence of discrimination furnished by the light blue-orange tests is not satisfactory because the conditions of the experiment did not permit the use of a sufficiently wide range of brightnesses. It is obvious, therefore, that a method of experimentation should be devised in which the experimenter can more fully control the brightness of the colors which he is using. I shall now describe a method in which this was possible.

CHAPTER X

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 gla.s.s, but unfortunately few gla.s.ses which are monochromatic are manufactured. Almost all of our so-called colored gla.s.ses transmit the light of two or more regions of the spectrum. After making spectroscopic examinations of all the colored gla.s.ses which were available, I decided that only the ruby gla.s.s 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 gla.s.s 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 part.i.tion 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 part.i.tion 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 s.p.a.ce 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 gla.s.s boxes 15 cm. long, 5 cm.

wide, and 15 cm. deep. Each box contained a substance which acted as a ray filter. Tightly fitted gla.s.s 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.

[Ill.u.s.tration: 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_.]

[Ill.u.s.tration: 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 gla.s.s in one of the gla.s.s 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 gla.s.ses as filters, if it is in any way possible to obtain or have manufactured blue, green, and yellow gla.s.ses 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 gla.s.s 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 pa.s.sage 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.