Closely allied to the question of parallax is the determination of the exact positions of the stars and the formation of star-catalogues. In this branch, too, much is due to the genius of Bessel. The observations of Bradley at Greenwich from 1750 to 1762 were reduced by Bessel into the form of a catalogue, which was published in 1818, with the title of ‘Fundamenta Astronomiæ.’ During the years 1821 to 1823 Bessel took 75,011 observations, by which he brought up the number of accurately known stars to 50,000. At the same time notable catalogues had been constructed, particularly by the English astronomer, _Francis Baily_ (1774-1844), and by _Giovanni Santini_ (1786-1877), director of the observatory at Padua; but Bessel’s successor in this branch of research was _Friedrich Wilhelm August Argelander_ (1799-1875). In 1821 he became assistant to Bessel at Königsberg, in 1823 director of the Observatory at Abo, in Finland, and in 1837 of that at Bonn. Here he commenced in 1852 the great ‘Bonn Durchmusterung,’ a catalogue and atlas of 324,198 stars visible in the northern hemisphere. The great catalogue was published in 1863. After Argelander’s death it was extended so as to include 133,659 stars in the southern hemisphere, by his assistant _Eduard Schönfeld_ (1828-1891), who succeeded him in 1875 as director of Bonn Observatory, where he died in 1891. Meanwhile a greater undertaking was commenced in 1865 by the Astronomische Gesellschaft. This was the co-operation of thirteen observatories in Europe and America for the exact determination of the places of 100,000 of Argelander’s stars. In the southern hemisphere, working at Cordova in Argentina, was the great American astronomer, _Gould_, whose ‘Uranometria Argentina,’ published in 1879, gives the magnitudes of 8198 stars, and whose Argentine General Catalogue, containing reference of 32,448 stars, was published in 1886. The late Radcliffe observer, Stone, published a useful catalogue in 1880 from his observations at the Cape. The application of photography to the work of star-charting dates from 1882, when Gill photographed the comet of 1882, and was struck with the distinctness of the stars on the background. For some time he had contemplated the extension of the ‘Durchmusterung,’ from the point where Schönfeld left it, to the southern pole, and the idea struck him to utilise photography for the purpose. In 1885, accordingly, Gill commenced work, and in four years all the photographs were taken. The reduction of the observations into the form of a catalogue was spontaneously undertaken by the great Dutch astronomer, Kapteyn, who was occupied with the work for fourteen years, until in 1900 the great catalogue, known as the ‘Cape Photographic Durchmusterung,’ was completed. Half a million stars are represented on the plates taken at the Cape.