olours on the optic nerve.

In the experiment just described we began with a platinum wire at an
ordinary temperature, and gradually raised it to a white heat. At the
beginning, and even before the electric current had acted at all upon
the wire, it emitted invisible rays. For some time after the action
of the current had commenced, and even for a time after the wire had
become intolerable to the touch, its radiation was still invisible.
The question now arises, What becomes of these invisible rays when the
visible ones make their appearance? It will be proved in the sequel
that they maintain themselves in the radiation; that a ray once
emitted continues to be emitted when the temperature is increased, and
hence the emission from our platinum wire, even when it has attained
its maximum brilliancy, consists of a mixture of visible and invisible
rays. If, instead of the platinum wire, the earth itself were raised
to incandescence, the obscure radiation which it now emits would
continue to be emitted. To reach incandescence the planet would have
to pass through all the stages of non-luminous radiation, and the
final emission would embrace the rays of all these stages. There can
hardly be a doubt that from the sun itself, rays proceed similar in
kind to those which the dark earth pours nightly into space. In fact,
the various kind of obscure rays emitted by all the planets of our
system are included in the present radiation of the sun.

The great pioneer in this domain of science was Sir William Herschel.
Causing a beam of solar light to pass through a prism, he resolved it
into its coloured constituents; he formed what is technically called
the solar spectrum. Exposing thermometers to the successive colours
he determined their heating power, and found it to augment from the
violet or most refracted end, to the red or least refracted end of the
spectrum. But he did not stop here. Pushing his thermometers into
the dark space beyond the red he found that, though the light had
disappeared, the radiant heat falling on the instruments was more
intense than that at any visible part of the spectrum. In fact, Sir
William Herschel showed, and his results have been verified by various
philosophers since his time, that, besides its luminous rays, the sun
pours forth a multitude of other rays, more powerfully calorific than
the luminous ones, but entirely unsuited to the purposes of vision.

At the less refrangible end of