598 



SCIENCE. 



[N. S. Vol. XII. No. 303. 



Ms invention of tlie micrometer and appli- 

 cation of telescopic sights to instruments of 

 precision. 



It was, however, not until about the mid- 

 dle of the seventeenth century that Kep- 

 ler's telescope came to be nearly universal, 

 and then chiefly because its field of view 

 exceeded that of the Galilean. 



The first powerful telescopes were made 

 by Huyghens, and with one of these he dis- 

 covered Titan (Saturn's brightest satellite) : 

 his telescopes magnified from forty-eight to 

 ninety-two times, were about 2J inches 

 aperture, with focal lengths ranging from 

 12 to 23 feet. By the aid of these he gave 

 the first explanation of Saturn's ring, 

 which he published in 1659. 



Huyghens also states that he made ob- 

 ject-glasses of 170 feet and 210 feet focal 

 length ; also one 300 feet long, but which 

 magnified only 600 times ; he also presented 

 one of 123 feet to the Koyal Society of Lon- 

 don. 



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 

 600 feet focus, but it is improbable that 

 they were ever put to practical use. Cas- 

 sini discovered Saturn's fifth satellite 

 (Rhea) in 1672, with a telescope made 

 by Campani, magnifying about 150 times, 

 whilst later, in 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 in 1722 

 with a telescope of 212 feet focal length. 



With such cumbersome instruments many 

 devices were invented of pointing these 

 aerial telescopes, as they were termed, to 

 various parts of the sky. Huyghens con- 

 trived some ingenious arrangements for 

 this purpose, and also for adjusting and 

 centering the eyepiece, the object-glass and 



eyepiece being connected by a long braced 

 rod. 



It was not, however, until DoUond's in- 

 vention of the achromatic object-glass in 

 1757-58 that the refracting telescope was 

 materially improved, and even then the dif- 

 ficulty of obtaining large blocks of glass free 

 from striae limited the telescope as regards 

 aperture, for even at the date of Airy's re- 

 port 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 Guinand began experiment- 

 ing 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 9.6 inches object- 

 glass for the Dorpat telescope. 



Merz and Madler the successors of Fraun- 

 hofer, carried out successfully the methods 

 handed down to them by Guinand and 

 Fraunhofer. 



Guinand communicated his secrets to his 

 family before his death in 1823, 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. Fell, of Paris, 

 who are direct descendants of Guinand and 

 Messrs. Chance Bros., of Birmingham, are 

 the best-known manufacturers 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 appa- 

 ratus for discovering vessels a long way oflF ; 

 it has also been maintained that the instru- 

 ment 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 concave 



