HEESCHEL. 221 



diaphanous atmosphere by which the head of the comet was surrounded, 

 at a distance of 518,000 kilometers (322,000 English miles) from the 

 nucleus. 



This distance was not constant. The matter of the semi-annular en- 

 velope seemed even to be precipitated by slow degrees through the 

 diaphanous atmosphere; finallj'^ it reached the nucleus; the earlier ap- 

 pearances vanished ; the comet was reduced to a globular nebula. 



During its period of dissolution, the ring ax)peared sometimes to have 

 several branches. 



The luminous shreds of the tail seemed to undergo rapid, frequent, 

 and considerable variations of length. Herschel discerned symptoms of 

 a movement of rotation both in the comet and in its tail. This rota- 

 tory motion carried unequal shreds from the center toward the border, 

 and reciprocally. On looking from time to time at the same region of 

 the tail — at the border, for example — sensible changes of length must 

 have been perceptible, which, however, had no real existence. Herschel 

 thought, as I have already said, that the beautiful comet of 1811 and 

 that of 1807 were self-luminous. The second comet of 1811 appeared to 

 him to shine only by borrowed light. It must be acknowledged that 

 these conjectures did not rest on anything demonstrative. 



In attentively comparing the comet of 1807 with the beautiful comet 

 of 1811, relative to the changes of distance from the sun, and the modi- 

 fications resulting thence, Herschel put it beyond doubt that these modi- 

 fications 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 insignificant. 



OPTICAL LABORS. 



I shall say very little as to the discoveries that Herschel made in phys- 

 ics, since every one is iamiliar with them. They are to be found in all 

 elementary works, and are given in verbal instruction ; they must be con- 

 sidered as the starting-point of a multitude of important labors with 

 which the sciences have been enriched during later years. 



The chief of these is that of the dark radiating heat which is found 

 mixed with light. 



In studying the phenomena, not with the eye, as Newton did, but 

 with a thermometer, Herschel discovered that the solar spectrum is pro- 

 longed on the red side far beyond the visible limits. The thermometer 

 sometimes rose higher in the dark region than in the midst of brilliant 

 zones. The light of the sun, then, contains, besides the colored rays so 

 well characterized by Newton, invisible rays, still less refrangible 

 than the red, and whose warming power is very considerable. A world 

 of discoveries has arisen from this fundamental (act. 



Tiie dark ray emanating from terrestrial objects more or less heated 

 also became the subject of Herschel's investigations. His work contained 



