Parsons, William, Third Earl of Rosse

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(b. York, England, 17 June 1800; d. Monkstown, Ireland, 31 October 1867)


William Parsons was the eldest son of Lawrence Parsons, second Earl of Rosse, and a descendant of the Sir William Parsons who had gone to Ireland in the sixteenth century. Prior to the death of his father, in 1841, he held the title Lord Oxmantown, under which style some of his scientific papers were published. (His own eldest son, Lawrence Parsons, held the same succession of titles; since he too was an astronomer, this has given rise to some confusion.) He received his early education privately at Birr Castle, the family seat, then in 1818 went for a year to Trinity College, Dublin. He next attended Magdalen College, Oxford, matriculating in 1821 and graduating with first-class honors in mathematics in 1822.

Since the nobility of his time took their responsibilities seriously, it was expected that Oxmantown would follow his father’s example and take his place in the Irish government. As an undergraduate, in 1821, he was returned as a member of parliament for King’s County, a seat that he held until 1834. He proved to be an able political economist and an effective committee member. He was appointed to further civil duties; in 1831 he was named lord lieutenant of County Offaly, in which Birr is situated, and in 1834 he became colonel of the local militia. In 1845, as the earl of Rosse, he was elected Irish representative peer and sat in the English House of Lords.

Rosse’s scientific achievements were all the more remarkable in light of his activities as an administrator and public servant. His chief contributions were to astronomical instrumentation, particularly the design and construction of large telescopes. He early realized the need for instruments of greater aperture and light-grasp than were provided by William Herschel’s forty-eight-inch aperture telescope of 1789; his own experiments, which he began in about 1826, were first concentrated on instruments incorporating the new optically excellent small-aperture Fraunhofer refractors. Having without success investigated the possibility of devising large fluid lenses, he was soon convinced that large apertures could be achieved only with reflectors. He therefore took up the search for an appropriate material for casting large mirrors and, after a number of experiments, decided to use an alloy of four parts of copper and one of tin. This alloy was both harder and more brittle than steel, it crystallized easily, and thus casting it was difficult. Rosse first tried making sectional mirrors composed of annular rings surrounding a central disk, all soldered to a brass disk having the same coefficient of expansion, but these proved ineffective in instruments of greater than eighteen-inch aperture.

Rosse was thus forced to develop a technique for casting solid disks. Having designed a mold ventilated so as to permit the mirror to cool evenly all over in an annealing oven, he finally achieved his aim. He completed a sectional thirty-six-inch speculum in 1839 and a superior solid mirror of the same size in 1840. In 1842 he cast the first seventy-two-inch disk, which, mounted in the meridian between two brick walls nearly sixty feet high, became known as the “Leviathan of Parsonstown” (Parsonstown being an old name of Birr). The telescope was completed in 1845; it had a focal length of fifty-four feet (with a nominal maximum magnification of 6,000), and a tube about seven feet in diameter. The mirror itself weighed nearly four tons, and its flexure under gravitation was controlled by twenty-seven felt-covered cast iron platforms. While it was not completely maneuverable, it was mounted so that a considerable portion of the sky was visible, and Rosse and his collaborator, the Reverend Thomas Romney Robinson, were able to utilize it to carry out, especially between 1848 and 1878, a number of important observations of nebulae.

With the new telescope Rosse and his co-workers were able to see hitherto unsuspected detail in many hundreds of nebulae, and to resolve many of these nebulae into stars. They abolished some of the existing distinctions (annular/planetary, for example) and added some new classes. Rosse himself was the first to detect the spiral nature of some nebulae, of which he published a number of fine drawings that clearly demonstrated the value of a large reflector of high optical quality.

In addition to overcoming the problems inherent in casting large solid mirrors (he eventually cast one of eighty-four inches), Rosse devised improvements in grinding and polishing techniques. Although he had initially believed that only hand finishing would be delicate enough to give a good conformation, he found this to be incorrect. He then designed an apparatus in which the mirror was rotated horizontally in a water bath (for constant temperature) beneath a grinding and polishing tool that could be moved in either a straight line or an ellipse of any centricity. The mirror could be tested in situ by observing its image in a watch dial fixed some fifty feet above it. The machine was driven by steam and was widely copied. Rosse also designed and executed a simple but effective clockwork drive for a large (eighteen-inch) equatorial. His interests extended further to the building of iron-armored ships, on which some correspondence is printed in his Scientific Papers He took some of the earliest lunar photographs.

Rosse was married in 1836 to a Yorkshirewoman Mary Field; they had four sons. He was president of the British Association at its 1843 meeting in Cork in 1852 he served as chancellor of Trinity College Dublin, and he was members of the board of visitors of both Greenwich Observatory and Maynooth College. Following the potato famine of 1846 Rosse devoted the major part of the rents from his Irish properties to alleviating the poverty of the local inhabitants; he was held in great affection by his tenants, some 4,000 of whom attended his funeral.


I. Origignal Works. All of Rosse’s scientific papers have been reprinted in Chalres Parsons, ed., The Scientific Papers of William parsons, Third Earl of Rosse 1800–1867 (London, 1926). Rosse’s chief publications are “An Account of Experiments on the Reflecting Telescope,” in Philosophical Transactions of the Royal society130 (1840), 503–528, “ Observations of some of the Nebuale, “ ibid., 134 (1844), 321–323; “Observations of the Nebulae,” ibid., 140 o(1850), 499–514; and “On the Constructions of specula of Six-Feet Aperture; and a selection From the Observations of Nebulae Made With Them, “ibid., 151 ’ 1861), 381–754

II. Secondary Literature. Two useful notes on Rosse are in Proceedings of the Royal Society16 (1868) xxxvi–xlii; and Monthly Notices of the Royal Astronomical Society29 (1869), 123–130. See also J. D. North , The Measure of the Universe (Oxford, 1965), esp. ch. 1.

J. D. North

Colin A. Ronan

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Parsons, William, Third Earl of Rosse

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