62 SPECIAL RESULTS IN THE URANOLOGICAL 



terrupted labours of more than forty years, to the most 

 important extension of all parts of physical astronomy, in the 

 planetary system as well as in the nebulae and double stars. 

 A long reign of reflectors was again followed, in the first 

 twenty years of the nineteenth century, by a happy emula- 

 tion in the construction of achromatic refractors and helio- 

 meters, moved equatorially by clock-work. Eor object- 

 glasses of extraordinary size, a homogeneous flint glass, free 

 from striae, was supplied in Germany by the Munich esta- 

 blishment of Utzschneider and Praunhofer, subsequently 

 of Merz and Mahler; and in Switzerland and France (for 

 Lerebours and Cauchoix), in the manufactories of Guinaud 

 and Bontems. It is sufficient for the purpose of this 

 historical review, to name here, as instances, the great refrac- 

 tors made, under Fraunhofer's superintendence, for the ob- 

 servatories of Dorpat and Berlin, having 9 Parisian inches 

 free aperture, with a focal length of 13 J (14 Engish) feet j 

 the refractors of Merz and Mahler, in the observatories of 

 Pulkova, and Cambridge in the United States of North 

 America, ( 121 ) both furnished with object-glasses of 14 

 Parisian inches aperture, and 21 feet focal length (14*9 

 English inches, and 22 English ieet 4'6 inches).* The 

 heliometer of the Konigsberg Observatory, which was for a 

 long time the largest in existence, has 6 Parisian inches aper- 

 ture, and has become celebrated by the memorable labours 

 of Bessel. The short dialytic refractors, advantageous in 

 respect of light, which Plosl, in Vienna, was the first to 

 execute, the merits of which were recognised almost at the 



* [The exact focal length of the refractor at Cambridge, U.S. is 22 English 

 feet 6 inches. ED.] 



