PEOGEESS IN ASTEONOMY. 127 



tors are the more suitable instruments. This form suffers less from 

 the vicissitudes of weather and temperature, and is therefore more 

 suited where exact measurements are required. 



Toward the end of the eighteenth centur}^ a Swiss artisan, Pierre 

 Guinard, after many 3^ears of patient labor, succeeded in producing- 

 pure disks of flint glass as large as inches in diameter. The modern 

 refracting telescope thus became possible. 



In 1804 there was started at Munich the famous optical and mechan- 

 ical institute which soon made its presence felt in the astronomic:il 

 world. Reforms in instrument making were soon taken in hand, and 

 under the leadership of the great German astronomer, Bessel, great 

 strides were made in instruments of precision. Fraunhofer, who had 

 been silently working away at the theory of lenses and making various 

 experiments in the manufacture of glass, was joined in 1805 by Guinard. 

 In 1800 Troughton invented a new method of graduating circles, 

 according to Air}" the greatest improvement ever achieved in the art 

 of instrument making. 



In 1821: Fraunhofer successful!}' completed and perfected an object 

 glass of 9.9 inches in diameter for the Dorpat Observatory. This 

 objective might literally have been called a "giant," for nothing 

 approaching it in size had been previously made. 



England, which was at one time the exclusive seat of the manufacture 

 of refracting telescopes, was now completely outstripped by both 

 Germany and France, and for this we had to thank "the short-sighted 

 policy of the Government, which had placed an exoi'bitant duty on the 

 manufacture of flint glass." In 1833 the Dorpat refractor was eclipsed 

 by one of 15 inches' aperture, made for the Pulkowa Observatory by 

 Merz & Mahler, Fraunhofer's successors, who, about ten years later, 

 supplied a similar instrument to Harvard College. At this time Lord 

 Rosse emulated with success the efi'orts of Herschel, and rehabilitated 

 the reflector hy producing a metallic mirror of (J-foot aperture and 

 54-foot focal length, which he mounted at ParsonstoAvn. The speculum 

 weighed no less than 4 tons. To mount this immense mass efficiently 

 and safely was a work of no light nature, but he successfully accom- 

 plished it, and eventually both mirror and the telescope, which weighed 

 now altogether 14 tons, were so well counterpoised that they could be 

 easily moved in a limited direction by means of a windlass, worked by 

 two men. The perfection of the "seeing'' qualities of this instrument 

 and its enormous light-grasping powers were particuhirly .striking, and 

 observational astronomy was considerably enriched by the discoveries 

 made with it. 



Speculum metal was not destined to stay. Ten years later (1857) the 

 genius of Leon Foucault introduced glass mirrors with a thin coating 

 of silver deposited chemically, and these have now universally super- 

 seded the metallic ones. 



