graph 17.

+A prismatic color sphere.+

(98) With a little effort of the imagination we can picture a prismatic
color sphere, using only the colors of light. In a cylindrical chamber
is hung a diaphanous ball similar to a huge soap bubble, which can
display color on its surface without obscuring its interior. Then, at
the proper points of the surrounding wall, three pure beams of colored
light are admitted,--one red, another green, and the third violet-blue.

(99) They fall at proper levels on three sides of the sphere, while
their intermediate gradations encircle the sphere with a complete
spectrum plus the needed purple. As they penetrate the sphere, they
unite to balance each other in neutrality. Pure whiteness is at the top,
and, by some imaginary means their light gradually diminishes until they
disappear in darkness below.

(100) This ideal color system is impossible in the present state of our
knowledge and implements. Even were it possible, its immaterial hues
could not serve to dye materials or paint pictures. Pigments are, and
will in all probability continue to be, the practical agents of
coloristic productions, however reluctant the scientist may be to accept
them as the basis of a color system. It is true that they are chemically
impure and imperfectly represent the colors of light. Some of them fade
rapidly and undergo chemical change, as in the notable case of a green
pigment tested by this measured system, which in a few weeks lost four
steps of chroma, gained two steps of value, and swung into a bluer hue.

(101) But the color sphere to be next described is worked out with a few
reliable pigments, mostly natural earths, whose fading is a matter of
years and so slight as to be almost imperceptible. Besides, its
principal hues are preserved in safe keeping by imperishable enamels,
which can be used to correct any tendency of the pigments to distort the
measured intervals of the color sphere.

This meets the most serious objection to a pigment system. Without it a
child has nothing tangible which he can keep in constant view to imitate
and memorize. With it he builds up a mental image of measured relations
that describe every color in nature, including the fleeting hues of the
rainbow, although they appear but for a moment at rare intervals.
Finally, it furnishes a simple notation which records every color
sensation by a letter and two numerals. With the enlargement of his
mental power he will unit