d color expression, is the best approach to a new art of
mobile color. There is a deep and abiding conviction, justified by the
history of aesthetics, that each art-form must progress from its
own beginnings and unfold in its own unique and characteristic way.
Correspondences between the arts--such a correspondence, for
example, as inspired the famous saying that architecture is frozen
music--reveal themselves usually only after the sister arts have
attained an independent maturity. They owe their origin to that
underlying unity upon which our various modes of sensuous perception
act as a refracting medium, and must therefore be taken for granted.
Each art, like each individual, is unique and singular; in this
singularity dwells its most thrilling appeal. We are likely to miss
light's crowning glory, and the rainbow's most moving message to the
soul if we preoccupy ourselves too exclusively with the identities
existing between music and color; it is rather their points of
difference which should first be dwelt upon.

Let us accordingly consider the characteristic differences between
the two sense-categories to which sound and light--music and
color--respectively belong. This resolves itself into a comparison
between time and space. The characteristic thing about time is
succession--hence the very idea of music, which is in time, involves
perpetual change. The characteristic of space, on the other hand, is
simultaneousness--in space alone perpetual immobility would reign.
That is why architecture, which is pre-eminently the art of space, is
of all the arts the most static. Light and color are essentially
of space, and therefore an art of mobile colour should never lack a
certain serenity and repose. A "tune" played on a color organ is only
distressing. If there is a workable correspondence between the musical
art and an art of mobile color, it will be found in the domain of
harmony which involves the idea of simultaneity, rather than in
melody, which is pure succession. This fundamental difference between
time and space cannot be over-emphasized. A musical note prolonged,
becomes at last scarcely tolerable; while a beautiful color, like the
blue of the sky, we can enjoy all day and every day. The changing hues
of a sunset, are _andante_ if referred to a musical standard, but to
the eye they are _allegretto_--we would have them pass less swiftly
than they do. The winking, chasing, changing lights of illuminated
sky-signs