Page:Dictionary of National Biography volume 38.djvu/145

 under the tuition of Dr. William Palliser [q. v.], afterwards archbishop of Cashel (, Dublin Univ. p. 377). Having graduated B.A. he quitted the university with credit, and proceeding to London entered the Middle Temple as a student of law on 23 June 1675. The heir to an easy fortune, and having no particular predilection for law, he devoted himself chiefly to philosophy and applied mathematics. In June 1678 he returned to Dublin, and with his father's consent married, on 19 Sept., Lucy, youngest daughter of Sir William Domvile, attorney-general of Ireland. Mrs. Molyneux was a lady of remarkable beauty and of an amiable disposition, but unfortunately, only three months after her marriage, she was attacked by an illness which not only deprived her of sight, but until her death, thirteen years later, caused her intolerable pain. Molyneux himself suffered from an hereditary affection of the kidneys, which seriously interfered with his enjoyment of life, and was eventually the cause of his premature death.

After some time spent in England in the vain endeavour to obtain medical relief for his wife, Molyneux settled down in Dublin. He resumed his philosophical studies, and during the winter of 1679 he made an English version of Descartes's 'Meditations,' which was published in London in April 1680. His interest in optics and astronomy was stimulated by a correspondence which he opened with John Flamsteed [q. v.], astronomer royal, in 1681. This intercourse continued till 1692, when, according to Molyneux, Flamsteed broke off relations with him owing to some offence Molyneux had given him in his 'Dioptrica Nova.' In the summer of 1682 he was engaged in collecting materials for a 'Description of Ireland,' to form part of Moses Pitt's 'Atlas;' it was never published owing to Pitt's failure to carry out his project. Among others with whom he in this way became acquainted was Roderick O'Flaherty [q. v.], whom he assisted in the publication of his 'Ogygia,' and Peter Walsh [q.v.], to whom he owed an introduction to the Duke of Ormonde. His interest in science, and the example furnished by the Royal Society, led him to take an active part in the foundation in 1683 of the Dublin Philosophical Society, the precursor of the Royal Irish Academy, of which he was the first secretary, and Sir William Petty [q. v.], the first president.

By the influence of the Duke of Ormonde Molyneux was in 1684 appointed, jointly with (Sir) William Robinson, chief engineer and surveyor-general of the king's buildings and works, in which capacity he built that part of Dublin Castle which stands upon the Piazza, with the turrets to the south; but he was ejected from office in 1688 by Tyrconnel on account of his religion. In 1685 he was elected a fellow of the Royal Society; and it being his intention that summer to visit his brother, Thomas (afterwards Sir Thomas) Molyneux [q. v.], at Leyden, he received a concordatum of 100l. from the Irish government to enable him to view and make draughts of the principal fortresses in Flanders. He left Dublin on 13 May, and meeting at Calais Viscount Mountjoy he travelled with him through the Netherlands and parts of Germany and France, including Paris, where, by means of letters of recommendation from Flamsteed, he made the acquaintance of the astronomer Cassini and other eminent men of science.

He returned to Ireland at the end of September, and was almost immediately prostrated by a severe illness. Early in the following year (1686) he published his 'Sciothericum Telescopicum: or, A New Contrivance of adapting a Telescope to a Horizontal Dial,' with a dedication to the lord-lieutenant, the Earl of Clarendon, in which he raised the question 'whether the natural philosophy formerly professed in the schools or that which is at present prosecuted by the societies lately instituted in several of the most noted parts of Europe be the true philosophy or method of investigating nature?' The telescopic dial itself never came into general use, and was practically condemned by Flamsteed. On the appearance of Sir Isaac Newton's 'Principia' in 1687 Molyneux candidly admitted that his knowledge of mathematics was not sufficient to enable him to understand it. Becoming alarmed at the policy of proscription pursued by Tyrconnel, and dreading a repetition of the horrors of 1641, he retired on 31 Jan. 1689, with his wife, to Chester, where he resided in a little house outside the north gate for nearly two years. There he wrote the greater part of his 'Dioptrica Nova,' in which he was assisted by Flamsteed. The book, which was for a long time the standard work on optics, was published at London in 1692, the sheets being revised by Edmund Halley [q. v.] the astronomer, who, at Molyneux's request, allowed his celebrated theorem for finding the foci of optic glasses to be printed in the appendix. A passage in the Epistle Dedicatory in warm commendation of Locke's 'Essay on the Human Understanding' obtained grateful acknowledgment from that philosopher, and was the beginning of a long and friendly correspondence between them (see Some Familiar Letters between