e the
product of wave-motion. But what, in this case, could be the material
forming the waves? For the waves of sound we have the air of our
atmosphere; but the stretch of imagination which filled all space with
a _luminiferous ether_ trembling with the waves of light was so bold
as to shock cautious minds. In one of my latest conversations with Sir
David Brewster, he said to me that his chief objection to the
undulatory theory of light was, that he could not think the Creator
capable of so clumsy a contrivance as the filling of space with ether
to produce light. This, I may say, is very dangerous ground, and the
quarrel of science with Sir David, on this point as with many
estimable persons on other points, is, that they profess to know too
much about the mind of the Creator.

This conception of an ether was advocated, and successfully applied to
various phenomena of optics, by the illustrious astronomer, Huyghens.
He deduced from it the laws of reflection and refraction, and applied
it to explain the double refraction of Iceland spar. The theory was
espoused and defended by the celebrated mathematician, Euler. They
were, however, opposed by Newton, whose authority at the time bore
them down. Or shall we say it was authority merely? Not quite so.
Newton's preponderance was in some degree due to the fact that, though
Huyghens and Euler were right in the main, they did not possess
sufficient data to _prove_ themselves right. No human authority,
however high, can maintain itself against the voice of Nature speaking
through experiment. But the voice of Nature may be an uncertain voice,
through the scantiness of data. This was the case at the period now
referred to, and at such a period, by the authority of Newton, all
antagonists were naturally overborne.

The march of mind is rhythmic, not uniform, and this great Emission
Theory, which held its ground so long, resembled one of those circles
which, according to your countryman Emerson, the intermittent force of
genius periodically draws round the operations of the intellect, but
which are eventually broken through by pressure from behind. In the
year 1773 was born, at Milverton, in Somersetshire, a circle-breaker
of this kind. He was educated for the profession of a physician, but
was too strong to be tied down to professional routine. He devoted
himself to the study of natural philosophy, and became in all its
departments a master. He was also a master of letters. Langu