However, the theory alone, unexpect ed and surprising as it was, did not satisfy him : he rather considered the proper use that might be made of it for improv ing telescopes, which was his first design. To this end, having now discovered that light was not homogeneous, but an he terogeneous mixture of differently re. frangible rays, he computed the errors arising from this different refrangibility ; and, finding them to exceed some hun dreds of times those occasioned by the circular figure of the glasses, he threw aside his glass works, and began to con sider the subject with precision. He was now sensible that optical instru ments might be brought to any degree of perfection desired, in case there could be found a reflecting substance which could polish as finely as glass, and reflect as much light as glass transmits, and the art of giving it a parabolical figure he also attained; but these at first seemed to him very great difficulties; nay, he thought them almost insuperable, when he fur ther considered, that every irregularity in a reflecting superficies makes the rays stray five or six times more from their due course, than the like irregularities in a refracting one.
Amidst these speculations, he was forced from Cambridge, in 1665, by the plague ; and it was more than two years before he made any fyrther progress in the subject. However, he was far from passing his time idly in the country ; on the contrary, it was here, at this time, that he first started the hint that gave rise to the system of the world, which is the main subject of the Principia. In his retirement he was sitting alone in a garden, when some apples falling from a tree, led his thoughts upon the subject of gravity; and reflecting on the power of that principle, he began to consider, that, as this power is not found to be sen sibly diminished at the remotest distance from the centre of the earth, to which we can Rim, neither at the tops of the loftiest buildings, nor on the summits of the highest mountains, it appeared to him reasonable to conclude, that this power must extend much farther than is usually thought. "Why not as high as the moon ?" said he to himself; " and if so, her motion must be influenced by it; perhaps she is retained in her orbit by it; however, though the power of gravity is not sensibly weakened in the little change of distance at which we can place our selves from the centre of the earth, yet it is very possible that, at the height of the moon, this power may differ in strength much from what it m here " To make an estimate of what might be the degree of this diminution, he considered • with himself, that if the moon be retain ed in her orbit by the force of gravity, no doubt the primary planets are carried about the sun by the like power ; and by comparing the periods of the severs] planets with their distances from the sun, he found, that if any power like gravity held them in their courses, its strength must decrease in the duplicate proportion of the increase of distance. Thisbe con
cluded by supposing them to move in perfect circles, concentric to the sun, from which the orbits of the greatest part of them do not much differ. Sup posing, therefore, the force of gravity, when extended to the moon, to decrease in the same manner, he computed whe ther that force would be sufficient to keep the moon in her orbit.
In this computation, being absent from books, he took the common estimate in use aitiong the geographers and our sea men, before Norwood had measured the earth, namely, that sixty miles make one degree of latitude; but as that is a very erroneous supposition, each degree con taining about sixty-nine and one-third of our English miles, his computation upon it did not make the power of gravity, de creasing in a duplicate proportion to the distance, answerable to the power which retained the moon in her orbit; whence be concluded, that some other cause must at least join with the action of the power of gravity on the moon. For this reason he laid aside, for that time, any further thoughts upon the matter. Mr. Whiston (in his Memoirs, p. 33.) says, he told him that he thought Des Cartes's vortices might concur with the action of gravity.
Nor did he resume this enquiry on his return to Cambridge, which was shortly after. The truth is, his thoughts were now engaged upon his newly projected reflecting telescope, of which he made a small specimen with a metallic reflector spherically concave. It was but a rude essay, chiefly defective by the want of a good polish for the metal. This instru ment is now in the nossession of the Roy al Society. In 1667, be was chosen fel low of his college, and took the degree of master of arts. And in 1669, Dr. Bar row resigned to him the mathematical chair at Cambridge, the business of which appointment interrupted, for a while, his attention to the telescope ; however, as his thoughts had been for some time chiefly employed upon optics, he made his discoveries in that science the subject of his lectures for the first three years after he was appointed mathematical pro fessor; and having now brought his the. ory of light and colours to a considera ble degree of perfection, and having been elected a Fellow of the Royal Socie ty, in January, 1672, he communicated it to that body, to have their judgment upon it ; and it was afterwards published in their Transactions, viz. of February 19, 1672. This publication occasioned a dispute upon the truth of it, which gave him so much uneasiness, that he resolved not to publish any thing further for a while upon the subject ; and in that re solution he laid by his optical lectures, al though he had prepared them for the press. And the analysis by infinite series, which he had intended to subjoin to them, unhappily for the world, under went the same fate, and for the same reason.