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 are only annoying, and for the same reason.  The eye longs for repose in some serene radiance or stately sequence, while the ear delights in contrast and continual change.  It may be that as the eye becomes more educated it will demand more movement and complexity, but a certain stillness and serenity are of the very nature of light, as movement and passion are of the very nature of sound.  Music is a seeking—­“love in search of a word”; light is a finding—­a “divine covenant.”

With attention still focussed on the differences rather than the similarities between the musical art and a new art of mobile color, we come next to the consideration of the matter of form.  Now form is essentially of space:  we speak about the “form” of a musical composition, but it is in a more or less figurative and metaphysical sense, not as a thing concrete and palpable, like the forms of space.  It would be foolish to forego the advantage of linking up form with colour, as there is opportunity to do.  Here is another golden ball to juggle with, one which no art purely in time affords.  Of course it is known that musical sounds weave invisible patterns in the air, and to render these patterns perceptible to the eye may be one of the more remote and recondite achievements of our uncreated art.  Meantime, though we have the whole treasury of natural forms to draw from, of these we can only properly employ such as are abstract.  The reason for this is clear to any one who conceives of an art of mobile color, not as a moving picture show—­a thing