ASTRO N O M Y of the northern ones. When we recall the expedition of Sir John Herschel to the Cape of Good Hope in 1832-34, and the rather limited work of the observatories at the Cape, Melbourne, Paramatta, &c., before 1870, we shall have included the most important researches in this part of the sky. The first step towards a marked extension of our knowledge in this field dates from the foundation of the National Argentine Observatory at Cordoba by Dr Gould in 1871. This observatory has, since its foundation, been devoted to the work of cataloguing and mapping the southern heavens with instruments of precision. The results of this work are found in the splendid volumes, now eighteen in number, published by the observatory. A few years later, Sir David Gill, F.R.S., on his appointment as astronomer of the Cape Observatory, so enlarged the means of that establishment and gave such an impetus to its work that it is now one of the leading observatories of the world. In the field of astronomy his work has differed from that of the Cordoba Observatory in being directed very largely towards the most precise measurements of a limited number of stars, Cordoba having devoted most attention to cataloguing all the stars which could be advantageously reached with the instruments. Gill’s most remarkable work has been done with the heliometer, an instrument which he has done much to bring to its present state of perfection. Determinations of stellar parallax have been pursued by him with such success that we have now, so far at least as the brightest stars are concerned, a better knowledge of this subject for the southern heavens than for the northern. Yet more recently the foundation of the Arequipa Observatory, a branch of the Harvard Observatory, has marked an epoch in stellar photometry, to which branch it has been very largely devoted. Through its work, our knowledge of the magnitudes of the stars of the southern heavens now surpasses that of the northern heavens as it was in 1880. Another prominent feature of the advance we are making has been the application of photography to astronomical investigation. From the time, about 1840, that Dr Draper took a daguerreotype of the moon, up to the year 1882, the photographic art played only a minor part in astronomical work. Perhaps Rutherfurd’s work, that of Gould at Cordoba in photographing clusters of stars, the practice of photographing the sun at Greenwich in order to have a record of the changes of its surface, and the application of the art to the transits of Yenus in 1874 and 1882, were the most important astronomical applications of photography during this interval. In 1882 Gill, in having the great comet of that year photographed with a portrait lens, found that all the stars in the field of view, down to the 9th or 10th magnitude, were imprinted upon the negatives. Following up the idea thus suggested, he undertook the enterprise of forming by photography a catalogue of all the stars down to the 10th magnitude between the south pole and 23° of south declination. This work has been brought to a completion with the aid of Professor Kapteyn of Groningen, who, with a disinterestedness to which it would be difficult to find a parallel in any other field than that of science, has devoted years of patient toil to the measurements of the positions of the stars on Gill’s plates. As a result of this work, and that of Thome, Gould’s successor at Cordoba, it may fairly be said that the stars are now even more completely catalogued for the southern heavens than for the northern. Gill’s demonstration, and the use in photography of more sensitive chemicals than had formerly been known, led to the idea on the part of the Paris astronomers of photographing the entire heavens by an international association of observatories. A conference to decide upon the methods of doing this was invited by Admiral
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Mouchez, director of the Paris Observatory, in April 1887. Here was formulated the plan for the international Carte Photographique du Ciel, which was undertaken by some eighteen observatories in both hemispheres. The work is all being done with similar instruments on a uniform plan. The charts in question form two series—one including only the stars down to about the 11th magnitude, the positions of which were to be determined by measurement, so as to form a catalogue, and the other all the stars which could be photographed with the longest available exposure. This work is not yet finished. Some portions of it were undertaken by observatories which appear not to have had the means for its successful prosecution, but other portions have been brought nearly or quite to a completion. The application of the photographic method has been a prominent feature in the remarkable work of the Harvard Observatory. On assuming the direction of that establishment in 1876 Professor Pickering decided to make astrophysics its speciality. The munificence of those interested in his work has enabled him to command a wealth of appliances almost without example, and to devote to their use one of the largest and best-trained corps of assistants ever engaged in astronomical observation. Before the Paris Conference he had planned a work somewhat similar to that undertaken by the conference and begun to put it in execution. His object was, however, in one point at least, essentially different from that of the international enterprise, for his main idea was to photograph the sky from time to time with a view of detecting at the earliest possible moment any change that might take place among the stars. A striking result of this system has already been mentioned—the discovery of the new planet Eros on a great number of plates taken before the existence of the body was known. Another result has been the discovery of several so-called new stars which have suddenly blazed forth, although, up to the present time, none have become objects that would strike the ordinary spectator. Another speciality of this observatory has been the study of stellar spectra by photography. In this connexion, the aid afforded by the Draper Memorial Fund is worthy of mention. The wealth of material thus accumulated is so vast that its complete publication is scarcely possible. Yet every spectrum photographed is subjected to careful scrutiny, and any remarkable peculiarity that may be exhibited is published and discussed. Photometry has been one of the main branches of Professor Pickering’s work, and the volumes already published contain estimates of the magnitudes of stars measured photometrically, now including the entire heavens in the scope of their contents. Two other important photometric works have appeared which are especially noteworthy, because made with instruments radically different from each other and from that of Pickering. The photometric work of the late Professor Charles Pritchard, F.R.S., at Oxford, was carried out by the use of the extinction wedge, of which a description will be found in the article Photometry, Stellar. The same subject has been a leading feature of the work at the Astrophysical Observatory at Potsdam. The founding of this institution by the German Government soon after the close of the Franco-German war marked an epoch in the development of astrophysics, both because of being contemporaneous with the beginning and early growth of that science and because of the wisdom which has marked its administration. In stellar photometry the special feature of its work has been a very careful determination, by two observers, of a list of stars more limited than that investigated at the Harvard Observatory. The field to be covered includes the
