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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 vari-
ous 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 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 refran-
gible 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, there»
fore, 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
colors, 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

2. Origin and Character of Radiation. The Ether.

When we see a platinum wire raised gradually to a
white heat, and emitting in succession all the colors 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 colors originate, but the mind

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