GREGORY, JAMES, an eminent Scotch mathemati cian and natural philosopher, was born at Aberdeen in the month of November 1638. He was a son of the Rev. Mr John Gregory of Drumoak, in Aberdeenshire,* who was married to the daughter of Mr David Ander son of Finshaugh, the brother of Alexander Anderson, who was professor of mathematics at Paris : (See AN DERSON.) Having lost his father in the 13th year of his age, the charge of his education devolved upon his elder brother David Gregory, who put into his hand the Ele ments of Euclid, and stimulated the ardour which he had begun to show for mathematical learning. After having completed his philosophical studies at the Ma.
t ischal College of Aberdeen, James Gregory directed his particular attention to the subject of optics, and he pub lished the results of his labours in a work, entitled Optica promota seu abdita radiorum refiexorum et refractorum mysteria, geometrice enucleata, cui subnectitur 4pliendix subtilissimorum astronomiee problernatum resolutionem exhi bens, Loud. 1663. It was dedicated to Charles II. and was completed with the assistance and encouragement of his brother David, after he had been stopped a long time at the twenty-sixth This work is remarkable, as containing the description of a new reflecting telescope, and the deduction of the true law of refraction. The method in which he has investigated this law is remark able for its elegance and originality ; and, in his experi mental demonstration of it, we are furnished with a strik ing proof of the accuracy of his observations. In com paring the refractions calculated by the law with the ex periments of Vitellio on the refractions of water and glass, the greatest error amounts to 61' in water, and 94' in re fractions from glass into water. When compared with the experiments of Athanasius Kircher, the greatest error in water is 89', in wine 110', in oil 104', and in glass 93'; but in comparing them with his own observations, the greatest error is only 15' ; and it is remarkable that he made the index of refraction for water 1.3347, differing only 0.0018 from 1.3358 the most accurate measure, where as Vitellio made it 1.306. Before the publication of the
Optica Promota, Gregory was informed, that he was an ticipated in the discovery of the law of refraction by Des cartes, and he thus alludes to it in his preface :—Et ex analogiis in prima hujus tractatuli propositione declaratis, inveni primain hujus optice partem, de genuina refractionum hypothesi et mensura ncscius scilicet (pro/ter inopiam no-) vorum librorum Mathematicorum in alias inclyta Bibliotheca ?bredoizensi) &et eadcm a Cartesio fuisse inventa.
The invention of the reflecting telescope formed an epoch in optics and astronomy. Gregory was not acquainted with the errors in dioptric instruments, arising from the unequal refrangibility of the rays of light ; and his principal object in proposing• this new instrument, was to avoid the 'error arising from the spherical figure of the lenses. It consist ed of a parabolic concave mirror, near whose focus is placed a small concave elliptic speculum, having a com mon focus with the parabolic one. This instrument gave rise to the Newtonian telescope, in which the small mirror is plane, and reflects the image to the side of the tube where the eye-glass is placed ; and also to the Cassegrai nian telescope, which differs only from that of Gregory in having the small speculum convex instead of concave. The Gregorian principle has been almost universally used for telescopes of a moderate size. The Newtonian form has been adopted in the magnificent instruments used by Dr Herschel ; but it is extremely probable, that the Casse grainian telescope, in which the rays never cross each other at a focus,* will hereafter be considered as the most valu able of the reflecting telescopes. The Optica Promota is terminated by a collection of astronomical problems. The object of one of these, is to determine the paralaxes of two planets from their conjunction ; and, in a scholium to this problem, he points out the great use of the transits of Ve nus and Mercury, in determining the sun's parallax.f This happy idea, which has since been of such service to astro nomy, has always been ascribed to Dr Halley.