Part 12 (1/2)
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.