2) All light, even though it be made up of all the different kinds of vibration, is simple in sensation, just as much as objectively simple light, which is made up of only one kind of waves, as is immediately apparent if we make a subjective comparison of sensations of chromatic light with those of achromatic light. From the first of these facts it follows that light which is physically simple may produce not only chromatic, but also achromatic sensations, for it approaches white when the amplitude of its vibrations increases, and black when the amplitude decreases. The quality of an achromatic sensation does not, therefore, determine unequivocally its source; it may be produced either through a change in the amplitude of objective light-vibrations or through a mixture of simple vibrations of different wave-lengths. In the first case, however, there is always connected with the change in amplitude a change in the grade of brightness, which does not necessarily take place when a mixture is made.
22. Even when the grade of brightness remains constant, this achromatic sensation may have one of several sources. A sensation of pure brightness of a given intensity may result not only from a mixture of all the rates of vibration contained in solar light, as, for example, in ordinary daylight, but it may also result when only two kinds of light-waves, namely those which correspond to sensations sub- [p. 66] jectively the most different, that is, to opposite colors, are mixed in proper proportions. Since opposite colors, when mixed objectively, produce white, they are called complementary colors. As examples of such opposite or complementary colors we may mention spectral red and green-blue, orange and sky-blue, yellow and indigo-blue.
Like achromatic sensations, each of the color-sensations may also, though to a more limited extent, have one of several sources. When two objective colors which lie nearer each other in the color-circle than opposites, are mixed, the mixture appears, not white, but of a color which in the series of objectively simple qualities lies between the two with which we started. The saturation of the resulting color is, indeed, very much diminished when the components of the mixture approach opposite colors; but when they are near each other, the diminution is no longer perceptible, and the mixture and the corresponding simple color are generally subjectively alike. Thus, the orange of the spectrum is absolutely indistinguishable from a mixture of red and yellow rays. In this way,,all the colors in the color-circle between red and green can be obtained by mixing red and green, all between green and violet by mixing green and violet, and, finally, purple, which is not in the solar spectrum, can be produced by mixing red and violet. The whole series of color-tones possible in sensation can, accordingly, be obtained from three objective colors. By means of the same three colors we can also produce white with its intermediate stages.
The mixture of red and violet gives purple, and this is the complementary color of green; and the white secured by mixing these complementary colors, when mixed in different proportions with the various colors, gives the different grades of saturation.
23. The three objective colors that may be used in this way to produce the whole system of light-sensations, are [p. 67] called fundamental colors . In order to indicate their significance, a triangular surface is chosen to represent the system of saturation, rather than the circular surface which is derived from the psychological relations alone. The special significance of the fundamental colors is then expressed by placing then at the angles of the triangle. Along the sides are arranged the color-tones in their maximal saturation, just as on the circumference of the color-circle, while the other grades of saturation in their transitions to white, which lies in the centre, are on the triangular surface. Theoretically, any set of three colors could be chosen as fundamental colors, provided they were suitably distant from one another. Practically, those mentioned, red, green, and violet, are preferable for two reasons. First, by using them we avoid having as one of the three, purple, which can not be produced by objectively simple light. Secondly, at the two ends of the spectrum sensations vary most slowly in proportion to the period of vibration, so that when the extreme colors of the spectrum are used as fundamental colors, the result obtained by mixing two neighboring ones is most like the intermediate, objectively simple color. [ 5 ]
24. These phenomena show that in the system of light sensations a simple relation does not exist between the physical stimuli and the sensations.
This can be understood from what has been said above (3) as to the, character of the physiological stimulation. The visual sense is to be [p.