COMETS 193 



In 1807 Herschel concluded one of his papers in 

 these words: "I find that out of the sixteen comets 

 which I have examined, fourteen have been without 

 any visible solid body in their centre, and that the 

 other two had a very ill-defined small central light, 

 which might perhaps be called a nucleus, but did not 

 deserve the name of a disk." In the end of September 

 that year a comet was discovered by Mr. Pigott, to 

 which Herschel at once turned his attention in the 

 hope of wresting from it information regarding its 

 nature. By careful observations, continued over five 

 months, he felt himself warranted in claiming for it 

 " a visible, round and well-defined disk," 538 miles in 

 diameter, and " shining in every part of it with equal 

 brightness." He came also to the conclusion "that 

 the body of the comet on its surface is self-luminous, 

 from whatever cause this quality may be derived." 

 He inferred besides that " the changes in the brightness 

 of the small stars, when they are successively im merged 

 in the tail or coma of the comet, or cleared from them, 

 prove evidently, that they are sufficiently dense to 

 obstruct the free passage of star-light." The tail of 

 this comet, three weeks after its discovery, was more 

 than nine millions of miles in length, and Herschel 

 was inclined to think that it "consisted of radiant 

 matter, such as, for instance, the aurora borealis." It 

 was not bifid or split in two, as that of the comet of 1769 

 had been, but it presented a peculiarity seen also in others 

 of these bodies : " The south-preceding side, in all its 

 length, except towards the end, is very well defined : 

 but the north-following side is everywhere hazy and irre- 

 gular, especially towards the end; it is also shorter than 

 the south-preceding one, . . . even to the naked eye," 



