In 1695 David Gregory married Elizabeth, a daughter of Mr Oliphant of Langton. His marriage is commemorated in a Latin ode written by his friend Anthony Alsop, a student of Christ Church, and published in his works. Shortly after his marriage he brought out his great book, _Catoptricae et Dioptricae Sphericae Elementa_, which turns out for the comfort of the ignorant to be a great work on looking-glasses and lenses. The book came as a revelation to many men in that day, for in it Gregory tried to simplify his subject, and to make it clear to the many instead of to the few. He was rewarded with praise, and his book was promised immortality. How changed are things in the present day, when to none of our writers will criticism promise celebrity exceeding at the outside two generations. _Keill_ blossomed out into poetry: ‘It will last as long as the sun and moon endure,’ and it is just possible that it may—in the Bodleian Library!—only that was not what Keill meant.[2] Footnote 2: John Keill, 1671–1721, was born in Edinburgh. Was Professor of Astronomy at Oxford and an active member of the Royal Society. He died of a ‘violent fever’ at Oxford on Thursday, August 31st 1721, a few days after entertaining ‘the Vice Chancellor and other academic dignitaries at his house in Holywell Street with wine and punch.’ He is buried in St Mary’s Church. Comparatively unnoticed at the time was a suggestion made in this book about mirrors and lenses with regard to following Nature in the construction of a telescope. It was almost certainly Pitcairne who had explained to Gregory the strange mechanism of the human eye, and how in Nature objects before they fall on the retina pass through both the vitreous humour and the crystalline lens. Gregory pointed out that Nature does nothing in vain, and suggested that, in imitation of Nature, the object glasses of telescopes might be composed of media of different density, and that an instrument made on this principle would probably produce much clearer vision than any then in use. After Dollond had brought out his beautiful achromatic glasses the meaning of Gregory’s suggestion became clear, but it is a curious fact that neither James Gregorie, who invented the reflecting telescope, nor David Gregory, who suggested the achromatic telescope, should ever have seen the practical result of their imaginations.