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172 FRAGMENTS OF SCIENCE.

place in the eye. But what connects the wire with this
organ ? By what means does it send such intelligence of
its varying condition to the optic nerve ? Heat being, as
defined by Locke, “ a very brisk agitation of the insensible
parts of an object,” it is readily conceivable that on touch-
ing a heated body the agitation may communicate itself to
the adjacent nerves, and announce itself to them as light
or heat. But the optic nerve does not touch the hot plati-
num, and hence the pertinence of the question, By' what
agency are 'the vibrations of the wire.transmitted to the
eye ?

The answer to this question involves, perhaps, the most
important physical conception that the mind of man has
yet achieved: the conception of a medium filling space and
fitted mechanically for the transmission of the vibrations
of light and heat, as air is fitted for the transmission of
sound. This medium is called the luminéferous ether.
Every vibration of every atom of our platinum wire raises
in this ether a wave, which speeds through it at the rate
of 186,000 miles a second. The ether suffers no rupture
of continuity at the surface of the eye, the inter-molecular
spaces of the various humors are filled with it; hence the
waves generated by the glowing platinum can cross these
humors and impinge on the optic nerve at the back of the
eye. Thus the sensation of light reduces itself to the com-
munication of motion. Up to this point we deal with pure
mechanics; but the subsequent translation of the shock of
the ethereal waves into consciousness eludes the analysis
of science. As an oar dipping into the Cam generates
systems of waves, which, speeding from the centre of dis-
turbance, finally stir the sedges on the river’s bank, so do
the vibrating atoms generate in the surrounding ether un-
dulations, which finally stir the filaments of the retina.
The motion thus imparted is transmitted with measurable
and not very great velocity to the brain, Where, by a pro-

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