The German term Durchmusterung, now natu ralized in scientific English, was first applied to Argelander's (q.v.) great catalogue of stars in the northern half of the sky, a work which has rendered possible many statistical and other researches of far-reaching importance in stellar astronomy. But Argelander and his successor, Schlinfeld, observing at Bonn, in Germany, were able to carry on their survey only a short way beyond the northern half of the sky. Gill, in his observatory at the Cope of Good Hope, far south of the equator, saw the possibility of extending Argelander's work, by means of pho tography, to the South Pole. To have so ex tended it by the visual methods that were used by Argelander would have required an enor mous expenditure of time and labor; but a small photographic telescope was procured, and, iu the hands of C. Bay Woods, a scientific pho tographer who was summoned from England, a complete collection of plates of the southern sky was Made. The negatives were sent to Groningen, Holland, and were there measured by Kapteyn. The resulting catalogue, pub lished in three large volumes by the British Government, is found to possess even greater completeness than Argelander's; for the fallible human eye could not avoid omitting a star now and then, while the photographic plate is of coin-se subject to no such error.
The Cape Durchmusterung is, however, not the only great result to which Gill's 1882 comet photograph has led. The Durchmusterung was carried out much on the same lines as Arge lander's older work in the Northern Hemisphere, the idea being to make a census of all the stars in the sky down to a given magnitude, even if it should be impossible to give more than a rough approximation of their positions in the sky. But the success of the first enterprise led Gill to conceive the idea of preparing a new star catalogue that should satisfy the condition of high precision as well as that of complete ness as to numbers. So vast an undertaking could be rendered possible only by the com bined efforts of many astronomers and many observations, and, as the result of preliminary correspondence. a meeting to consider the sub ject was called at Paris by the French Go•ern ment in 1887. Delegates from all the civilized nations attended, and it was decided that the proposed photographic' catalogue of precision should really be made, that it might stand for all time as the foundation of research in sid ereal astronomy. Eighteen observatories have
since been engaged in carrying out this astro nomical enterprise. the greatest the world has ever seen. To make error of any kind practi cally impossible. the photographs are taken in duplicate. the total number of plates required being 44,108, each representing a surface of four square degrees. All stars down to the eleventh magnitude. numbering about 2,000,000, will thus be subjected to precise measurement. A fur ther series of plates is being made for the myriads of stars of still lower brilliancy. But as their precise measurement would involve an amount of labor surpassing the possibilities even of elaborate governmental cohperatiou, they will be merely reproduced in charts for the use J f astronomers who may need them in their special researches.
We have already called special attention to the pioneering work of Rutherfurd in the pho tography of stellar clusters, etc. It may be of interest also to mention some other American as tronomers who have distinguished themselves in the new methods of observation. Probably the first really successful celestial photograph was made in 1840 by Draper. He was able to obtain negatives of the moon, showing the prin cipal formations of her surface. Ten years later, in 1850, Bond and Whipple made still better moon pictures at Harvard College, Cam bridge, Mass. Coming clown to more recent years, we find among American photographic astronomers, Pickering, Avho has done much in the photography of stellar spectra and in other departments of astrophysical photography; Hale, who has succeeded in photographing the solar prominences and facuhe in full sunlight; and Campbell, who has determined photo graphically the velocities of motion in many of the stellar systems.
More detailed information on the subject of astro-photography may be found in the Bulletin du Corn ire permanent de la carte du riel (Paris, Institut de Frame, Vols. I. and II.), and in Seheimer's Photographic der Gestirne (Leipzig, 1898).