20a. From the early origin of the names for the six qualities mentioned, the conclusion has been drawn that they are fundamental qualities of vision, and that the others are compounded from them. Grey is declared to be a mixture of black and white, violet and purple-red to be mixtures of blue and red, etc. Psychologically there is no justification for calling any light-sensations compound in comparison with others. Grey is a simple sensation just as much as white or black; such colors as orange and purple-red are just as much simple colors as red and yellow; and any grade of saturation which we have placed in the system between a pure color and white, is by no means, for that reason, a compound sensation. The closed, continuous character of the system makes it necessary for language to pick out certain especially marked differences in reference to which all other sensations are then arranged, for the simple reason that it is impossible to have an unlimited number of names. It is most natural that white and black should be chosen as such points of reference for the achromatic series, since they designate the greatest differences. When once these two are given, however, all other achromatic sensations will be considered as transitional [p. 63] sensations between them, since the extreme differences are connected by a series of all possible grades of brightness. The case of color-sensations is similar; only here, on account of the circular form of the color-line, it is impossible to choose directly two absolutely greatest differences.
Other motives besides the necessary qualitative difference, are decisive in the choice of the principal colors. We may regard as such motives, the frequency and affective intensity of certain light-impressions due to the natural conditions of human existence. The red color of blood, the green of vegetation, the blue of the sky, and the yellow of the heavenly bodies in contrast with the blue of the sky, and the yellow heavenly bodies may well have furnished the earliest occasions for the choice of certain colors as those to receive names. Language generally names the sensation from the object that produced it, not the object from the sensation. In this case too, when certain principal qualities were once determined, all others must, on account of the continuity of the series of sensations, seem to be intermediate color-tones. The difference between principal colors and transitional colors is, therefore, very probably due entirely to external conditions. If these conditions had been other, red might have been regarded as a transitional color between purple and orange, just as orange is now placed between red and yellows [ 3 ]
21. The attributes of the system of light-sensations above described, are so peculiar as to lead us to expect a priori that the relation between these psychological attributes and the objective processes of stimulation, is essentially different from that in the cases of the sensational systems discussed before, especially those of the general and auditory senses.
Most [p. 64] striking, in this respect, is the difference between the system in question and that of tones. In the latter case, the principle of parallelism between sensation and stimulus (p. 45), holds not only for the physiological processes of. stimulation, but to a great extent for the physical processes as well. A simple sensation corresponds to a simple form of sound-vibration, and a plurality of simple sensations to compound form. Furthermore, the intensity of the sensation varies in proportion to the amplitude of the vibrations, and its quality with their form, so that in both directions the subjective difference between sensations increases with the growing difference between the objective physical stimuli. The relation in the case of light-sensations is entirely different. Like objective sound, objective light also consists of vibrations in some medium. To be sure, the actual form of these vibrations is still a question, but from physical experiments on the phenomena of interference we know that the consist of very short and rapid waves. Those seen as light vary in wave-length from 688 to 393 millionths of a millimetre, and in rate from 450 to 790 billion vibrations per second. In this case, too, simple sensations correspond to simple vibrations, that is, vibrations of like wave-length; and the quality of the sensation varies continuously with the rate: red corresponds to the longest and slowest wives, and violet to the shortest and most rapid, while the other color-tones form a continuous series between these, varying with the changes in wave-length.
Even here, however, an essential difference appears, for the colors red and violet, which are the most different in wave-length, are more similar in sensation than those which lie between [ 4 ]
There are also other differences. 1) Every change in the amplitude of the physical vibrations corresponds to a subjective change in both intensity and quality, as we noted above in the discussion of sensations of brightness.