BOSSE, WTI:LIAM PARSONS, third Earl of, a well-known practical astronomer, was born in York in 1800, and educated first at Trinity college, Dublin, and afterward at Magdalen college, Oxford, where he graduated first-class in mathematics in 1822. Dur ing the life of his father he sat in the house of commons as lord Oxmantown, represent ing King's county from 1821 to 1831; he succeeded to the peerage in 1841, and was elected a representative peer for Ireland in 1845. At an early age Rosse had devoted much attention to the study of practical science, and especially to the improvement of the telescope, and had commenced as far back as 1826 to make experiments in the construc tion of fluid lenses (see Philosophical Transactions for 1840), but he subsequently relin quished those investigations, to engage himself with the problem of the best mode of• the speculum of the reflecting telescope. The two great defects which had hitherto baffled opticians were "spherical aberration" and absorption of light by specula; and in the casting of these of large size, there was the apparent impossibility of preventing cracking and warping of the surface on cooling. However, by a long series of carefully conducted experiments, he succeeded in discovering a mode of operation by which the last defect was wholly obviated, and the two others greatly diminished in amount. The metal for the speculum of his great telescope (see TELESCOPE), three tons'
weight, was poured into the iron mold April, 1842, the crucibles being lifted and emptied by means of cranes: and the mold was kept in an annealing oven for 16 weeks, so that the metal should cool equably. It was then polished and mounted in his park at Parsonstown, at a cost of £30,000, the adjustments consisting of a system of chains, pulleys, and counterpoising weights, so complete in all its parts, that the ponderous instrument of 12 tons' weight can lie moved so as to point in any direction, and with almost as much precision as the ordinary equatorial of the observatory. The first addi tion to the body of astronomical knowledge made by this telescope was the resolution of certain nebulte, which had defied Herschel's instrument, into groups of stars; next callus the discovery of numerous binary and trinary stars, find a description of the moon's sur face. This telescope, constructed under Rosse's personal directions, is described in the Philosophical Transactions. Ile died in 1867, and a statue to lds memory was erected in Parsonstown in 1876.