The various notes as sounded by a great variety of musical instruments constitute the raw material from which the composer and performer produce melodies and harmonies absolutely unknown to nature, and which--judged by the only possible standard, their emotional and intellectual effects--are incomparably finer than any sounds in nature, finer because a human utterance, the play of soul upon soul. * * * * * The eye has a range of color-notes from four hundred millions of millions of ether-vibrations per second, the rate of the deepest red of the spectrum, to seven hundred and fifty millions of millions, the rate of the violet rays. The following table of vibration rates of the colors of the spectrum shows the vibration intervals which divide the pronounced colors:[36] Color-sensation. Ether-vibrations per second. Deep red 400 millions of millions. Red-orange 437 “ “ Yellow-orange 457 “ “ Yellow 509 “ “ Green 570 “ “ Blue-green 617 “ “ Blue-violet 696 “ “ Violet 750 “ “ This color-scale, as produced by a great variety of agents,--such as colored lights, glass, stones, metals, fabrics, dyes, stains, pigments, etc.,--constitutes the raw material from which the color-composer, painter, and decorator produce melodies and harmonies absolutely unknown to nature, and, judged as musical sounds are judged, are incomparably finer than effects in nature, because essentially human, because produced by man for their emotional and intellectual effect upon man. Theoretically the variation of a single ether-vibration per second changes the shade of the color; but while the trained ear can detect the variation in pitch due to a half-vibration of air per second more or less, ether-vibrations are so incomparably more rapid that the best the trained eye can do is detect about one thousand different tints in the spectrum. In other words, there must be an increase or decrease of three hundred and fifty thousand millions of ether-vibrations before even the practised eye is consciously affected. It is, however, altogether likely that while the eye is not consciously affected without these great variations in frequency, it is _unconsciously_ affected, and susceptibility to and skill in handling color depend upon this unconscious susceptibility. It is pretty well established that the range of color-vision cannot be materially extended below the red or above the violet by practice, but susceptibility to color variations and the ability to distinguish gradations of tone within the scale can be increased almost indefinitely. Education of the color-sense is the development of this _unconscious_ susceptibility,--of the _feeling_ for, as distinguished from a knowledge of, color.