By RICHARD A. PROCTOR. During the last two years several comets—some telescopic, others visible to the naked eye, and even conspicuous objects in the heavens—have been observed, not only by the older methods, but by some which have only been available within recent years. It is naturally expected, therefore, by the general public that some new light should be thrown on these mysterious objects, whose phenomena still remain among the unexplained, seemingly the inexplicable problems of the celestial depths. We propose to consider here what has thus been learned, and what also (unfortunately it is much more) remains still to be learned, respecting comets. But first it will be well to show what are the special phenomena which present themselves for explanation. A comet apparently comes out from the remote depths of space in a condition of comparative calm. It appears as a small round nebulous object, looking like a tiny cloud of extreme tenuity—the idea of tenuity being suggested by the exceeding faintness of the comet’s light. This cloud appears somewhat condensed toward the middle. As the comet draws nearer to the sun, it usually grows somewhat long in the direction of the sun; and before long a portion within the part nearest the sun is seen to be brighter than the rest, and to have a more or less defined outline. This is the _nucleus_—sometimes seen as a dull disc of nearly uniform brightness, at others as a mere bright point not unlike a star. The fainter light around this is the _coma_, or hair, which resembles a luminous fog round the nucleus, usually brighter on the side toward the sun, and on the other side growing fainter and fainter till it can no longer be seen. Later this lengthening of the comet in directions toward and from the sun becomes more marked, until at length the comet may fairly be said to have a head directed toward the sun and a tail directed from him. Nucleus, coma, and tail may be very different in appearance in different comets, and in particular the tail may be more or less complicated in structure, being sometimes a mere straight streak, at others twofold, multiple, curved, with thwart streaks, and so forth—no two comets, in fine, having tails resembling each other except in general details.