TRANSACTIONS OF SECTION A. 667 



Auzout states that the best telescopes of Campani at Rome magnified 

 150 times, and were of 17 feet focal length. He himself is said to have made 

 telescopes of from 300 to COO feet focus, but it is improbable that they were ever 

 put to practical use. Ca?sini discovered Snturn's fifth satellite (Rhea) in 1672, 

 •with a telescope made by Campani, magnifying about 150 times, whilst later, iu 

 1684, he added the third and fourth satellites of the same planet to the list of his 

 discoveries. 



Although these telescopes were unwieldy, Bradley, with his usual persistency, 

 actually determined the diameter of Venus iu 1722 with a telescope of 212 feet 

 focal length. 



AVith such cumbersome instruments many devices were invented of pointing 

 these mrial telescojies, as they were termed, to various parts of the sky. Huyghens 

 contrived some ingenious arrangements for this purpose, and also for adjusting and 

 centreing the eyepiece, the object-glass and eyepiece being connected by a long 

 braced rod. 



It was not, however, until DoUond's invention of the achromatic object-glass 

 in 1757-58 that the refracting telescope was materially inqjroved, and even then 

 tlie difficulty of obtaining large blocks of glass free from striaa limited the telescope 

 as regards aperture, for even at the date of Airy's report we have seen that 

 12 inches was about the maximum aperture for an object-glass. 



The work of improving glass dates back to 1784, when Guiuand began 

 e:^perimenting with the manufacture of optical flint glass. 



He conveyed his secrets to the firm of Fraunhofer and Utzschneider, whom he 

 joined in 1805, and during the period he was there they made the O'G inches 

 object-glass for the Dorpat telescope. 



Merz and Miidler, the successors of Fraimhorer, carried out successfally the 

 methods handed down to them by Gulnand and Fraunhofer. 



Guinand communicated his secrets to his family before his death in 182-3, and 

 they entered into partnership with Bontemps. the latter afterwards joined the 

 firm of Chance Bros., of Birmingham, and so some of Guinand's work came to 

 England. 



At the present day MM. Feil, of Paris, who are direct descendants of 

 Guinand and Messrs. Chance Bros., of Birmingham, are the best known manu- 

 facturers of large discs of optical glass. 



It is related in history that Ptolemy Euergetes had caused to be erected on a 

 lighthouse at Alexandria a piece of apparatus for discovering vessels a long way 

 off; it has also been maintained that the instrument cited was a concave reflecting 

 mirror, and it is possible to observe with the naked eye images formed by a 

 concave mirror, and that such images are very bright. 



Also the Romans were well acquainted with the concentrating power of con- 

 cave mirrors, using them as burning mirrors, as they were called. The first 

 application of an eye lens to the image formed by reflection from a concave mirror 

 appears to have been made by Father Zucchi, an Italian .Jesuit. Ilis work was 

 published in 1652, though it appears lie employed such an instrument as early as 

 1616. The priority, however, of describing, if not making, a practical reflecting 

 telescope belongs to Gregory, who, in his ' Optica Promota,' 1663, discusses the 

 forms of images of objects produced by mirrors. He was well aware of the 

 failure of all attempts to perfect telescopes by using lenses of various curvature, 

 and proposed the form of reflecting telescope which bears his name. 



Newton, however, was the first to construct a reflecting telescope, and with it 

 he could see Jupiter's satellites, &c. Encouraged by this he made another of 

 6^ inches focal length, which magnified thirty-eight times, and this he presented 

 to the Royal Society on the day of his election to the Society iu 1671. 



To Newton we owe also the idea of employing pitch, used in the working of 

 the surfaces. 



A third form of telescope was invented by Cassegrain in 1672. He substituted 

 a small convex mirror for the concave mirror in Gregory's form, and thus rendered 

 the telescope a little shorter. 



Short also, from 1730-1768, displayed uucommon ability iu the manufacture of 



