rvations of a comet, when it comes
within the range of his telescope, an astronomer can calculate its
path in regions which no telescope can reach: and in like manner, by
means of data furnished in the narrow world of the senses, we make
ourselves at home in other and wider worlds, which are traversed by
the intellect alone.

From the earliest ages the questions, 'What is light?' and 'What is
heat?' have occurred to the minds of men; but these questions never
would have been answered had they not been preceded by the question,
'What is sound?' Amid the grosser phenomena of acoustics the mind was
first disciplined, conceptions being thus obtained from direct
observation, which were afterwards applied to phenomena of a character
far too subtle to be observed directly. Sound we know to be due to
vibratory motion. A vibrating tuning-fork, for example, moulds the
air around it into undulations or waves, which speed away on all sides
with a certain measured velocity, impinge upon the drum of the ear,
shake the auditory nerve, and awake in the brain the sensation of
sound. When sufficiently near a sounding body we can feel the
vibrations of the air. A deaf man, for example, plunging his hand
into a bell when it is sounded, feels through the common nerves of his
body those tremors which, when imparted to the nerves of healthy ears,
are translated into sound. There are various ways of rendering those
sonorous vibrations not only tangible but visible; and it was not
until numberless experiments of this kind had been executed, that the
scientific investigator abandoned himself wholly, and without a shadow
of misgiving, to the conviction that what is sound within us is,
outside of us, a motion of the air.

But once having established this fact--once having proved beyond all
doubt that the sensation of sound is produced by an agitation of the
auditory nerve--the thought soon suggested itself that light might be
due to an agitation of the optic nerve. This was a great step in
advance of that ancient notion which regarded light as something
emitted by the eye, and not as anything imparted to it. But if light
be produced by an agitation of the retina, what is it that produces
the agitation? Newton, you know, supposed minute particles to be shot
through the humours of the eye against the retina, which he supposed
to hang like a target at the back of the eye. The impact of these
particles against the target, Newton believed to