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HISTORY OF PHYSICAL ASTRONOMY.
of their due course than in a refracting telescope *. While his mind wasoccupied with these reflections, he was compelled, by the prevalence ofthe plague, to quit Cambridge, and more than two years elapsed before heagain turned his attention to the subject. Having now reflected upon theconstruction of Gregory’s instrument, he came to the conclusion that itwould be preferable to place the eye-glass in the middle of the tube. Ac-cordingly, having succeeded, after much trouble, in discovering a substancethat was adapted for the formation of the larger speculum, and having alsodevised a delicate mode of polishing the surface, he actually executed a tele-scope upon this principle. He wisely abstained from attempting to makethe speculum truly parabolic, on account of the extreme difficulty of theoperation, contenting himself with giving it the figure of a segment of asphere. With this instrument he could see the satellites of Jupiter and thehorns of Venus; but the latter were not very distinct, nor could they beperceived at all without some nicety of adjustment. Encouraged by thesuccess of his efforts, he made a second telescope, which proved to bestill better than the former one. The focal length of the speculum was0-]- inches, and that of the eye-glass (which was a plano-convex lens, withthe flat side turned towards the eye) was £th of an inch ; the magnifyingpower of the instrument was therefore 38.
In the month of December, 1671, Newton transmitted to London thesecond reflecting telescope executed by him, in order that it might besubmitted to the inspection of the Royal Society . At the meeting of theSociety, held on the 11th of January, 1672, it was announced that Mr.Newton’s instrument had been examined by the President, Sir RobertMoray , Sir Paul Neile, Dr. Wren, and Mr. Hooke, and that it had re-ceived their unanimous commendation. The Society at the same timeresolved, that in order to secure the right of the inventor, a description ofthe instrument should be sent to Huyghens , who was then residing atParis f. Huyghens communicated an account of it to the Academy ofSciences , and in consequence it soon became generally known upon theContinent t-
* Phil. Tran., 1672, p. 3080.
+ Birch, Hist. Roy. Soc., vol. iii. p. 1. The same telescope was afterwards presentedto the Royal Society . This interesting relic of the immortal philosopher is still to beseen at the apartments of the Society in Somerset House.
$ Huyghens addressed a letter to the Academy of Sciences , in which he gives a de-scription of Newton ’s reflecting telescope, and enumerates its peculiar advantages. Thefollowing passage, extracted from this letter, is worthy of notice.—“ Je compte pour untroisieme avantage, que par la reflexion du miroir de metal, il ne se perdpoint de rayons,comme aux verres qui en reflechissent une quantite notable, par ehacune de leurs sur-faces, et en interceptent encore une partie par l’obscurite de leur matiere.”— {Mem.Acad, des Sciences, tome x., p. 506.) M. Arago cites this passage as a proof thatHuyghens was not aware of any light being lost in the course of its reflexion from ametallic surface, and he naturally expresses his astonishment that the Dutch philosophershould have been unacquainted with so obvious a principle of physics. {Amiuaire, 1842, p.293.) It would appear, however, from the following passage of a letter addressed byHuyghens to the Royal Society in answer to their communication respecting Newton ’stelescope, that the case cannot be made out so clearly against him:—“Again, by themeer reflexion of the metallic speculum, there are not so many rays lost as in glasses,which reflect a considerable quantity by each of their surfaces, and besides intercept manyof them by the obscurity of their matter.”— {Phil. Trans., 1672, p. 4008.) WhetherHuyghens had corrected his mistake in the latter of the two passages just cited, orwhether his views on the subject have been misrepresented in one or other of the two pas-sages, we leave our readers to decide. The communication inserted in the Memoirs ofthe Academy of Sciences, bears no date. The letter td the Roval Society is dated Feb.13, 1672.