It would serve no useful purpose to repeat here the stories of any of the great comets. These may be found in considerable detail in such volumes as Chambers's 'Handbook of Astronomy,' vol. i., or Miss Agnes Clerke's 'History of Astronomy.' Attention must rather be turned to the question, 'What are comets?' It is a question to which no answer of a satisfactory character could be given till within the last fifty years. Even the great comet of 1858, the Donati, which made so deep an impression on the public mind, and was so closely followed and studied by astronomers, was not the medium of any great advance in the knowledge of cometary nature. The many memoirs which it elicited disclosed nothing fundamentally new, and broke out no new lines of inquiry. Two things have since then revolutionized the study of the subject--the application of the spectroscope to the various comets that have appeared in the closing years of the nineteenth century, and the discovery of the intimate connection between comets and meteors. It was in 1864, a year further made memorable astronomically by Sir William Huggins's discovery of the gaseous nature of some of the nebulæ, that the spectroscope was first applied to the study of a comet. The celestial visitor thus put to the question, a comet discovered by Tempel, was in nowise a distinguished object, appearing like a star of the second magnitude, or less, with a feeble though fairly long tail. When analyzed by Donati, it was found to yield a spectrum consisting of three bright bands, yellow, green, and blue, separated by dark spaces. This observation at once modified ideas as to cometary structure. Hitherto it had been supposed that comets shone by reflected light; but Donati's observation revealed beyond question that the light of the 1864 comet at all events was inherent, and that, so far as the observation went, the comet consisted of glowing gas. [Illustration: PLATE XXV. Great Comet. Photographed May 5, 1901, with the 13-inch Astrographic Refractor of the Royal Observatory, Cape of Good Hope.]