William, a learned mathematician, was descended from a family of fortune, and was born at Dublin in April 1656. He entered the university of his native city in 1671, and after graduating as B.A., he repaired to London in 1675 to study law at the Middle Temple. The greater part of his time, however, was devoted to philosophy and mathematics. On his return to Ireland in 1678, his independent fortune enabled him to marry, and to devote all his time to the cultivation and advancement of his favourite studies. It was at his suggestion that the Dublin Philosophical Society was instituted in 1683. He became its first secretary; and by the zeal and talent with which he discharged this office, he speedily attracted the notice of some of the most influential men in the country. The consequence was, that in 1685 he was appointed by the Irish government to inspect the fortresses in Flanders; and in the following year he was elected a member of the Royal Society of London. During the commotions that prevailed in Ireland in 1689 and 1690, Molyneux was in England engaged in completing his principal work, the treatise on Optics. In 1692 it was published in 4to at London, under the title of Dioptrica Nova, and with an appendix containing the theorem, newly discovered by Dr Halley, for finding the foci of optic glasses. Molyneux sat in the Irish Parliament in 1692 for the city, and in 1695 for the university of Dublin. He was a bosom friend of Locke, and while suffering under his last illness, he undertook a journey to England to visit that philosopher. His death took place in October 1698. Molyneux also wrote a Translation of the Six Metaphysical Meditations of Descartes, together with the objections against them by Thomas Hobbes, Lond., 1680, and twenty-seven papers on miscellaneous subjects in the Philosophical Transactions.