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in optics and chemistry and continued his mathematical speculations. From this forced retirement in 1666 he dated his discovery of the gravitational theory: "In the same year I began to think of gravity extending to the orb of the Moon, . . . compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the earth and found them to answer pretty nearly." At about the same time his work on optics led to his explanation of the composition of white light. Of the work he accomplished in these years Newton later remarked: "All this was in the two years of 1665 and 1666, for in those years I was in the prime of my age for invention and minded Mathematics and Philosophy more than at any time since."

374 HOW TO READ A BOOK

On the re-Qpening of Trinity College in 1667, Newton was elected a fellow, and two years later, a little before his twentyseventh birthday, he was appointed Lucasian professor of mathematics, succeeding his friend and teacher, Dr. Barrow. Newton had already built a reflecting telescope in 1668; the second telescope of his making he presented to the Royal Society in December, 1671. Two months later, as a fellow of the Society, he communicated his discovery on light and thereby started a controversy which was to run for many years and to involve Hooke, Lucas, Linus, and others. Newton, who always found controversy distasteful, "blamed my own imprudence for parting with so substantial a blessing as my quiet to run after a shadow." His papers on optics, the most important of which were communicated to the Royal Society between 1672 and 1676, were collected in the Optics ( 1704 ) .

It was not until 1684 that Newton began to think of making known his work on gravity. Hooke, Halley, and Sir Christopher Wren had independently come to some notion of the law of gravity but were not having any success in explaining the orbits of the planets. In that year Halley consulted Newton on the problem and was astonished to find that he had already solved it. Newton submitted to him four theorems and seven problems, which proved to be the nucleus of his major work. In some seventeen or eighteen months during 1685 and 1686 he wrote in Latin the Mathematical Principles of Natural Philosophy. Newton thought for some time of suppressing the third book, and it was only Halley's insistence that preserved it. Halley also took upon himself the cost of publishing the work in 1687 after the Royal Society proved unable to meet its cost. The book caused great excitement throughout Europe, and in 1689 Huygens, at that time the more famous scientist, came to England to make the personal acquaintance of Newton.

While working upon the Principles, Newton had begun to take a more prominent part in university affairs. For his opposition to the attempt of James II to repudiate the oath of allegiance and supremacy at the university, Newton was elected parliamentary member for Cambridge. On his return to the university, he suffered a serious illness which incapacitated him for most of 1692

and 1693 and caused considerable concern to his friends and fellow workers. After his recovery, he left the university to work for the government. Through his friends Locke, Wren, and Lord Appendix B 375

Halifax, Newton was made Warden of the Mint in 1695 and four years later, Master of the Mint, a position he held until his death.

For the last thirty years of his life Newton produced little original mathematical work. He kept his interest and his skill in the subject; in 1696 he solved overnight a problem offered by Bernoulli in a competition for which six months had been allowed, and again in 1716 he worked in a few hours a problem which Leibniz had proposed in order to "feel the pulse of the English analysts." He was much occupied, to his own distress, with two mathematical controversies, one regarding the astronomical observations of the astronomer royal, and the other with Leibniz regarding the invention of calculus. He also worked on revisions for a second edition of the Principles, which appeared in 1713.

Newton's