TELESCOPES. 65 



object-glasses of 15 inches in diameter, and a focal length 

 of 22-5 feet. The heliometer at the Konigsberg Observa- 

 tory, which continued for a long time to be the largest in 

 existence, has an aperture of 6*4 inches in diameter. This 

 instrument has been rendered celebrated by the memorable 

 labors of Bessel. The well-illuminated and short dyalitic 

 refractors, which were first executed by Plosl in Vienna, 

 and the advantages of which were almost simultaneously 

 recognized by Rogers in England, are of sufficient merit to 

 warrant their construction on a large scale. 



During this period, to the efforts of which I have refer- 

 red, because they exercised so essential an influence on the 

 extension of cosmical views, the improvements made in in- 

 struments of measurement (zenith sectors, meridian circles, 

 and micrometers) were as marked in respect to mechanics as 

 they were to optics and to the measurement of time. Among 

 the many names distinguished in modern times in relation 

 to instruments of measurement, we will here only mention 

 those of Ramsden, Troughton, Fortin, Reichenbach, Gam- 

 bey, Ertel, Steinheil, Repsold, Pistor, and Oertling ; in rela- 

 tion to chronometers and astronomical pendulum clocks, we 

 may instance Mudge, Arnold, Emery, Earnshaw, Breguet, 

 Jiirgensen, Kessels, Winnerl, and Tiede ; while the noble la- 

 bors of William and John Herschel, South, Struve, Bessel, 

 and Dawes, in relation to the distances and periodic motions 

 of the double stars, specially manifest the simultaneous per- 

 fection acquired in exact vision and measurement. Struve's 

 classification of the double stars gives about 100 for the num- 

 ber whose distance from one another is below 1", and 336 

 for those between V and 2" ; the measurement in every case 

 being several times repeated.* 



During the last few years, two men, unconnected with 

 any industrial profession — the Earl of Rosse, at Parson's 

 Town (about fifty miles west of Dublin), and Mr. Lassell, at 

 Starfield, near Liverpool, have, with the most unbounded 

 liberality, inspired with a noble enthusiasm for the cause of 

 science, constructed under their own immediate superintend- 

 ence two reflectors, which have raised the hopes of astron- 

 omers to the highest degree. f Lassell's telescope, which has 



Lerebours have also constructed object-glasses of more than 13-3 inches 

 in diameter, and nearly 25 feet focal length. 



* Struve, Stellarum duplicium et multiplicium Mensura; Micrometriccc, 

 p. 2, 41. 



t Mr. Airy has recently given a comparative description of the meth« 

 ods of constructing these two telescopes, including an account of the 



