HISTORY OF PHYSICAL ASTRONOMY.

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In 1672 Cassegrain, a Frenchman , devised a new form of the reflect-ing telescope. It differed from the Gregorian telescope in the small mirrorbeing convex , and in its being placed so as to intercept the rays beforethey came to a focus. This telescope is more convenient than the Gre-gorian , inasmuch as it is shorter, cmteris paribus, by twice the focal lengthof the small mirror. Ramsden remarked, that the effect of sphericalaberration was also less than in the Gregorian telescope *, and morerecently, Captain Kater shewed, by a series of experiments, that it wasgreatly superior in point of light and distinctness to any telescope of thelatter construction, possessing an equal aperture j-. Notwithstanding thesealleged advantages, this form of the reflecting telescope has been com-paratively seldom used since its invention. As has been already hintedat, the Newtonian telescope exhibits objects not in their true direction,but at right angles to it. Hooke was the first person who executed atelescope, the observations with which were made by viewing the objectdirectly. This he effected in 1674, by perforating the centre of the largerspeculum agreeably to the suggestion of Gregory J.

Although both Newton and Hooke shewed that the construction ofreflecting telescopes was perfectly practicable, it does not seem that for along time any further attempts were made to execute such instruments ;at all events no progress was made in bringing them to perfection. Thefirst person who succeeded in accomplishing this object was Hadley, theindividual whom we have had occasion to allude to, as one of the inde-pendent inventors of the reflecting octant. In 1723 he presented to thoRoyal Society a reflecting telescope of the Newtonian form, which heexecuted with his own hands. The diameter of the speculum was6 inches, and its focal length w r as 5 feet 3 inches. Pound and Bradley,having compared it with the refracting telescope of 123 feet focallength, which Huyghens had presented to the Society , found that itbore as high a magnifying power as that instrument, and exhibited ob-jects equally distinct though not quite so bright. Notwithstanding its in-feriority in the latter respect, they were enabled to perceive with it allthe phenomena which they had hitherto discovered with the Iiuygeniantelescope, such as the transits of Jupiter ’s satellites ; their shadows uponthe disk of the planet; the blade list on Saturn ’s ring; and the edgeof his shadow upon the ring. On several occasions they succeeded inseeing with it the five satellites of Saturn §. Reflecting telescopeswere executed by Hawksbee, about the time that Hadley was prosecutinghis labours with such success. This individual made a telescope of3| feet focal length, which proved to be even more perfect than any ofHadley’s. About the same time Bradley and Molyneux, having beeninstructed by Hadley in the grinding and polishing of metallic specula,began to turn their attention to the construction of reflecting tele-scopes. After much trouble, they finally succeeded in executing severaltelescopes of considerable magnitude. One of these had a focal lengthof 8 feet ||.

Hitherto the execution of reflecting telescopes was confined exclusivelyto persons engaged in scientific pursuits, whence it may naturally be

* Phil. Trans., 1779, p. 427.

+ Phil. Trans., 1813, p. 206; 1814, p, 231.

J Birch, Hist. Roy. Soc., vol. iii., p. 122.

§|Phii. Trans., 1723, p. 382.

)j Smith’s Optics, Book III., chap. 1.

M M