Rosse, William Parsons, Third Earl of (1800-1867), a British astronomer and engineer. His invention of a machine for grinding and polishing mirrors made it possible for him to construct a 72-inch (183-cm) reflecting telescope, the largest in the world when it was completed in 1845.
Parsons was born in England, the son of an Irish peer. After study at Dublin and Oxford universities, he served in the House of Commons, 1821-34. Rosse was president of the Royal Society, 1849-54, and chancellor of the University of Dublin, 1862-67.