osites { Green Red-purple } Each pair of which unites
{ Blue Yellow-red } in neutral gray.
{ Purple Green-yellow }

(63) But if, instead of mixing these opposite hues, we place them side
by side, the eye is so stimulated by their difference that each seems to
gain in strength; _i.e._, each _enhances_ the other when separate, but
_destroys_ the other when mixed. This is a very interesting point to be
more fully illustrated by the help of a color wheel in Chapter V.,
paragraph 106. What we need to remember is that the mixture of
neighborly hues makes them less stimulating to the eye, because they
resemble each other, while a mixture of opposite hues extinguishes both
in a neutral gray.

+Hues once removed, and their mixture.+

[Illustration: Fig. 8.]

(64) There remains the question, What will happen if we mix, not two
neighbors, nor two opposites, but _a pair of hues once removed in the
circle_, such as red and green? A line joining this pair does not pass
through the neutral centre, but to one side nearer yellow, which shows
that this mixture falls between neutral gray and yellow, partaking
somewhat of each. In the same way a line joining yellow and blue shows
that their mixture contains both green and gray. Indeed, a line joining
any two colors in the circuit may be said to describe their union.
A radius crossing this line passes to some hue on the circumference, and
describes by its intersection with the first line the chroma of the
color made by a mixture of the two original colors.

Red & Green make Yellow-gray }
Yellow Blue Green-gray } Each pair unites in a _colored_
Green Purple Blue-gray } gray, which is an intermediate hue
Blue Red Purple-gray } of weak chroma.
Purple Yellow Red-gray }

+Mixture of white and black: a scale of grays.+

(65) So far we have thought only of the plane of the equator, with its
circle of middle hues in ten steps, and studied their mixture by drawing
lines to join them. Now let us start at the neutral centre, and think
upward to white and downward to black (Fig. 9.)

[Illustration: Fig. 9.]

This vertical line is the _neutral axis_ joining the poles of white and
black, which represent the opposites of light and darkness. Middle gray
is half-way between. If black is called 0, and white is 10, then the
middle point is 5, with 6, 7, 8, and 9 above, while 4, 3, 2, and 1 are
below, thus