Waning interest in the ever-increasing family of asteroids was revived in 1898 by the discovery by _Karl Gustav Witt_ (born 1866) of a small planet, to which he gave the name of Eros, which comes nearer to the Earth than Mars, and which is of great assistance to astronomers in the determination of the solar parallax. For some time prior to 1898 astronomers had considered it a waste of time to search for new asteroids; but this idea is not now so popular, in view of the benefit conferred on astronomy by the discovery of Eros. Of the physical nature of the asteroids astronomers know nothing. Only the four largest have been measured. For many years it was supposed that Vesta, the brightest of the asteroids, was also the largest. The measures of Barnard with the great Lick refractor in 1895, however, showed that Ceres is the largest, with a diameter of 477 miles. Pallas comes next, with a diameter of 304 miles; while the diameters of Vesta and Juno are respectively 239 and 120 miles. Barnard saw no traces of atmosphere round any of the asteroids. It should be stated that in 1872 Vogel thought he could detect an “air-line” in the spectrum of Vesta: he admitted that the observation required confirmation, but it has not been corroborated either by himself or any other observer. CHAPTER VI. THE OUTER PLANETS. Jupiter, the greatest planet of the Solar System, has perhaps been more persistently studied by astronomers than any other. In the early nineteenth century the prevalent idea was that Jupiter was a world similar to the Earth, only much larger,—a view held by Herschel and other famous astronomers, and put forward by Brewster in ‘More Worlds than One.’ This view prevailed for many years, although Buffon in 1778, and Kant in 1785, had stated their belief in the idea that Jupiter was still in a state of great heat—in fact, that the great planet was a semi-sun. This idea, however, was long in being adopted by astronomers, and very little attention was paid to Nasmyth’s expression of the same opinion in 1853. The older view still held the field—namely, that the belts of Jupiter represented trade-winds, and that a world similar to the terrestrial lay below the Jovian clouds. In 1860 _George Philip Bond_ (1826-1865), director of the Harvard Observatory, found from experiments that Jupiter seemed to give out more light than it received, but he did not dare to suggest that Jupiter was self-luminous, considering that the inherent light might result from Jovian auroras.