Page:A color notation (Munsell).djvu/93

 practical color workers. The chart forms a rectangle whose length equals the equator of the color sphere and its height equals the axis (a proportion of 3.14: 1), representing a union and balance of the scales of hue and of value. This provides for two color dimensions; but, to be complete, the chart must provide for the third dimension, chroma.

(138) Replacing the chart around the sphere and joining its ends, so that it re-forms the transparent envelope, we may thrust a pin through at any point until it pierces the surface of the sphere. Indeed, the pin can be thrust deeper until it reaches the neutral axis, thus forming a scale of chroma for the color point where it enters (see paragraph 12). In the same way any colors on the sphere, within the sphere, or without it, can have pins thrust into the chart to mark their place, and the length by which each pin projects can be taken as a measure of chroma. If the chart is now unrolled, it retains the pins, which by their place describe the hue and value of a color, while their length describes its chroma.

(139) With this idea of the third color dimension incorporated in the score we can discard the pin, and record its length by a numeral. Any dot placed on the score marks a certain degree of hue and value, while a numeral beside it marks the degree of chroma which it carries, uniting with the hue and value of that point to give us a certain color. Glancing over a series of such color points, the eye easily grasps their individual character, and connects them into an intelligible series.

(140) Thus a flat chart becomes the projection of the color solid, and any color in that solid is transferred to the surface of the chart, retaining its degrees of hue, value, and chroma. So far the scales have been spoken of as divided into ten steps, but