On March 28, 1802, while observing Ceres from his house at Bremen, Olbers was struck by the presence of a strange object near the path of the planet. At first he supposed it to be a variable star at maximum brilliance, but a few hours showed him that it was in motion, and was therefore another planet. He named it Pallas, and propounded the theory that the two “Asteroids”—so named by Herschel—were fragments of a trans-Martian planet, which, through some accident, had been shattered to pieces in the remote past. Olbers urged the necessity of searching for more small planets. His advice was taken. In 1804 _Karl Ludwig Harding_ (1765-1834), Schröter’s assistant, discovered Juno, and Olbers himself detected Vesta, March 29, 1807. After 1816 the search was relinquished, as no more planets were discovered. In 1830, however, a German amateur, _Karl Ludwig Hencke_ (1793-1866), ex-postmaster of Driessen, commenced a search for new planets, which was rewarded, after fifteen years, by the discovery of Astræa, December 8, 1845. On July 1, 1847, he made another discovery, that of Hebe. A few weeks later, _John Russell Hind_ (1823-1895), the English astronomer, discovered Iris. Since 1847 not a year has passed without one or more planets being found, sometimes as many as twenty being discovered in a single year. Some astronomers have made the search for asteroids their chief business. The principal asteroid discoverers have been _Christian H. F. Peters_ (1813-1890), Henri Perrotin, _Paul Henry_ (1848-1905), _Prosper Henry_ (1849-1903), James Watson, _Robert Luther_ (1822-1900), _Johann Palisa_ (born 1848), and _Max Wolf_ (born 1863). In 1891 a new impulse was given to asteroid study by the application of photography by Max Wolf to the discovery of the minor planets. It occurred to Wolf that the asteroid would be represented on the plate by a trail, caused by its motion during the time of exposure; and assisted by _Arnold Schwassmann_ (born 1870), _Luigi Carnera_ (born 1875), and others, Wolf has discovered over a hundred asteroids, and he has the whole field of asteroid hunting to himself. Few minor planets are now discovered by the older method. In 1901 Wolf invented his new instrument of research, the stereo-comparator, which, on the principle of the old-fashioned stereoscope, represents the planetary bodies as suspended in space far in front of the stars. In this way this ingenious astronomer has been enabled to discover asteroids at the first glance: year by year fresh discoveries are announced from the Heidelberg Observatory, until more than five hundred asteroids are now known.