There is one type of celestial object which seldom fails to stir up the mind of even the most sluggishly unastronomical member of the community and to inspire him with an interest in the science--an interest which is usually conspicuous for a picturesque inaccuracy in the details which it accumulates, for a pathetic faith in the most extraordinary fibs which may be told in the name of science, and for a subsidence which is as rapid as the changes in the object which gave the inspiration. The sun may go on shining, a perpetual mystery and miracle, without attracting any attention, save when a wet spring brings on the usual talk of sun-spots and the weather; Jupiter and Venus excite only sufficient interest to suggest an occasional question as to whether that bright star is the Star of Bethlehem; but when a great comet spreads its fiery tail across the skies everybody turns astronomer for the nonce, and normally slumber-loving people are found willing, or at least able, to desert their beds at the most unholy hours to catch a glimpse of the strange and mysterious visitant. And, when the comet eventually withdraws from view again, as much inaccurate information has been disseminated among the public as would fill an encyclopædia, and require another to correct. Comets are, however, really among the most interesting of celestial objects. Though we no longer imagine them to foretell wars, famines, and plagues, or complacently to indicate the approbation of heaven upon some illustrious person deceased or about to decease, and have almost ceased to shiver at the possibilities of a collision between a comet and the earth, they have within the last half century taken on a new and growing interest of a more legitimate kind, and there are few departments of science in which the advance of knowledge has been more rapid or which promise more in the immediate future, given material to work upon. The popular idea of a comet is that it is a kind of bright wandering star with a long tail. Indeed, the star part of the conception is quite subsidiary to the tail part. The tail is _the_ thing, and a comet without a tail is not worthy of attention, if it is not rather guilty of claiming notice on false pretences. As a matter of fact, the tail is absent in many comets and quite inconspicuous in many more; and a comet may be a body with any degree of resemblance or want of resemblance to the popular idea, from the faint globular stain of haze, scarcely perceptible in the telescopic field against the dark background of the sky, up to a magnificent object, which, like the dragon in the Revelation, seems to draw the third part of the stars of heaven after it--an object like the Donati comet of 1858, with a nucleus brighter than a first-magnitude star, and a tail like a great feathery plume of light fifty millions of miles in length. It seems as impossible to set limits to the variety of form of which comets are capable as it is to set limits to their number.