the schemes of Runge and Chevreul, as shown in the
Appendix to this chapter.]

+A chromatic tuning fork.+

(117) The five principal steps in this color equator are made in
permanent enamel and carefully safeguarded, so that, if the pigments
painted on the globe should change or become soiled, it could be at once
detected and set right. These five are middle red (so called because
midway between white and black, as well as midway between our strongest
red and the neutral centre), middle yellow, middle green, middle blue,
and middle purple. They may be called the CHROMATIC TUNING FORK, for
they serve to establish the pitch of colors, as the musical tuning fork
preserves the pitch of sounds.

+Completion of a pigment color sphere.+

(118) When the chromatic tuning fork has thus been obtained, the
completion of the globe is only a matter of patience, for the same
method can be applied at any level in the scale of value, and a new
circuit of balanced hues made to conform with its position between the
poles of white and black.

[Illustration: Fig. 19.]

(119) The surface above and below the equatorial band is set off by
parallels to match the photometric scale, making nine bands or value
zones in all, of which the equator is fifth, the black pole being 0 and
the white pole 10.

(120) Ten meridians carry the equatorial hues across all these value
zones and trace the gradation of each hue through a complete scale from
black to white, marked by their values, as shown in paragraph 68. Thus
the red scale is R1, R2, R3, R4, R5 (middle red), R6, R7, R8, and R9,
and similarly with each of the other hues. When the circle of hues
corresponding to each level has been applied and tested, the entire
surface of the globe is spread with a logical system of color scales,
and the eye gratified with regular sequences which move by measured
steps in each direction.

(121) Each meridian traces a scale of value for the hue in which it
lies. Each parallel traces a scale of hue for the value at whose level
it is drawn. Any oblique path across these scales traces a regular
sequence, each step combining change of hue with a change of value and
chroma. The more this path approaches the vertical, the less are its
changes of hue and the more its changes of value and chroma; while, the
nearer it comes to the horizontal, the less are its changes of value and
chroma, while the greater become its changes of hue. Of these two
oblique pat