In the present century, the comets most remarkable for brilliancy have been the C. of 1811, that of 1843, and that of 1858 (Donati's).
Spectroscopic investigation, so far as yet pursued, points to the conclusion that the nucleus is self-luminous, but that the tail shines with reflected light. It has been dis covered recently, in determining the tracks of those streams of dark bodies that cause meteoric showers, that some of the tracks coincide with the orbits of well-known comets. From this, it is inferred that star-showers and comets may be only different manifesta tions of the same thing. See METEORS.
What the matter of the comets consists of is, of course, only a subject for specula tion. The composition of the nebulosity and the tail is, at all events, something of almost inconceivable tenuity, as shown by three consideration. 1. Stars seen through them suffer no diminution of brightness, though the light must have to traverse some times millions of miles of the cometary atmosphere. 2. Though the thickness of the tail of a C. may be millions of miles, and its length of course much greater, the comets have never been observed to cause any sensible disturbance of the planetary motions, though approaching near enough to be themselves so much affected as to change the entire character of the orbit. 3. The curvature of the tails, and the acceleration of the periodic time, in the case of Encke's C., indicate their being affected by a resisting medium, which has never been observed to have the slightest influence on the planetary periods, though so long observed. Even the nuclei of comets appear to,be of extremely small density. This may be inferred; though with less force than regards the tails, from the two last considerations above mentioned: and, moreover, there are reliable accounts of stars of a very low order of magnitude being seen through the nuclei themselves.
Comets have been alternately regarded with terror and with welcome in the popular mind. The appearance of Halley's C., in 1456, just as the Turks had become masters
of Constantinople, and threatened an advance into Europe, was regarded by Christen dom with a superstitious dread, and to the Ave Maria was added the prayer: "Lord save us from the devil, the Turk, and the comet." At Constantinople, the occurrence of a lunar eclipse at the same time, increased the portentousness of the event. The discoveries of science of the magnitude of the space filled by their bodies, and their pro digious velocity, together with the confessed impossibility of always predicting their approach, produced fet.rs of another kind, which have sometimes been, especially in France, extravagantly exaggerated in 'the public mind. The groundlessness of such alarms from the extreme improbability of collision with the nucleus, the probable innocuousness of a contact with the extremely attenuated surrounding matter, and, pos sibly. to the greater part of the world, of a collision with the nucleus itself, will be suf ficiently evident from what has been said above. It is probable that already, on many occasions, some of the attenuated vapor in the tail of comets must have come within the earth's attraction, and been absorbed in its atmosphere. Whether the effect is deleteri ous or salubrious, or whether it has any perceptible influence at all, is only matter of speculation. The salubrity of cometary influence is now a popular idea; and the vin tages of 1811 and 1858 were favorable seasons, whose produce is often advertised as the C. wines. It is scarcely worth while, however, to follow further speculation ou these subjects, and it has been considered preferable to confine this article chiefly to the description of the general characteristics of comets, and the facts respecting them afforded by science.