Perhaps the most elaborate investigations on the structure of the Universe have been those of Kapteyn, commenced in 1891. In that year he demonstrated that stars are bluer and more easily photographed in the Galaxy than elsewhere, a discovery independently made by Gill at the Cape, and Pickering at Harvard. In 1893 Kapteyn announced his conclusions, derived from a novel method of studying the distance of the stars from their proper motions. In order to reach a definite idea of the distances of the stars, he made use of the component of the proper motion, measured at right angles to a great circle of the sphere which passes through a given star and the apex of the solar motion. He found that stars of the first spectral type have smaller proper motions than those of the second, indicating that stars of the second type are on the average nearer to the Solar System than those of the first, the near vicinity containing almost exclusively second-type stars. Kapteyn concluded that the group of second-type stars formed one system, named the solar cluster, which he considered to be roughly spherical in shape. In 1902 he abandoned this idea, retaining, however, his opinions as to the relative distances of the different types. That the second-type stars are nearer to the Sun than the first is, he remarked in a letter to the writer, incontrovertible. In the investigation of the motions in, and extent of, the Universe, the name of Simon Newcomb stands out pre-eminently. Born in 1835 at Wallace, in Nova Scotia, he went to the States in 1853. In 1862 he received an appointment at Washington Observatory, and he retained an official position until 1897. Throughout his scientific career he has been specially attracted by the question of the construction of the heavens, which he fully discussed in his book on ‘The Stars’ in 1901. Newcomb’s investigations have shown that some of the stars are not permanent members of the Sidereal System, among them the swiftly-moving 1830 Groombridge. He has shown that the Stellar Universe does not possess that form of stability which is seen in the Solar System. Newcomb considers the Universe to be limited in extent, as opposed to the opinions of Struve and others, who believed it to be infinite. He has brought clearly before his readers a calculation, based on the known law that there are three times as many stars of any given magnitude as of that immediately brighter, the increase of number compensating for the decrease of brilliance. Were the Universe infinitely extended, the whole heavens would shine with the brilliance of the Sun. Newcomb, therefore, concludes that “that collection of stars which we call the Universe is limited in extent.”