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RADIATION. 1 6 9

in fact produced by the simultaneous action of all these
colors on the optic nerve.

In the eXperiment just described we began with a plat-
inum 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 kinds 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 colored constituents;
he formed what is technically called the solar spectrum.
Exposing thermometers to the successive colors he deter-
mined their heating power, and found it to augment from
the violet or most refracted end, to the red or least refracted

8

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