_In 1665 he discovered his method of fluxions, not greatly unlike Leibnitz’s Differential Calculus. In 1672 he was elected a fellow of the Royal Society and shortly afterwards sent them a paper describing how he had broken up light by means of a prism, demonstrating by that means the compound nature of the sun’s rays._ _In 1687 elaborated his theory of gravitation in “Philosophiæ Naturalis Principia Mathematica.” This was the result of his reflections and researches dating from 1666, when he attempted to explain the moon’s motion by the hypothesis of the assumed influence of gravitation. In the meantime, through the use of telescopic instruments, French geographers had tested the spherical shape of the earth and had made a new and more accurate triangulation. Using the data which they supplied, Newton perceived that these data agreed with his theory that the force varied inversely as the square of the distance. Overcome with the emotion incident to the solution of a great problem, he begged a friend to complete his calculations, with the result that the new astronomy begun by Copernicus, and continued by Brahe, Kepler, and Galileo, was formulated and mathematically interpreted by a single mechanical principle._ _Although he later made some chemical investigations, his papers were accidentally destroyed, and it is said that he never recovered from the shock of losing them. In 1695 he was made warden, and in 1699 promoted to the mastership of the mint, which office he retained at a munificent salary until his death on March 20, 1727._ THE THEORY OF GRAVITATION[11] BOOK III. PROPOSITION V. THEOREM V. SCHOLIUM The force which retains the celestial bodies in their orbits has been hitherto called centripetal force; but it being now made plain that it can be no other than a gravitating force, we shall hereafter call it gravity. For the cause of that centripetal force which retains the moon in its orbit will extend itself to all the planets. BOOK III. PROPOSITION VI. THEOREM VI. _That all bodies gravitate towards every planet; and that the weights of bodies towards any the same planet, at equal distances from the centre of the planet, are proportional to the quantities of matter which they severally contain._