_Star Catalogues._—Since the days of very accurate observations numerous star-catalogues have been produced by individuals or by observatories. Bradley’s monumental work may be said to head the list. Lacaille’s, in the Southern hemisphere, was complementary. Then Piazzi, Lalande, Groombridge, and Bessel were followed by Argelander with his 324,000 stars, Rumker’s Paramatta catalogue of the southern hemisphere, and the frequent catalogues of national observatories. Later the Astronomische Gesellschaft started their great catalogue, the combined work of many observatories. Other southern ones were Gould’s at Cordova and Stone’s at the Cape. After this we have a new departure. Gill at the Cape, having the comet 1882.ii. all to himself in those latitudes, wished his friends in Europe to see it, and employed a local photographer to strap his camera to the observatory equatoreal, driven by clockwork, and adjusted on the comet by the eye. The result with half-an-hour’s exposure was good, so he tried three hours. The result was such a display of sharp star images that he resolved on the Cape Photographic Durchmusterung, which after fourteen years, with Kapteyn’s aid in reducing, was completed. Meanwhile the brothers Henry, of Paris, were engaged in going over Chacornac’s zodiacal stars, and were about to catalogue the Milky Way portion, a serious labour, when they saw Gill’s Comet photograph and conceived the idea of doing the rest of their work by photography. Gill had previously written to Admiral Mouchez, of the Paris Observatory, and explained to him his project for charting the heavens photographically, by combining the work of many observatories. This led Admiral Mouchez to support the brothers Henry in their scheme.[21] Gill, having got his own photographic work underway, suggested an international astrographic chart, the materials for different zones to be supplied by observatories of all nations, each equipped with similar photographic telescopes. At a conference in Paris, 1887, this was decided on, the stars on the charts going down to the fourteenth magnitude, and the catalogues to the eleventh. [Illustration: GREAT COMET, NOV. 14TH, 1882. (Exposure 2hrs. 20m.) By kind permission of Sir David Gill. From this photograph originated all stellar chart-photography.] This monumental work is nearing completion. The labour involved was immense, and the highest skill was required for devising instruments and methods to read off the star positions from the plates. Then we have the Harvard College collection of photographic plates, always being automatically added to; and their annex at Arequipa in Peru. Such catalogues vary in their degree of accuracy; and fundamental catalogues of standard stars have been compiled. These require extension, because the differential methods of the heliometer and the camera cannot otherwise be made absolute.