ations are
increased. The current does this, however, without altering the
periods of the old vibrations, or the times in which they were
executed. But besides intensifying the old vibrations the current
generates new and more rapid ones, and when a certain definite
rapidity has been attained, the wire begins to glow. The colour first
exhibited is red, which corresponds to the lowest rate of vibration of
which the eye is able to take cognisance. By augmenting the strength
of the electric current more rapid vibrations are introduced, and
orange rays appear. A quicker rate of vibration produces yellow, a
still quicker green; and by further augmenting the rapidity, we pass
through blue, indigo, and violet, to the extreme ultra-violet rays.

Such are the changes recognised by the mind in the wire itself, as
concurrent with the visual changes taking 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 touching 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 platinum, 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 one of the most important
physical conceptions 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 _luminiferous
aether_. Every vibration of every atom of our platinum wire raises in
this aether a wave, which speeds through it at the rate of 186,000
miles a second.

The aether suffers no rupture of continuity at the surface of the eye,
the inter-molecular spaces of the various humours are filled with it;
hence the waves generated by the glowing platinum can cross these
humours and impinge on the optic nerve at the back of the eye.
[Footnote: The action here described is analogous to the passage of
sound-waves through thick felt whose interstices are occupied by air.]
Thus the sensation of light reduces itself to the acceptance of
motion. Up to this point we deal with pure mechanics; b