be the cause of
light. But Newton's notion has not held its ground, being entirely
driven from the field by the more wonderful and far more philosophical
notion that light, like sound, is a product of wave-motion.

The domain in which this motion of light is carried on lies entirely
beyond the reach of our senses. The waves of light require a medium
for their formation and propagation; but we cannot see, or feel, or
taste, or smell this medium. How, then, has its existence been
established? By showing, that by the assumption of this wonderful
intangible aether, all the phenomena of optics are accounted for, with
a fulness, and clearness, and conclusiveness, which leave no desire of
the intellect unsatisfied. When the law of gravitation first
suggested itself to the mind of Newton, what did he do? He set
himself to examine whether it accounted for all the facts. He
determined the courses of the planets; he calculated the rapidity of
the moon's fall towards the earth; he considered the precession of the
equinoxes, the ebb and flow of the tides, and found all explained by
the law of gravitation. He therefore regarded this law as
established, and the verdict of science subsequently confirmed his
conclusion. On similar, and, if possible, on stronger grounds, we
found our belief in the existence of the universal aether. It
explains facts far more various and complicated than those on which
Newton based his law. If a single phenomenon could be pointed out
which the aether is proved incompetent to explain, we should have to
give it up; but no such phenomenon has ever been pointed out. It is,
therefore, at least as certain that space is filled with a medium, by
means of which suns and stars diffuse their radiant power, as that it
is traversed by that force which holds in its grasp, not only our
planetary system, but the immeasurable heavens themselves.

There is no more wonderful instance than this of the production of a
line of thought, from the world of the senses into the region of pure
imagination. I mean by imagination here, not that play of fancy which
can give to airy nothings a local habitation and a name, but that
power which enables the mind to conceive realities which lie beyond
the range of the senses--to present to itself distinct images of
processes which, though mighty in the aggregate beyond all conception,
are so minute individually as to elude all observation. It is the
waves of air excited by a