cult problems in astronomy." In this work, which forms an era in the history
of the science of that century which its author so eminently adorned, and which
was published Avhen he was only twenty-four, there was first given to the
world a description of the reflecting telescope, of which Gregory is the in-
disputable inventor. He proposed to himself no other advantage from using
mirrors instead of glasses in the construction of telescopes, than to correct the
error arising from the spherical figure of the lenses, and by forming the reflectors
of a parabolic figure, to bring the rays of light into a perfect focus, being ignorant of the far greater error arising from the unequal refrangibility of the
rays of light, Avhich it was reserved for Newton afterwards to discover. Gregory went to London a year after the publication of his work on optics, with a
view to the construction of his telescope, and was introduced to Mr Rieves, an
optical instrument maker, by Mr Collins, secretary to the Royal Society.
Rieves could not finish the mirrors on the tool so as to preserve the figure, and
so unsuccessful was the trial of the new telescope that the inventor was deterred
from making any farther attempts towards its improvement, nor were these
reflectors ever mounted in a tube. Sir I. Newton objected to this telescope, that the hole in the centre of the large speculum would be the cause of the loss of so much light, and invented one in which this defect was remedied. The Gregorian form is universally preferred to the Newtonian, when the instrument is of moderate size, the former possessing some material advantages; yet the latter was always employed by Dr Herschel, in those large instruments, by which the field of discovery has, of late, been so much extended. Although the inventor of the reflecting telescope has received all the honour which posterity can bestow, yet it is lamentable to think that he never had the satisfaction of seeing an instrument completed in his own lifetime. It is only necessary to remark farther, on this subject, that some papers of great interest passed between Gregory and Sir Isaac Newton, concerning the reflecting telescope, which may be consulted with advantage by those who would wish to investigate the subject. His work on optics contains, besides the discovery of the reflecting telescope, that of the law of refraction. Descartes had made a similar discovery long ere this, but Gregory had not heard of it till his own work was ready for publication to which circumstance he alludes in his preface. Playfair, in considering this subject, very justly remarks, that "though the optics of Descartes had been published twenty-five years, Gregory had not heard of the discovery of the law of refraction, and had found it out only by his own efforts; happy in being able, by the fertility of his genius, to supply the defects of an insulated and remote situation."[1] The method in which Gregory investigated the law of
refraction is truly remarkable, not only for its singular elegance, but originality, and the series of experiments which he instituted for the purpose of demonstration, affords an indelible proof of the accuracy of his observations. It is truly
remarkable, that the calculations by this law differ so little from those obtained by the most accurate experiments. There is yet another discovery of the very highest importance to the science of astronomy, which is falsely and, we would
hope, unknowingly attributed to another philosopher, whose manifold brilliant discoveries throw an additional lustre over the country which gave him birth. We allude to the employment of the transits of Mercury and Venus, in the determination of the sun's parallax, the merit of which is always ascribed to Dr Halley, even by that eminent astronomer Laplace. But it is plainly pointed out in the scholium to the 28th proposition of Gregory's work, published many years prior to Halley's supposed discovery. The university of Padua was at
- ↑ Playfair's Dissertation, in the Supplement to the Encyclopaedia Britannica, part 1st, page 25, 6th edition,
