86 FRAGMENTS OF SCIENCE 



which the dark earth pours nightly into space. In fact, 

 the various kinds of obscure rays emitted by all the plan- 

 ets of our system are included in the present radiation 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 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 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 veri- 

 fied 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 refran- 

 gible end. Bitter 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 



