The Galaxy 3, 1871
What is color? This is a question that may seem very simple, but is not so easy to answer. The question raises another question, where is color, or what kind of phenomena does it involve? The common notion is that is something external to us, or objective, and is to be investigated by analyzing the rays of light or studying the principals of pigments. Yet this is not the true view. Dr Thomas Young, in the beginning of the century, developed the doctrine that color is a sensation; its phenomena are therefore subjective and psychological, so that the "science of color must therefore be regarded as a mental science." In a late lecture before the Royal Institution of Great Britain, Mr. G. Clark Maxwell observes: "The science of color differs from the greater part of what is called mental science in the large use which it makes of the physical sciences, and in particular of optics and anatomy. But it gives evidence that it is a mental science by the numerous illustrations which it furnishes of the various operations of the mind." The demonstrations were chiefly experimental, and during their progress Mr. Maxwell incidentally corrected a common error in regard to the production of colors, by mixing their pigments; as for example, mingling blue and yellow powders to make green. He says: "Here are two transparent solutions (exhibited). One appears yellow, it contains bichromate of potash; the other appears blue, it contains sulphate of copper. If I transmit the light of the electric lamp through the two solutions at once, the spot on the screen appears green. By means of the spectrum we shall be able to explain this. The yellow solution cuts off the blue end of the spectrum, leaving only the red, orange, yellow, and green. The blue solution cuts off the red end, leaving only the green, blue, and violet. The only light which can get through both is the green light, as you see. In the same way most blue and yellow paints when mixed appear green. The light which falls on the mixture is so beaten about between the yellow particles and the blue, that the only light which survives is the green. But yellow and blue light when mixed do not make green, as you will see if we allow them to fall on the same parts of the screen together. It is a striking illustration of our mental processes that many persons have not only gone on believing, on the evidence of the mixture of pigments, that blue and yellow make green, but they have even persuaded themselves that they could detect the separate sensations of blueness and of yellowness in the sensation of green."