EADIATION. 31 



planets of our system are included in the present radia- 

 tion 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 coloured 

 constituents ; he formed what is technically called the 

 solar spectrum. Exposing thermometers to the suc- 

 cessive colours he determined their heating power, and 

 found it to augment 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 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 

 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 refrangible end. Kitter 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 ad- 

 mirable researches of Professor Stokes. The complete 

 spectrum of the sun consists, therefore, 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 colours, 

 red, orange, yellow, green, blue, indigo, violet ; thirdly, 

 of ultra-violet rays which, like the ultra-red ones, are 



