[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.