In this temper he resumed his teles cope ; and observing that there was no absolute necessity for the parabolic fi gure of the glasses, since, if metals could be ground truly spherical, they would be able to bear as great apertures as men could give a polish to, he completed ano ther instrument of the same kind. This answering the purpose so well, as, though only half a foot in length, to show the planet Jupiter distinctly round, with his four satellites, and also Venus horh ed, he sent it to the Royal Society, at their request, together with a description of it, with further particulars ; which were published in the Philosophical Transactions for March, 1672. Several attempts were also made by that society to bring it to perfection; but for want of a proper composition of metal, and a Food polish, nothing succeeded, and the invention lay dormant till Hadley made his Newtonian telescope in 1728. At the request of Leibnitz, in 1676, he ex plained his invention of Infinite Series, and took notice how far he had improved it by his method of Fluxion, which, however, he still concealed, and particu larly on this occasion, by a transposition of the letters that make up the two fun damental propositions of it, into an al phabetical order ; the letters concerning which are inserted in Collins's " Com mercium Epistolicum," printed 1712. In the winter, between the year 1676 and 1677, he found out the grand proposi tion, that by a centripetal force acting reciprocally as the square of the distance, a planet must revolve in an ellipsis, about the centre of force placed in its lower focus, and, by a radius drawn to that centre, describe areas proportional to the times. In 1680 he made several astronomical observations upon the co met that then appeared ; which, for some considerable time, he took not to be one and the same, but two different comets; and upon this occasion several letters passed between him and Mr. Flamstead.
He was still under this mistake, when he received a letter from Dr. Hook, ex plaining the nature of the line described by a falling body, supposed to be moved circularly by the diurnal motion of the earth, and perpendicularly by the power of rarity. This letter put him upon en quiring anew what WAS the real figure in which such a body moved; and that en quiry convincing him of another mistake which he had before fallen into concern ing that figure, put him upon resuming his former thoughts with regard to the moon ; and Picart having not long before, -air. in 1679, measured a degree of the earth with sufficient accuracy, by using his measures, that planet appeared to be retained in her orbit by the sole power of gravity and, consequently, that this power decreases in the duplicate ratio of the distance ; as he had formerly conjec tured. . Upon this principle he found the line described by a falling body to be an ellipsis, having one focus in the centre of the earth. And finding by this means that the primary planets really moved in such orbits as Kepler had supposed, he had the satisfaction to see that this en quiry, which he had undertaken at first out of mere curiosity, could be applied to the greatest purposes. Hereupon he drew up about a dozen propositions, re lating to the motion of the primary pla nets round the sun, which were commu nicated to the Royal Society in the latter end of 1683. This coming to be known
to Dr. Halley, that gentleman, who had attempted the demonstration in vain, ap plied, in August, 1684, to Newton, who assured him that he had absolutely com pleted the proof. This was also register ed in the books of the Royal Society ; at whose earnest solicitation Newton finish ed the work, which was printed under the care of Dr. Halley, and came out about Midsummer, 1687, under the title of "Philosophic: Naturalis Principia Mathe matics," containing, in the third book, the cometic astronomy, which had been lately discovered by him, and now made its first appearance in the world: a work which may he looked upon as the pro duction of a celestial intelligence rather than of a man.
This work, however, in which the great author has built a new system of natural philosophy, upon the most sublime geo metry, did not meet at first with all the applause it deserved, and was one day to receive. Two reasons concurred in producing this effect : Des Cartes had then got full possession of the world. His philosophy was indeed the creature of a fine imagination, gaily dressed out : he had given her likewise some of na ture's fine features, and painted the rest to a seeming likeness of her. On the other hand, Newton had, with an unpa ralleled penetration and force of genius, pursued nature up to her most secret abode, and was intent to demonstrate her residence to others, rather than anx ious to describe particularly the way by which he arrived at it himself: he finish ed that piece in that elegant conciseness which had justly gained the ancients a universal esteem. In fact, the conse quences flow with such rapidity from the principles, that the reader is often left to supply a long chain of reasoning to con nect them, so that it required some time before the world could understand it. The best mathematicians were obliged to study it with care, before they could make themselves masters of it ; and those of a lower rank durst not venture upon it, till encouraged by the testimonies of the more learned. But at last, when its value came to be sufficiently known, the approbation, which had been so slowly gained, became universal, and nothing was to be heard, from all quarters, but one general burst of admiration. " Does Mr. Newton eat, drink, or sleep, like other men 1" says the Marquis De ]'Hos pital, one of the greatest mathemati cians of the age, to the English who vi sited him. " I represent him to myself as a celestial genius, entirely disengaged from matter." In the midst of these profound mathe matical researches, just before his Prin cipia went to the press in 1686, the pri vileges of the university being attacked by James the Second, Newton appeared among its most strenuous defenders, and was on that occasion appointed one of their delegates to the High-commission Court; and they made such a defence, that James thought proper to drop the affair. Our author was also chosen one of their members for the Convention Par liament, in 1688, in which he sat till it was dissolved.