We may perhaps begin with a brief historical account of the enterprise. Photographs of the stars were taken many years ago, but only by a few enthusiasts, and with no serious hope of competing with eye observations of the sky. The old wet-plate photography was, in fact, somewhat unsuited to astronomical purposes; to photograph faint objects a long exposure is necessary, and the wet plate may dry up before the exposure is concluded--nay, even before it is commenced, if the observer has to wait for passing clouds--and therefore it may be said that the successful application of photography to astronomy dates from the time when the dry plate was invented; when it became possible to expose a plate in the telescope for hours, or by accumulation even for days. The dry plate remains sensitive for a long period, and if it is desired to extend an exposure beyond the limits of one night, it is quite easy to close up the telescope and return to the operations again on the next fine night; and so on, if not perhaps indefinitely, at any rate so long as to transcend the limits of human patience up to the present. [Illustration: VII.--GREAT COMET OF NOV. 7TH, 1882 (_From a photograph taken at the Royal Observatory, Cape of Good Hope._)] [Sidenote: Comet of 1882.] [Sidenote: Stars shown on the pictures.] But to consider our particular project. We may assign, perhaps, the date 1882 as that in which it first began to take shape. In that year there was a magnificent bright comet, the last really large comet which we, in the Northern Hemisphere, have had the good fortune to see. Some of us, of course, were not born at that time, and perhaps others who were alive may nevertheless not have seen that comet; for it kept somewhat uncomfortably early morning hours, and I can well remember myself feeling rather more resentment than gratitude to the man who waked me up about four o'clock to see it. Many observations were of course made of this interesting visitor, and what specially concerns us is that at the Cape of Good Hope some enterprising photographers tried to photograph it. They tried in the first instance with ordinary cameras, and soon found--what any astronomer could have told them--that the movement of the earth, causing an apparent movement of the comet and the stars in the opposite direction, frustrated their efforts. The difficulties of obtaining pictures of moving objects are familiar to all photographers. A "snap-shot" might have met the difficulty, but the comet was scarcely bright enough to affect the plate with a short exposure. Ultimately Dr. David Gill, the astronomer at the Cape Observatory, invited one of the photographers to strap his camera to one of the telescopes at the Observatory, a telescope which could be carried round by clockwork in the usual way, so as to counteract the earth's motion, and in effect to keep the comet steadily in view, as though it were at rest. As a consequence, some very beautiful and