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tuning-fork, for example, moulds the air round it into un-
dulations 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 ren-
dering 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 uncer-
tainty, to the conviction that What is sound within us is,
outside of us, amotion 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 nerve of the ear, the
thought soon suggested itself that light might be due to
an agitation of the nerve of the eye. This was a great step
in advance of that ancient notion which regarded light as
something emitted by the eye, and not as any thing
imparted to it. But if light be produced by an agitation
of the optic nerve or retina, what is it that produces the
agitation ? Newton, you know, supposed minute particles
to be shot through the humors of the eye against the
retina, which hangs like a target at the back of the eye.
The impact of these particles against the target, Newton
believed to 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 philos0phical
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

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