630
HISTORY OF PHYSICAL ASTRONOMY.
presumed that the number of such instruments actually made continuedto be very limited. Molyneux, however, having acquainted Scarlett andHearne, two London opticians, with the mode of grinding and polishingspecula, these individuals commenced making reflecting telescopes forsale ; and, in consequence, this class of instruments soon afterwards cameinto general use*.
The next person who distinguished himself by his skill in the con-struction of reflecting telescopes, was James Short, of Edinburgh . In1732, when he was only twenty-two years of age, this individual turnedhis attention to the polishing of specula, in which he soon attained anunrivalled degree of excellence. He also discovered a method of givingthem a true parabolic figure, which had not hitherto been done by anyperson. In his first attempts he confined himself to glass specula, whichwere made to reflect the light by covering them behind with quicksilver.James Gregory had recommended the use of glass for reflectors, on theground of its absorbing less light than metal, and being at the sametime less liable to tarnish f. With one of Short ’s glass reflectors, whichhad a focal length of only 15 inches, Maclaurin asserts that it was pos-sible to read in the Philosophical Transactions at a distance of 280 feet f.Finding that in such telescopes the light was fainter than in those inwhich the speculum was composed of metal, Short abandoned the use ofglass, and confined himself in future to metallic reflectors. The tele-scopes of the latter description made by him appear to have been exqui-site specimens of skill. Maclaurin states, that having compared some ofthem with instruments of much greater focal length, made by London artists, he found the former to be far superior in respect of brightness,distinctness, and magnifying power. With a metallic reflecting telescopeof Short 's, which had a focal length of only 15 inches, it was possible toread distinctly in the Philosophical Transactions at a distance of 500 feet,and to see the five satellites of Saturn all at the same time. In 1742Short removed to London , where he continued to prosecute his formervocation with great success till his death, in 1768. One of the lastinstruments executed by him was a large reflecting telescope, which hisbrother Thomas mounted equatorially for the observatory of Edinburgh ,and for which he was offered 1200 guineas by the King of Denmark.The telescopes of Short were all of the Gregorian construction.
It has been already mentioned, that as soon as Newton discovered the
* Smith’s Optics, Book III . chap, 1.
f Newton in his Optics (1704) states that, about five or six years previously, he triedto make a reflecting telescope 4 feet in length, that would magnify 150 times, thespeculum of which was of glass, and that he satisfied himself that there wanted nothingbut a good artist to bring the design to perfection. He recommends glass specula inpreference to metal, as being more easily polished, less liable to tarnish, and at the sametime capable of reflecting more light. ( Optics, part i. prop, vii.) Smith has inferredfrom the passages of Newton ’s work just cited that “ the method of making reflectingtelescopes with glass specutums, quicksilvered over,” was first recommended by that phi-losopher. {System of Optics, Remarks, No. 489.) This, however, is a mistake. Theperson who first recommended glass specula for telescopes, was James Gregory, as hasbeen mentioned in the text. Gregory made this suggestion in a letter to Collins, datedSt. Andrew’s, March 7, 1673, pointing out at the same time the peculiar advantages ofglass specula, and requesting that his views on the subject might be communicated toNewton. The letter of Gregory was read at the meeting of the Royal Society , heldon the 26th of March, 1673, and it was ordered, that it should be communicated toNewton as the person most concerned in it. {Birch, Hist, Roy. Soc. 9 vol. iii., p. 79.)
| Smith’s Optics (Remarks, No. 489).