ircular series.

[Illustration: Fig. 33.--The color circle. R, Y, G and B, stand for
the colors red, yellow, green and blue. The shaded portion corresponds
to the spectrum or rainbow. Complementary colors (see later) lie
diametrically opposite to each other on the circumference.]

A _saturation series_ runs from full-toned or saturated colors to pale
or dull. Since we can certainly say of a pale blue that it is less
saturated than a vivid red, etc., we could, theoretically, arrange our
whole collection of bits of color in a single saturation series, but
our judgment would be very uncertain at many points. The most
significant saturation series confine themselves to a single
color-tone, {208} and also, as far as possible, to a constant
brightness, and extend from the most vivid color sensation obtainable
with this color-tone and brightness, through a succession of less and
less strongly colored sensations of the same tone and brightness, to a
dead gray of the same brightness. Any such saturation series
terminates in a neutral gray, which is light or dark to match the rest
of the particular saturation series.

White, black and gray, which find no place in the color-tone series,
give an intensity series of their own, running from white through
light gray and darker and darker gray to black, and any gray in this
series may be the zero point in a saturation series of any color-tone.

A three-dimensional diagram of the whole system of visual sensations
can be built up in the following way. Taking all the colors of the
same degree of brightness, we can arrange the most saturated, in the
order of their color-tone, around the circumference of a circle, put a
gray of the same brightness at the center of this circle, and then
arrange a saturation series for each color-tone extending from the
most saturated at the circumference to gray at the center. This would
be a two-dimensional diagram for colors having the same brightness.
For a greater brightness, we could arrange a similar circle and place
it above the first, and for a smaller brightness, a similar circle and
place it below the first, and we could thus build up a pile of
circles, ranging from the greatest brightness at the top to the least
at the bottom. But, as the colors all lose saturation when their
brightness is much increased, and also when it is much decreased, we
should make the circles smaller and smaller toward either the top or
the bottom of the pile, so t