Sivilian professorship of astronomy at Oxford. Gregory had a formidable rival to contend with, as great interest was used for Halley at court, and he had besides rendered himself eminent by his numerous and important discoveries. Gregory in all likelihood would not have obtained this situation, notwithstanding the zealous intercession of Newton and Flamstead, had it not been for a circumstance which is stated by Whiston in his Memoirs of his Own Time, as follows: "Halley being thought of as successor to the mathematical chair at Oxford, bishop Stillingfleet was desired to recommend him at court; but hearing that he was a sceptic and a contemner of religion, the bishop scrupled to be concerned till his chaplain Mr Bentley should tally with him about it; which he did, but Halley was so sincere in his infidelity, that he would not so much as pretend to believe the Christian religion, though he was likely to lose a professorship by it—which he did, and it was given to Dr Gregory." To the honour of science let it be mentioned, that this circumstance, which opposed the interest of these two mathematicians so directly to each other, instead of becoming the cause of those petty jealousies or animosities, which in such cases, so commonly occur, was in the present instance the foundation on which was raised a firm and lasting friendship.
Nor is it perhaps too bold to suspect, that the liberality displayed in this instance by these two eminent men, proceeded not so much from themselves as from the science which they cultivated in common. The scruples of Stillingfleet in time lost their efficacy, and Gregory had soon after the pleasure of having Dr Halley as his colleague, he having succeeded Dr Wallis in the Savilian chair of Geometry.
In 1695, he published at Oxford a very valuable work on the reflection and refraction of spherical surfaces. This work is valuable as it contains the first hint for a practical method of improving the refracting telescope* and destroying the chromatic defect of these instruments. The difficulty to be avoided in those telescopes which operate by glasses instead of mirrors, lies in procuring a large field of view, and at the same time retaining distinctness of vision. Gregory drew an analogy from the construction of the eye, and by referring to the method by which this was effected in nature, gave the hint that the same principle might be applied in practice. This, perhaps, paved the way for the achromatic glasses, one of the finest triumphs of modern science. A simplicity pervades the whole work truly characteristic of the author's mind. But the work on which the fame of David Gregory must ultimately depend, was published in 1702, entitled " Elements of Physical and Geometrical Astronomy." This work was a sort of digest of Newton's Principia. Great originality was shown in the illustrations, and the arrangement was so adapted as to show the progress the science had made in its various gradations towards perfection; and it was allowed by Newton himself that Gregory's work was an excellent view of his system.
Sir Henry Savile had projected a design of printing a uniform series of the ancient mathematicians; in pursuance of which Gregory published an edition of Euclid, and in conjunction with Dr Halley, he commenced the Conies of Appollonius; but scarcely had he entered upon this interesting undertaking, when death put a period to his existence. He departed this life in 1701, at Maidenhead in Berkshire, where it is believed his body is interred. His wife erected a monument at Oxford to his memory, with a very simple and elegant inscription. Of the talents of Dr Gregory ample testimony is borne by the works which he bequeathed to posterity, and of his worth as a private individual by the respect in which he was held by his contemporaries, Flamstead, Keil, Halley, and above all, Sir Isaac Newton, who held him in the highest estimation. Of Newton's respect for him we shall add one other instance : Sir Isaac had in-
