end of the spectrum give the sensation of
warmth, while short waves at the violet end cause the sensation
of coolness. Midway between these extremes is the wave length of
green.]

[Illustration: Fig. 22.]

(137) The color score presents four large divisions or color fields made
by the intersection of the equator with the meridian of green. Above the
centre are all light colors, and below it are all dark colors. To the
right of the centre are all warm colors, and to the left are all cool
colors. Middle green (5G 5/5) is the centre of balance for these
contrasted qualities, recognized by all 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.

+Pins stuck into the score represent 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 divide