me three qualities, only in different degrees, and if each quality
can be measured by a scale, then there is a clue to this labyrinth.

+A COLOR SPHERE and COLOR TREE to unite hue, value, and chroma.+

(31) This clue is found in the union of these three qualities by
measured scales in a _color sphere and color tree_.[10] The equator of
the sphere[11] may be divided into ten parts, and serve as the scale of
hue, marked R, YR, Y, GY, G, BG, B, PB, P, and RP. Its vertical axis may
be divided into ten parts to serve as the scale of value, numbered from
black (0) to white (10). Any perpendicular to the neutral axis is a
scale of chroma. On the plane of the equator this scale is numbered 1,
2, 3, 4, 5, from the centre to the surface.

[Illustration: Fig. 3.]

[Footnote 10: See Color Tree in paragraph 14.]

[Footnote 11: Unaware that the spherical arrangement had been
used years before, I devised a double tetrahedron to classify
colors, while a student of painting in 1879. It now appears that
the sphere was common property with psychologists, having been
described by Runge in 1810. Earlier still, Lambert had suggested
a pyramidal form. Both are based on the erroneous assumption
that red, yellow, and blue are primary sensations, and also fail
to place these hues in a just scale of luminosity. My twirling
color solid and its completer development in the present model
have always made prominent the artistic feeling for color value.
It differs in this and in other ways from previous systems, and
is fortunate in possessing new apparatus to measure the degree
of hue, value, and chroma.]

(32) This chroma scale may be raised or lowered to any level of value,
always remaining perpendicular to the axis, and serving to measure the
chroma of every hue at every level of value. The fact that some colors
exceed others to such an extent as to carry them out beyond the sphere
is proved by measuring instruments, but the fact is a new one to many
persons. (Figs. 2 and 3.)

[Illustration: Fig. 2. (See Fig. 20) The Color Tree]

(33) For this reason the COLOR TREE is a completer model than the
sphere, although the simplicity of the latter makes it best for a
child's comprehension.

(34) The color tree is made by taking the vertical axis of the sphere,
which carries a scale of value, for the trunk. The branches are at right
angles to the trunk; and, as in the sphere, they carry