(151) In the same way the term _color harmony_, from association with musical harmony, presents to the mind an image of color arrangement,--varied, yet well proportioned, grouped in orderly fashion, and agreeable to the eye. But any attempt to define this image in terms of color is disappointing. Here is a beautiful Persian rug: why do we call it beautiful? One says “because its colors are _rich_.” Why are they rich? “Because they are _deep in tone_.” What does that mean? The double-bass and the fog-horn are _deep_ in tone, but not necessarily beautiful on that account. “Oh, no,” says another, “it is all in _one harmonious key_.” But what is a key of color? Is it made by all the values of one color, such as red, or by all the hues of equal value, such as the middle hues in our color solid? (152) Certainly it is neither, for the rug has both light and dark colors; and, of the reds, yellows, greens, and blues, some are stronger and others weaker. Then what do we mean by a key of color? One must either continue to flounder about or frankly confess ignorance. (153) Musical harmony explains itself in clear language. It is illustrated by fixed and definite sound intervals, whose measured relations form the basis of musical composition. Each key has an unmistakable character, and the written score presents a statement that means practically the same thing to every person of musical intelligence. But the adequate terms of color harmony are yet to be worked out. Let us leave these musical analogies, retaining only the clue that _a measured and orderly relation underlies the idea of harmony_. The color solid which has been the subject of these pages is built upon measured color relations. It unites measured scales of hue, value, and chroma, and gives a definite color name to every sensation from the maxima of color-light and color-strength to their disappearance in darkness. (154) Must not this theoretical color solid, therefore, locate all the elements which combine to produce color harmony or color discord?[32] [Footnote 32: Professor James says there are three classic stages in the career of a theory: “First, it is attacked as absurd; then admitted to be true, but obvious and insignificant; finally it is seen to be so important that its adversaries claim to be its discoverers.”] (155) Instead of theorizing, let us experiment. As a child at the piano, who first strikes random and widely separated notes, but soon seeks for the intervals of a familiar air, so let us, after roaming over the color globe and its charts, select one familiar color, and study what others will combine with it to please the eye.