Page:Popular Science Monthly Volume 57.djvu/395

Rh situated in the southern hemisphere were too ill-endowed to permit of their undertaking a complete census of this invisible region.

The first considerable work emanating from the Cordoba Observatory, under Gould, was a catalogue of all the stars from the south pole to 10° of north declination which could be seen with the naked eye. Another work, which was not issued until after Dr. Gould's death, was devoted to photographs of southern clusters of stars.

The work of the Cordoba Observatory, with which we are more especially concerned in the present connection, consists of a 'Durchmusterung' of the southern heavens, commencing at 22° of south declination, where Schönfeld's work ended, and continued to the south pole. This work is still incomplete, but two volumes have been published by Thome, extending to 41° of south declination. It is expected that the third is approaching completion. This catalogue is, in one point at least, more complete than that of Argelander and Schönfeld, as it contains all the stars down to the tenth magnitude. The two volumes give the positions and magnitudes of no less than 340,000 stars, and therefore more than the catalogue of Argelander gives for the entire northern hemisphere. If the remaining part of the heavens, from 42° to the south pole, is equally rich, it will contain nearly half a million stars, and the entire work will comprise more than 800,000 stars.

The Royal Observatory of the Cape of Good Hope, under the able and energetic direction of Dr. David Gill, has undertaken a work of the same kind, which is remarkable for being based on photography. The history of this work is of great interest. In 1882 Gill secured the aid of photographers at the Cape of Good Hope to take pictures of the brilliant comet of that year, with a large camera. On developing the pictures the remarkable discovery was made that not only all the stars visible to the naked eye, but telescopic stars down to the ninth or tenth magnitude were also found on the negatives. This remarkable result suggested to Gill that here was a new and simple method of cataloguing the stars. It was only necessary to photograph the heavens and then measure the positions of the stars on the glass negatives, which could be done with much greater ease and certainty than measures could be made on the positions of the actual stars, which were in constant apparent motion.

As soon as the necessary arrangements could be made and the apparatus put into successful operation, Gill proceeded to the work of photographing the entire southern heavens from 18° of south declination to the celestial pole. The results of this work are found in the 'Cape Photographic Durchmusterung' a work in three quarto volumes, in which the astronomers of all future time will find a permanent record of the southern heavens towards the end of the nineteenth century. The actual work of taking the photographs extended from 1887 to 1891.