hers the dispersed rays back into their original beam, making a
white spot on the floor. This proves that all the colored rays of light
combine to balance each other in whiteness. But if pigments which are
the closest possible imitation of these hues are united on a painter's
palette, either by the brush or the knife, they _make gray, and not
white_.

(93) This is another illustration of the behavior of pigments, for,
instead of uniting to form white, they form gray, which is a darkened or
impure form of white; and, lest this should be attributed to a chemical
reaction between the various matters that serve as pigments, the
experiment can be carried out without allowing one pigment to touch
another by using Maxwell discs, as will be shown in the next chapter.

(94) Before leaving these prismatic colors, let us study them in the
light of what has already been learned of color dimensions. Not only do
they present different values, but also different chromas. Their values
range from darkness at each end, where red and purple become visible, to
a brightness in the greenish yellow, which is almost white. So on the
color tree described in Chapter II., paragraph 34, yellow has the
highest branch, green is lower, red is below the middle, with blue and
purple lower down, near black.

[Illustration: Fig. 15.]

(95) Then in chroma they range from the powerful stimulation of the red
to the soothing purple, with green occupying an intermediate step. This
is also given on the color tree by the length of its branches.

(96) In Fig. 15 the vertical curve describes the values of the spectrum
as they grade from red through yellow, green, blue, and purple. The
horizontal curve describes the chromas of the spectrum in the same
sequence; while the third curve leaning outward is obtained by uniting
the first two by two planes at right angles to one another, and sums up
the three qualities by a single descriptive line. Now the red and purple
ends are far apart, and science forbids their junction because of their
great difference in wave length. But the mind is prone to unite them in
order to produce the red-purples which we see in clouds at sunset, in
flowers and grapes and the amethyst. Indeed, it has been done
unhesitatingly in most color schemes in order to supply the opposite of
green.

(97) This gives a slanting circuit joining all the branch ends of the
color tree, and has been likened to the rings of Saturn in Chapter I.,
para