It is a curious circumstance that Sir W. Herschel, during all his star-gaugings and sweeps for nebulæ, never discovered a comet. He found a nebula on Dec. 18, 1783, near δ Ceti, which he described as “small and cometic.” In Sir J. Herschel’s ‘General Catalogue of Nebulæ,’ 1864, p. 17, this object is presumed to have been a comet, as it could not be identified; but at p. 45 the doubts are cleared up, and Sir W. Herschel’s nebula, the position of which was only roughly given, is shown to be the same as another very near; it is No. 1055 of the new ‘General Catalogue’ published by the Royal Astronomical Society in January 1888. Quite possibly Sir W. Herschel’s lists of nebulæ contain several comets, as some of his objects are missing; but errors of observation in ascribing positions may explain this. Herschel himself, in speaking of a comet visible in the winter of 1807-8, says:—“If I had met the comet in one of my sweeps, as it appeared between Dec. 6 and Feb. 21, I should have put it down as a nebula. Perhaps my lists of nebulæ, then, contain some comets.”
_Large Comets._—The most widely observed and attractive class of comets includes those of large proportions, as they are not only visible to the naked eye, but exhibit features having the lustre necessary to permit of their examination with high magnifying powers. A brief summary of some of the finest comets of modern times is subjoined; but, to save space, a few only of the more salient facts concerning them are given:—
1577, Nov. and Dec.—Observed by Tycho Brahe. At the end of November it had a double tail; the longest of the two branches was about 20°. This comet was visible in the daytime.
1618 II., Nov.—“The length of its tail equalled in extent one sixth part of the zodiac.” On Nov. 18 it was estimated as 40°. Longomontanus, however, described it as 104° long, and Cysatus estimated it as 75°. Kepler referred to it as the largest comet that had appeared for a hundred and fifty years.
1680, Dec.—A fine comet, which on Dec. 12 had a narrow tail about 80° long. The nucleus was equal to a 1st mag. star. Hooke remarked jets of flame issuing from the nucleus. At perihelion the comet approached very near the Sun’s surface, similarly to the fine comets of 1843, 1880, and 1882.