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 immerged 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 irregular, especially towards the end; it is also shorter than the south-preceding one, . . . even to the naked eye,"
