WILLIAM PARSONS ROSSE, 3RD Earl Of (1800-1867), Irish astronomer and telescope constructor, was born at York on the 17th of June 1800, a son of the 2nd earl (see above). 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 representative peer from 1845, president of the British Association in 1843, president of the Royal Society from 1849 to 1854, and chancellor of the university of Dublin from 1862. From 1827 he devoted himself to the improvement of reflecting telescopes; in 1839 he mounted a telescope of 3 ft. aperture at his seat, Birr Castle, Parsonstown; and in February 1845 his celebrated 6-foot reflector was finished. Owing to the famine and the disturbed state of the country, which demanded his attention as a large landowner and lieutenant of King's County (from 1831), the instrument remained unused for nearly three years, but since 1848 it has been in constant use, chiefly for observations of nebulae, for which it was particularly suited on account of its immense optical power, nominally 6000. Lord Rosse died at Monkstown on the 31st of October 1867. He had four sons. The eldest, Lawrence Parsons, 4th earl of Rosse, and Baron Oxmantown, born on the 17th of November 1840, succeeded to the title on his father's death, and made many investigations on the heavenly bodies, particularly on the radiation of the moon and related physical questions; the youngest, the Hon. Charles Algernon Parsons, born on the 13th of June 1854, is famous for his commercial development of the steam turbine.
The first constructor of reflecting telescopes on a large scale, William Herschel, never published anything about his methods of casting and polishing specula, and he does not appear to have been very successful beyond specula of 18 in. diameter, his 4-foot speculum ("the 40-foot telescope") having been little used by him (see discussion between Sir J. Herschel and Robinson in The Athenaeum, Nos. 831-36, 1843). Lord Rosse had therefore 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, which resists tarnish better than any other compound tried. 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 (sixteen for a 3-foot mirror) soldered on the back of a strong but light framework made of a peculiar kind of 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. It imitates the motions made in polishing a speculum by hand by giving both a rectilinear and a lateral motion to the polisher, while the speculum revolves slowly; by shifting two eccentric pins the course of the polisher can be varied at will from a straight line to an ellipse of very small eccentricity, and a true parabolic figure can thus be obtained. The speculum lies face upwards in a shallow bath of water (to preserve a uniform temperature), and the polisher fits loosely in a ring, so that the rotation of the speculum makes it revolve also, but more slowly. Both the grinding and polishing tools are grooved, to obtain a uniform distribution of the emery used in the grinding process and of the rouge employed in polishing, as also to provide for the lateral expansion of the pitch with which the polisher is coated. In September 1839 a 3-foot speculum was finished and mounted on an altazimuth stand similar to Herschel's; but, though the definition of the images was good (except that the diffraction at the joints of the speculum caused minute rays in the case of a very bright star), and its peculiar skeleton form allowed the speculum to follow atmospheric changes of temperature very quickly, Lord Rosse decided to cast a solid 3-foot speculum. Hitherto it had been felt as a great difficulty in casting specula that the solidification did not begin at one surface and proceed gradually to the other, the common sand mould allowing the edges to cool first, so that the central parts were subject to great straining when their time of cooling came, and in large castings this generally caused cracking. By forming the bottom of the mould of hoop iron placed on edge and closely packed, and the sides of sand, while the top was left open, Lord Rosse overcame this difficulty, and the hoop iron had the further advantage of allowing the gas developed during the cooling to escape, thus preventing the speculum from being full of pores and cavities. This invention secured the success of the casting of a solid 3-foot speculum in 1840, and encouraged Lord Rosse to make a speculum of 6 ft. diameter in 1842. In the beginning of 1845 this great reflector was mounted and ready for work. The instrument has a focal length of 54 ft. and the tube is about 7 ft. in diameter; owing to these large dimensions it cannot be pointed to every part of the heavens, but can only be moved a short distance from the meridian and very little to the north of the zenith; these restrictions have, however, hardly been felt, as there is almost at any moment a sufficient number of objects within its reach.
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 remarkable "spiral" configuration prevailing in many of the brighter 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.) Lord Rosse gave a detailed account of the experiments which step by step had led to the construction of the 3-foot speculum in the Philosophical Transactions for 1840. In the same publication for 1844 and 1850 he communicated short descriptions and drawings of some of the more interesting nebulae, and in the volume for 1861 he published a paper "On the Construction of Specula of 6-ft. Aperture, and a Selection from the Observations of Nebulae made with them," with numerous engravings. The accounts of the observations given in these papers, however, were fragmentary; but in 1879-80 a complete account of them was published by the present earl ("Observations of Nebulae and Clusters of Stars made with the 6-foot and 3-foot Reflectors at Birr Castle from 1848 to 1878") in the Scient. Trans. R. Dublin Soc. vol. ii. The drawing of the nebula of Orion was published in the Phil. Trans. for 1868.
See obituary notice in the Proc. Roy. Soc. (1868), 16, 36, and in the Monthly Notices of Roy. Astr. Soc. vol. 29, p. 123.
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