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LEFT ASTRONOMY 319 ASTRONOMY the distinguishing letter "H," is the greatest of the results of his labors. Newton's fame rests on his discovery of the law of gravitation, announced in the "Principia" in 1677. Newton also made the important discovery of the revolution of comets around the sun in conic sections, proved the earth's form to be that of an oblate spheroid, gave a theory of the moon and tides, invented fluxions and wrote on optics. While the foundations of gravitational astron- omy were thus broadly laid by Newton, Flamsteed, the first astronomer-royal at Greenwich, and Halley were greatly im- proving and extending the practical de- partment of the science. To Flamsteed we are indebted for numerous observa- tions on the fixed stars, on planets, sat- ellites and comets, and for a catalogue of 2,884 stars. His "Historia Ccelestis" formed a new era in sidereal astron- omy. Dr. Halley, who succeeded Flam- steed as astronomer-royal, discovered the accelerated mean motion of the moon, and certain inequalities in Jupiter and Saturn, but he is most famed for his successful investigations into the motions and nature of comets. His suc- cessor was Dr. Bradley, who, in the year of Newton's death, made the im- portant discovery of the aberration of light, which furnishes the most con- clusive proof we have of the earth's an- nual motion. While Bradley was at work at Green- wich, at the middle of the 18th century, Lacaille, a celebrated French astrono- mer, undertook a voyage to the Cape of Good Hope to determine the sun's par- allax, by observations of Mars and Venus simultaneously with similar ones in Europe, and to form a catalogue of southern circumpolar stars. In a sin- gle year and single-handed he observed the positions of over 10,000 stars and computed the places of 1,942 of them. The latter half of the 18th century was marked by the brilliant work of Sir William Herschel, who discovered the planet Uranus and its four satellites, and two additional satellites of Saturn; determined the direction of the motion of the solar system in space; resolved the Milky Way into countless myriads of stars, and opened up a boundless field of discovery and research among the nebulae and double and multiple stars. Maskelyne perfected the method of reducing observations of lunar distances at sea for the determination of longi- tudes, and had tables of lunar distances first published in the British "Nautical Almanac." Lalande observed the posi- tions of by far the largest number of stars that had been catalogued up to the end of the 18th century. These wfere afterward reduced and published by Baily in a catalogue which contains over 47,000 star positions. Mayer, besides making a valuable catalogue of zodiacal stars at about the same time as those of Bradley and Lacaille, perfected lunar tables which were for many years the most accui'ate in existence. The 18th MT. WILSON OBSERVATORY and 19th centuries were astronomically connected by the work of Piazzi in the observatory established at Palermo in 1790, where on the night of Jan. 1, 1801, a new planet, the first of the numerous belt of planetoids between Mars and Jupiter, was discovered. Modern Astronovvy. — Friedrich Wil- helm Bessel contributed more than any other to the solid advancement of the science in the 19th century. Bessel com- bined in an extraordinary degree the qualities of an able mathematician and a skillful observer. Before mentioning particularly any of the other prominent astronomers of the early part of the 19th century, the celebrated optician Fraunhofer, who contributed so much to their success, deserves special notice. In connection with his experiments on light for the further perfection of his lenses, Fraunhofer was led to the discovery of