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(10) The fruit is then filled with assorted colors, graded from white to black, according to their values, and disposed by their hues in the five sections. A slice near the top will uncover light values in all hues, and a slice near the bottom will find dark values in the same hues. A slice across the middle discloses a circuit of hues all of middle value; that is, midway between the extremes of white and black.

(11) Two color dimensions are thus shown in the orange, and it remains to exhibit the third, which is called Chroma, or strength of color. To do this, we have only to take each section in turn, and, without disturbing the values already assorted, shove the grayest in toward the narrow edge, and grade outward to the purest on the surface. Each slice across the fruit still shows the circuit of hues in one uniform value; but the strong chromas are at the outside, while grayer and grayer chromas make a gradation inward to neutral gray at the centre, where all trace of color disappears. The thin edges of all sections unite in a scale of gray from black to white, no matter what hue each contains.

The curved outside of each section shows its particular hue graded from black to white; and, should the section be cut at right angles to the thin edge, it would show the third dimension,—chroma,—for the color is graded evenly from the surface to neutral gray. A pin stuck in at any point traces the third dimension.

A color sphere can be used to unite the three dimensions of hue, value, and chroma.

(12) Having used the familiar structure of the orange as a help in classifying colors, let us substitute a geometric solid, like a sphere,[1] and make use of geographical terms. The north pole is white. The south pole is black.

  1. See frontispiece.