the solar spectrum, then, the range of
the sun's radiation is not limited by that of the eye. The same
statement applies to the more refrangible end. Ritter discovered the
extension of the spectrum into the invisible region beyond the violet;
and, in recent times, this ultra-violet emission has had peculiar
interest conferred upon it by the admirable researches of Professor
Stokes. The complete spectrum of the sun consists, therefore, of
three distinct parts: first, of ultra-red rays of high heating power,
but unsuited to the purposes of vision; secondly, of luminous rays
which display the succession of colours, red, orange, yellow, green,
blue, indigo, violet; thirdly, of ultra-violet rays which, like the
ultra-red ones, are incompetent to excite vision, but which, unlike
the ultra-red rays, possess a very feeble heating power. In
consequence, however, of their chemical energy these ultra-violet rays
are of the utmost importance to the organic world.

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2. Origin and Character of Radiation. The Aether.

When we see a platinum wire raised gradually to a white heat, and
emitting in succession all the colours of the spectrum, we are simply
conscious of a series of changes in the condition of our own eyes. We
do not see the actions in which these successive colours originate,
but the mind irresistibly infers that the appearance of the colours
corresponds to certain contemporaneous changes in the wire. What is
the nature of these changes? In virtue of what condition does the
wire radiate at all? We must now look from the wire, as a whole, to
its constituent atoms. Could we see those atoms, even before the
electric current has begun to act upon them, we should find them in a
state of vibration. In this vibration, indeed, consists such warmth
as the wire then possesses. Locke enunciated this idea with great
precision, and it has been placed beyond the pale of doubt by the
excellent quantitative researches of Mr. Joule. 'Heat,' says Locke,
'is a very brisk agitation of the insensible parts of the object,
which produce in us that sensation from which we denominate the object
hot: so what in our sensations is _heat_ in the object is nothing but
_motion_.' When the electric current, still feeble, begins to pass
through the wire, its first act is to intensify the vibrations already
existing, by causing the atoms to swing through wider ranges.
Technically speaking, the _amplitudes_ of the oscill