22 FRAGMENTS OF SCIENCE. 
tain 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 incandes- 
cence, 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 Will- 
iam Herschel. Causing a beam of solar light to pass 
through a prism, he resolved it into its colored constitu- 
ents; he formed what is technically called the solar spec- 
trum. Exposing thermometers to the successive colors 
he determined their heating power, and found it to aug- 
ment 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 be- 
yond the red he found that, though the light had disap- 
peared, 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 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 refrangi- 
ble 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 
