"In the fourth place," says Newton, writing in 1675, "I sup pose light is neither aether nor its vibrating motion but something of a different kind propagated from lucid bodies. . . . Fifthly it is to be supposed that light and aether mutually act upon one an other." Did he here build more truly than he knew? Who can say, but we must revert to other work, to other discoveries on which his fame will rest secure till time shall end.
In 1666, when at Woolsthorpe on account of the plague at Cambridge, he "began to think of gravity extending to the orb of the moon," though of this nothing was published for 18 years.
Discussions went on in London at the Royal Society or in the houses of the members, Wren, Seth Ward, Bishop of Salisbury, Robert Boyle, Hooke, Halley, and the others who were active in the Society, until one Wednesday in January 1684 Halley met Wren and Hooke and the latter declared "that he had demon strated all the laws of the celestial motions." Halley confessed his ignorance and Sir Christopher "to encourage enquiry said he would give Hooke or me"—the quotation is from a letter of Halley to Newton—"two months to bring him a convincing demonstration." Sir Christopher offered to give "a book of 40 shillings" to the one who first found the solution. He was not convinced of Hooke's assertion that he had done it, but wished to conceal the result "that others trying and failing might know how to value it when he would make it public." So it remained till August, when Halley visited Newton at Cambridge and put the question, what would be the path of a body moving under the action of a central force which varied as the inverse square of the distance from the centre. "I then learned," writes Halley in the same letter, "that you had brought this demonstration to perfec tion." Newton promised to look for the old proof already men tioned but could not find it, "and not finding it did it again and reduced the work into the proposition," which he sent in Novem ber to Halley, who immediately returned to Cambridge and per suaded Newton to put them in form for the Royal Society.
On December loth, 1684, Halley informed the Society that he had lately seen Mr. Newton at Cambridge who had showed him a curious treatise De Motu which, upon Mr. Halley's desire, was sent to the Society to be entered on their Register "and a tract, Propositiones de Motu" was registered in February 1685 with the date in the margin, so Dec. 1684.
But the early months of 1685 were fertile with a new discovery. Newton was away from Cambridge; hitherto his calculations had proceeded on the assumption that the sun and the planets could each be treated as though they were points, concentrated at their respective centres, through which the various forces were assumed to act; but was this true or was it merely an approximation due to the fact that the planetary distances were so immense that even a great sphere like the sun could in comparison be treated as a point? What will be the force with which the sun attracts an exterior particle? Newton proceeded to work this out, on the assumption that each particle of the sun attracted the external particle with a force which was proportional to the product of the masses of the two and inversely proportional to the square of the distance between them and found (we know he had no expectation of the result until it emerged from his calculations), that if the sun were of uniform density, or consisted of a series of concentric shells each of uniform density, then the resultant force on the external particle was the same as that which would be exerted by the whole mass of that concentrated at the centre.
It was no approximation, the sun and the planets considered as spherical, really behaved as point centres of force.
In the opinion of Professor Adams it was the difficulty of solving this problem and not the uncertainty as to the moon's distance which caused Newton in 1665 to lay aside his astronom ical calculations, which were now resumed with a more correct knowledge of the moon's distance. Newton returned to Cambridge and the writing of the Principia was begun in March 1686.
The work is entitled Philosophise Naturalis Principia Mathe matica. It is in three books, the first De Motu Corporum was finished on April 28, 1686 and exhibited to the Royal Society on that day. On June 20, 1687, Newton wrote that the second book was ready. "The third I now design to suppress. Philosophy is such an impertinently litigious lady that a man had as good be engaged in lawsuits as have to do with her." Halley was able to prevent this, and on Sept. 6, 1687 the third book described as De Mundi Systemate was presented; the whole was published about Mid summer 1687.