OPTICAL LABOURS. 301 



beautiful comet of 1811, relative to the changes of dis- 

 tance from the sun, and the modifications resulting 

 thence, Herschel put it beyond doubt that these modifi- 

 cations have something individual in them, something 

 relative to a special state of the nebulous matter. On 

 one celestial body the changes of distance produce an 

 enormous effect, on another the modifications are insig- 

 nificant. 



OPTICAL LABOURS. 



I shall say very little on the discoveries that Herschel 

 made in physics. In short, everybody knows them. 

 They have been .inserted into special treatises, into ele- 

 mentary works, into verbal instruction ; they must be 

 considered as the starting-point of a multitude of im- 

 portant labours with which the sciences have been 

 enriched during several years. 



The chief of these is that of the dark radiating heat 

 which is found mixed with light. 



In studying the phenomena, no longer with the eye, 

 like Newton, but with a thermometer, Herschel discov- 

 ered that the solar spectrum is prolonged on the red side 

 far beyond the visible limits. The thermometer some- 

 times rose higher in that dark region, than in the midst 

 of brilliant zones. The light of the sun then, contains, 

 besides the coloured rays so well characterized by New- 

 ton, some invisible rays, still less refrangible than the 

 red. and whose warming power is very considerable. A 

 world of discoveries has arisen from this fundamental 

 fact. 



The dark heat emanating from terrestrial objects more 

 or less heated, became also subjects of Herschel's investi- 

 gations. His work contained the germs of a good num- 



