telescope, Herschel was enabled to take the whole field of observational astronomy for his province. He enormously extended knowledge of the Sun; and his solar theory, untenable though it proved, was the first attempt to systematise and co-ordinate the known facts concerning the Sun. His work on the various worlds of the Solar System marked the foundation of modern planetary topography. His paper on Mars in 1784 constituted Martian astronomy a distinct branch of the science. The scientific study of the Saturnian system may be said to have begun with him; he was the author of the first serious attempt to explain the atmospheric phenomena on Jupiter. He not only discovered Uranus—and thus prepared the way for the discovery of Neptune—but he persistently scrutinised its disc until he ascertained its size and mass. His discoveries of satellites did not exhaust his work on the secondary systems. His remarkable intuition that the rotation periods of satellites are equal to their times of revolution was one of the flashes of his genius.
In stellar astronomy he discovered binary stars, and thus proved what others had suspected—that the law of gravitation was of universal application. With consummate skill and audacity he attacked the highly difficult and elusive problem of the solar motion, and successfully measuring its rate and direction, he demonstrated further the essential kinship of Sun and stars. His efforts to know the construction of the heavens were unsuccessful, but he laid the foundation of a new branch of astronomy—that dealing with the distribution and motions of the stars; and in addition discovered suns and worlds in process of formation. Herschel was the first student of nature to point to evolution in progress and to classify natural objects in evolutionary array.
Above all, Herschel enormously widened the mental horizon of man. His researches extended the universe
