148) Harmony of color has been still further complicated by the use of
terms that belong to musical harmony. Now music is a _measured art_, and
has found a set of intervals which are defined scientifically. The two
arts have many points of similarity; and the impulses of sound waves on
the ear, like those of light waves on the eye, are measured vibrations.
But they are far apart in their scales, and differ so much in important
particulars that no practical relationship can be set up. The intervals
of color sensation require fit names and measures, ere their infinite
variety can be organized into a fixed system.

(149) Any effort to compare certain sounds to certain colors soon leads
to the wildest vagaries.

+Harmony of sound is unlike harmony of color.+

(150) The poverty of color language tempts to a borrowing from the
richer terminology of music. Musical terms, such as "pitch, key, note,
tone, chord, modulation, nocturne, and symphony," are frequently used in
the description of color, serving by association to convey certain vague
ideas.

(151) In the same way the term _color harmony_, from association with
musical harmony, presents to the mind an image of color
arrangement,--varied, yet well proportioned, grouped in orderly fashion,
and agreeable to the eye. But any attempt to define this image in terms
of color is disappointing. Here is a beautiful Persian rug: why do we
call it beautiful? One says "because its colors are _rich_." Why are
they rich? "Because they are _deep in tone_." What does that mean? The
double-bass and the fog-horn are _deep_ in tone, but not necessarily
beautiful on that account. "Oh, no," says another, "it is all in _one
harmonious key_." But what is a key of color? Is it made by all the
values of one color, such as red, or by all the hues of equal value,
such as the middle hues in our color solid?

(152) Certainly it is neither, for the rug has both light and dark
colors; and, of the reds, yellows, greens, and blues, some are stronger
and others weaker. Then what do we mean by a key of color? One must
either continue to flounder about or frankly confess ignorance.

(153) Musical harmony explains itself in clear language. It is
illustrated by fixed and definite sound intervals, whose measured
relations form the basis of musical composition. Each key has an
unmistakable character, and the written score presents a statement that
means practically the same thing to every person of music