a prism used to split a beam of sunlight into its various wave lengths.
Through a narrow slit there enters a straight pencil of light which we
are accustomed to think of as _white_, although it is a bundle of
variously colored rays (or waves of ether) whose union and balance is so
perfect that no single ray predominates.

[Illustration: Fig. 13.]

(88) Cover the narrow slit, and we are plunged in darkness. Admit the
beam, and the eye feels a powerful contrast between the spot of light on
the floor and its surrounding darkness. Place a triangular glass prism
near the slit to intercept the beam of white light, and suddenly there
appears on the opposite wall a band of brilliant colors. This delightful
experiment rivets the eye by the beauty and purity of its hues. All
other colors seem weak by comparison.

Their weakness is due to impurity, for all pigments and dyes reflect
portions of hues other than their dominant one, which tend to "gray" and
diminish their chroma.

(89) But prismatic color is pure, or very nearly so, because the shape
of the glass refracts each hue, and separates it by the length of its
ether wave. These waves have been measured, and science can name each
hue by its wave length. Thus a certain red is known as M. 6867, and a
certain green sensation is M. 5269.[21] Without attempting any
scientific analysis of color, let it be said that Sir Isaac Newton made
his series of experiments in 1687, and was privileged to name this color
sequence by seven steps which he called red, orange, yellow, green,
blue, violet, and indigo. Later a scientist named Fraunhofer discovered
fine black lines crossing the solar spectrum, and marked them with
letters of the alphabet from a to h. These with the wave length serve to
locate every hue and define every step in the sequence. Since Newton's
time it has been proved that only three of the spectral hues are
_primary_; viz., a red, a green, and a violet-blue, while their mixture
produces all other gradations. By receiving the spectrum on an opaque
screen with fine slits that fit the red and green waves, so that they
alone pass through, these two primary hues can be received on mirrors
inclined at such an angle as to unite on another screen, where, instead
of red or green, the eye sees only yellow.[22]

[Footnote 21: See Micron in Glossary.]

[Footnote 22: The fact that the spectral union of red and green
makes yellow is a matter of surprise to practical