170 FRAGMENTS OF SCIENCE. 



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 

 world. 



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 



