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HISTORY OF PHYSICAL ASTRONOMY.

of their being used without tubes, these contrivances were termed aerialtelescopes. Huyghens presented one having a focal length of 123 feet tothe Royal Society of London . This philosopher effected a capital im-provement in the construction of telescopes by the invention of an eye-piece consisting of a combination of lenses which, by their respectiverefractions, served to correct the effects of spherical aberration.

While Huyghens was engaged in these labours, the art of executingtelescopes was attaining a high 'degree of excellence in Italy. EustachioDivini at Rome, and Campani at Bologna , more especially signalisedthemselves in this department of practical optics. It was with telescopesexecuted by the latter that Cassini made the multitude of interesting dis-coveries in the heavens which have rendered the name of that astronomerso deservedly celebrated. The object-glasses had, in some instances,focal lengths of 90, and even 130, feet. Cassini was enabled, like Huy-ghens , to make use of such glasses without tubes.

Telescopes of great length were constructed in most other countries ofEurope during the seventeenth century, especially in France and England.Auzout is said to have made a telescope of 300 feet focal length. It doesnot appear, however, that he actually employed such an instrument inastronomical observations. In England the construction of telescopesseems to have attained considerable perfection at a comparatively earlyperiod. Among the instruments with which the observatory of Copenhagen was furnished, on the occasion of its original establishment in 1056, was atelescope magnifying 100 times, which Longomontanus brought with himfrom England in the year 1642. The telescopes of Sir Paul Neile, as wellas those of Pteeves and Cox, are said to have been equal to the best tele-scopes executed at the same time in any country of Europe . It speaksvery much in favour of the English telescopes made during the latterpart of the seventeenth century, that with one of such instruments, of 38feet focal length, William Ball of Mainhead, in Devonshire, having ob-served Saturn on the 13th October, 1665, perceived that the ring aroundthe planet was double *. This observation was made ten years beforeCassini remarked a similar appearance on the ring. It appears thatBradley occasionally employed telescopes of great length, in the early partof his career. On the 27th of December, 1722, while he was residingat Wanstead with Pound, his uncle, he measured the diameter of Venuswith a telescope, or rather object-glass, of 212j feet focal length f.

Notwithstanding the vast amount of labour and skill bestowed uponthe construction of refracting telescopes, it was still found that the imagesformed in these instruments were by no means so well defined as wasnecessary for delicate observations. This circumstance ivas attributed tothe spherical aberration of the lenses, but, although the indistinctness nodoubt partly arose from that cause, it was mainly due to the influence ofa physical principle which had not yet been discovered. When, to theimperfect formation of the image, the inconvenience attending the use oftelescopes of enormous focal length was taken into account, it obviouslybecame desirable to devise a different mode of construction. In 1663,James Gregory published his Optica Promota , in which he explained theconstruction of the reflecting telescope which has since been called by hisname. He proposed that the rays of light from a remote object should

* Phil. Trans. 1666, p. 153.

+ Bradley’s Miscellaneous Works, p. 354.