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HISTORY OF PHYSICAL ASTRONOMY.

powerful instruments of the same kind which have emanated from theworkshops on the Continent *.

While the refracting telescope has been rendered vastly more efficientin modern times, the reflecting telescope has received a correspondingdegree of improvement. During the present century several very largetelescopes were constructed upon this principle by the late Mr. Damage,of Aberdeen, an individual of considerable originality, who possessed aremarkable aptitude for mechanical inventions. The perfection of theseinstruments does not seem, however, to have been commensurate withtheir magnitude, for they have been employed only to a very limitedextent in astronomical observations. In more recent times, Mr. Lassell,of Liverpool, has especially distinguished himself by his skill in theconstruction of reflecting telescopes. With a Newtonian reflector of 2-feetaperture, which he executed with his own hands, he has discovered thesingle satellite of Neptune, and the eighth satellite of Saturn; while, atthe same time, he has been enabled by its use, to make several veryinteresting physical observations of the planetary bodies. In the con-struction of reflecting telescopes the Earl of Hosse has attained a degreeof excellence which far surpasses all previous efforts of the kind. It isimpossible here to give any account of the multitude of admirable con-trivances by means of which that distinguished nobleman has succeededin bringing his telescopes to so high a state of perfection. It must suf-fice to state, that in 1840, lie completed a Newtonian reflector of 3 feetaperture, which he subsequently employed in astronomical observations.Phenomena of a highly-interesting nature were soon disclosed by the useof this powerful instrument. Several nebulas, hitherto observed as suchin the most powerful telescopes, were resolved into clusters of stars, whileothers exhibited forms totally different from those which had been hithertoassigned to them. In 1845 Lord Kosse gave to the world a still morestriking proof of his practical talents, and his devotion to astronomicalscience, by the construction of a reflecting telescope of 6-feet aperture,and 54-feet focal length! This magnificent instrument, by far the mostpowerful which the genius of man has hitherto executed for the purposeof exploring the grand phenomena of the heavens has already, in the handsof its nol^e owner, done valuable service to astronomy, by the light whichit has thrown upon the structure of the nebular part of the universe-)-.

The brilliant success which has attended Lord Rosse ’s efforts toconstruct reflecting telescopes, has suggested to several eminent sci-entific men of the present day the expediency of transporting a powerfulinstrument of this description to the southern hemisphere, for the pur-

* At the meeting of the British Association, held in the month of July of the presentyear (1851), at Ipswich, the Astronomer Royal, in his opening address as President ofthe Association, stated that Mr. Simms had completed the flint-glass for an achromaticobject-glass of 13 inches in diameter, and that he was engaged in executing one of 16inches. He also mentioned that Mr. Ross was attempting to make an object-glass of2 feet in diameter! It is to be hoped that these glasses may turn out to be remarkablefor quality as well as size.

f Dr. Robinson, in a brief account of Lord Rosse ’s optical labours, delivered at themeeting of the British Association, held at Cork in 1843, has remarked that “ between thespherical and parabolic figures, the extreme difference is so slight, even in the telescopeof 6-feet aperture, that if the two surfaces touched at their vertex, the distance at theedge would not amount to the ijj^th of an inch, a space which few can measure,and none without the microscope.” This statement may give the reader some idea ofthe exquisite delicacy of the operation necessary for giving the speculum its true para-bolic figure.