During the last quarter of the Seventeenth Century, the telescope assumes importance and two great observatories begin their work. In 1670 the Paris Observatory, of which Cassini was made director, was finished, and five years later the Greenwich Observatory, where Flamsteed was installed as royal astronomer. Of Cassini, Lalande remarks that under him Astronomy underwent revolutions, and in France he was regarded as the “creator of the science.” Cassini discovered that Saturn’s ring was double and found four satellites of Jupiter. Flamsteed’s observations on planets, satellites, comets, “fixed stars,” and his catalogue of 2,884 stars were valuable contributions to science; and his _Historia Cœlestis_ is said to have “formed a new era in sidereal astronomy.” Flamsteed was succeeded by Halley, particularly famed for his investigations of comets. The next great astronomical event was the discovery of Uranus by Sir William Herschel in 1781. Sir William Herschel also discovered two more of Saturn’s satellites, and began the great work of resolving the Milky Way and other clusters into swarms of suns, single stars into double and triple stars, inquiries into the mysteries of the nebulæ, and in every way enlarging the general conception of the sidereal universe. To the end of the Eighteenth and beginning of the Nineteenth Centuries belongs the brilliant French astronomer and mathematician Laplace, who published in 1799-1808 his _Mécanique Céleste_, in which he announced his Nebular Hypothesis (described on page 433 of Vol. II. The discoveries of the Planetoids are described on pages 396-403, and that of Neptune in 1846 on pages 430-432). The latest important additions to the Solar System are the discovery by Prof. Barnard of Jupiter’s Fifth Satellite in 1892 and Saturn’s Ninth by Prof. W. H. Pickering in 1904. The discovery even of a Seventh Satellite of Jupiter has just been announced from the Lick Observatory. It would be impossible to mention the names of the astronomers whose work from the middle of the last century to its closing years has been distinguished in various fields. Space only permits brief mention of the new methods of research by means of the spectroscope and celestial photography. With the first the name of the English astronomer, William Huggins, is identified and has yielded most important and startling information regarding the composition of heavenly bodies, and with the application of the photographic telescope these new methods have created a revolution in astronomical observation. It may be interesting to gain a slight idea of the numbers of stars revealed by the camera by referring to Sir Robert Ball: