Page:The American Cyclopædia (1879) Volume XV.djvu/655

 TELESCOPE 625 figured as to collect any one sort of rays into one point, it could not collect those also into the same point which, having the same inci- dence upon the same medium, are apt to suffer a different refraction." Thus he was led "to take reflectors into consideration," since here there would be no separation of colors ; but in- asmuch as any irregularity of figure in a con- cave mirror would produce greater distortion in the image than would be the case with a lens, "a much greater curiosity [nicety] would be re- quisite than in figuring glasses for refraction." The Gregorian construction, mentioned above, appeared to him to have such disadvantages, that he " saw it necessary to alter the design, and place the eye glass at the side of the tube." Having then found an alloy of copper and tin which appeared to possess the requisite qualities for mirrors, and having also devised a " ten- der way of polishing proper for metal," he at- tempted the construction of a reflecting tele- scope upon the plan which has ever since borne the name of Newtonian, and soon produced an instrument with which he could discern the " concomitants " of Jupiter and the phases of Venus. Another one made soon after (1671), Fro. 4. Newton's Telescope. having a speculum of 1| in. diameter and 6 in. focus, was presented by him to the royal society of London, by whom it is still pre- served. In these telescopes the mirror M, fig. 4, is at the lower end of the tube, the mouth of which is directed toward the object to be observed. The rays 1 and 2 from one end of the object being reflected toward a, and the rays 3 and 4 from the other end toward 5, an inverted image of the object would be formed at 5 a ; but a small plane mirror M', interposed at an angle of 45, diverts the image to a' J', and the eye lens magnifies this into A B. In the same year that Newton's new telescopes were made, Cassegrain, a Frenchman, pro- posed still another construction. The large mirror was perforated, but the rays proceed- ing from it were, before reaching their focus, received upon a small convex mirror which sent them back with less convergence to form the image near the eye piece. It was asserted that this form, which like Gregory's was not immediately brought into use, would possess several advantages over the Newtonian; but the English philosopher showed that these advantages were rather objections, and that the difficulty of properly working the mirrors would always be a serious obstacle to their general acceptance. In fact, we hear little more of them until TO or 80 years later, when Short, a celebrated artist of Edinburgh, re- vived their manufacture, and, by his peculiar skill in figuring and mutually adapting the mirrors (" marrying them," as he termed it), brought them into favor for a time. But practical difficulties, especially in the manipu- lations of the large speculum, interposed for many years to prevent even the Newtonian construction from coming into general use. It was known indeed that in order to reflect all the rays accurately to the same focus, the figure of the mirror should be not spherical but parabolic ; but no method was known whereby this figure could be attained with certainty. At length, in 1718, Hadley made a mirror 6 in. in diameter and with a focal length of 62 in., which bore a magnifying power of 230. This instrument may be con- sidered to have established the reputation of reflectors ; for on being compared by Bradley and Pound with the 123-foot aerial telescope of Huygens, it proved fully a match for the refractor, 'except that the latter showed ob- jects somewhat brighter. After this period reflectors came rapidly into general use, and have ever since been the favorite kind of tele- scope in England. Their construction was greatly facilitated to practical men by the ap- pearance in 1777 of an elaborate memoir by Mudge, giving a detailed account of his pro- cess of making and finishing specula. Another important memoir upon the same subject, by the Kev. John Edwards, was published in the appendix to the "British Nautical Almanac " for 1787. (See SPECULUM.) About 1766 a small telescope, only 2 ft. long, fell into the hands of a German organist residing in Bath, Eng- land. He sent to London for a larger instru- ment, and, finding its cost too great, undertook to make one for himself. That organist was the elder Herschel. He devoted all the time at his command to the manufacture of reflect- ors. Improving continually upon his succes- sive results, and with increasing means at his disposal, he made many Newtonian reflectors, some even as large as 20 ft., as well as sev- eral of the Gregorian form of 10 ft. focus. His discovery of the planet Uranus, in 1781, brought him to the notice of George III., by whose liberality he was enabled in 1785 to undertake the construction of the celebrated 40-foot reflector, which was pronounced fin- ite. 5._ Herschers Telescope. shed in August, 1789; but it never accom- plished any work worthy of its dimensions. n it the mirror M, fig. 5, was slightly inclined,