ROSSE, WILLIAM PARSONS, 3RD EARL OF (1800–I867), Irish astronomer and telescope constructor, was born at York on June 17, 1800, a son of the 2nd earl, Lawrence. Until his father's death he was known as Lord Oxmantown. Entered at Trinity college, Dublin, in 1818, he proceeded to Magdalen college, Oxford, in 1821, and in the same year he was returned as M.P. for King's County, a seat which he resigned in 1834. He was Irish repre sentative peer from 1845, president of the British Association in 1843, president of the Royal Society from 1849 to 1854, being awarded the Royal Medal in 1851, and chancellor of the Uni versity of Dublin from 1862. He died at Monkstown on Oct. 31, 1867.
The first constructor of reflecting telescopes on a large scale, William Herschel, never published anything about his methods of casting and polishing specula, and Lord Rosse had no help towards his brilliant results. His speculum metal is composed of four atoms of copper (126.4 parts) and one of tin (58.9 parts), a brilliant alloy. Chiefly owing to the brittleness of this material, Lord Rosse's first larger specula were composed of a number of thin plates of speculum metal (16 for a 3-foot mirror) soldered on the back of a strong but light framework made of a brass (2.75 of copper to 1 of zinc), which has the same expansion as his speculum metal. In Brewster's Edinburgh Journal of Science
for 1828 he described his machine for polishing the speculum, which in all essential points remained unaltered afterwards. In Sept. 1839 a 3-foot speculum was finished and mounted, but, though the definition of the images was good, its skeleton form allowed the speculum to follow atmospheric changes of tem perature very quickly, so Lord Rosse decided to cast a solid 3-foot speculum. Hitherto a great difficulty in casting specula was the fact that they generally cracked while cooling. Rosse experimented, ingeniously overcame this difficulty, and success fully cast a solid 3-foot speculum in 1840. In 1842 he began a speculum of 6 ft. diameter, and in 1845 this great reflector was mounted and ready for work.
From 1848 to 1878 it was but with few interruptions employed for observations of nebulae (see NEBULA) ; and many previously unknown features in these objects were revealed by it, especially the similarity of "annular" and "planetary" nebulae, and the re markable "spiral" configuration in many of the nebulae. A special study was made of the nebula of Orion, and the resulting large drawing gives an extremely good representation of this complicated object. (See TELESCOPE.) See Ball, Great Astronomers (London, 1895).