84 COSMOS. 



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

 of 22 -5 feet. The heliometer at the Koriigsberg Observatory, 

 which continued for a long time to be the largest in exist* 

 ence, has an aperture of 6'4 inches in diameter. This in- 

 strument has been rendered celebrated by the memorable 

 labours 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 referred, 

 because they exercised so essential an influence on the ex- 

 tension of cosmical views, the improvements made in instru- 

 ments 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 in- 

 struments of measurement, we will here only mention those 

 of Ramsden, Troughton, Fortin, Reichenbach, Gambey, Ertel, 

 Steinheil, Repsold, Pistor, and Oertling ; in relation to chrono- 

 meters and astronomical pendulum clocks, we may instance 

 Mudge, Arnold, Emery, Earnshaw, Breguet, Jiirgensen, 

 Kessels, Winnerl, and Tiede ; while the noble labours 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 perfec- 

 tion acquired in exact vision and measurement. Struve' s 

 classification of the double stars gives about 100 for the 

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



and Lerebours have also constructed object-glasses of more 

 than 13 '3 inches in diameter, and nearly 25 feet focal 

 length. 



