cannot define these changes.

Science describes chroma as the purity of one wave length separated from
all others. Other wave lengths, INTERMINGLING, make its chroma less
pure. A beam of daylight can combine all wave lengths in such balance as
to give the sensation of whiteness, because no single wave is in
excess.[8]

[Footnote 8: See definition of White in Glossary.]

(24) The color sphere (see Fig. 1) is a convenient model to illustrate
these three qualities,--hue, value, and chroma,--and unite them by
measured scales.

(25) The north pole of the color sphere is white, and the south pole
black. Value or luminosity of colors ranges between these two extremes.
This is the vertical scale, to be memorized as _V_, the initial for both
value and vertical. Vertical movement through color may thus be thought
of as a change of value, but not as a change of hue or of chroma. Hues
of color are spread around the equator of the sphere. This is a
horizontal scale, memorized as _H_, the initial for both hue and
horizontal. Horizontal movement around the color solid is thus thought
of as a change of hue, but not of value or of chroma. A line inward from
the strong surface hues to the neutral gray axis, traces the graying of
each color, which is loss of chroma, and conversely a line beginning
with neutral gray at the vertical axis, and becoming more and more
colored until it passes outside the sphere, is a scale of chroma, which
is memorized as _C_, the initial both for chroma and centre. Thus the
sphere lends its three dimensions to color description, and a color
applied anywhere within, without, or on its surface is located and named
by its degree of hue, of value, and of chroma.

[Illustration: Fig. 1.]

+HUES first appeal to the child, VALUES next, and CHROMAS last.+

(26) Color education begins with ability to recognize and name certain
hues, such as red, yellow, green, blue, and purple (see paragraphs 182
and 183). Nature presents these hues in union with such varieties of
value and chroma that, unless there be some standard of comparison, it
is impossible for one person to describe them intelligently to another.

(27) The solar spectrum forms a basis for scientific color analysis,
taught in technical schools; but it is quite beyond the comprehension of
a child. He needs something more tangible and constantly in view to
train his color notions. He needs to handle colors, place them side by
side for comparison,