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 



