PORTION OF THE COSMOS. VISUAL POWER. 63 



same time by Eogers in England, deserve to be constructed 

 of larger dimensions. 



During the same period in which these endeavours 

 were made, to which I have thus referred because they 

 have so materially influenced the enlargement of cosmical 

 views, mechanical improvements in measuring-instruments 

 (zenith sectors, meridian circles, and micrometers) did not 

 remain behind the progress made in optical instruments, 

 and in those employed for measuring time. Amongst the 

 many distinguished names of the present or recent times, I 

 will mention only the following : for measuring instru- 

 ments, those of Eamsden, Troughton, Fortin, Eeichenbach, 

 Gambey, Ertel, Steinheil, Eepsold, Pistor, Oertling; for 

 chronometers and astronomical clocks Mudge, Arnold, 

 Emery, Earnshaw, Breguet, Jiirgensen, Kessels, Winnerl, 

 Tiede. In the valuable and extensive investigations into 

 the distances apart and the periodic movements of double 

 stars, which we owe to William and John Herschel, South, 

 Struve, Bessel, and Dawes, we specially remark this pro- 

 portionate and simultaneous advance in the perfection at 

 once of sight and of measurement. Stmve's classification 

 of double stars contains, of those which are less than 1' apart, 

 about 100, and of those between 1' and 2" apart, 336, 

 all by frequently repeated measurements. ( 122 ) 



Within the last few years, two men who, by station and 

 circumstances, were far removed from such occupations with 

 a view to pecuniary profit, the Earl of Eosse, at Parsonstown 

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

 near Liverpool, animated by a noble love of astronomy, 

 have, with devoted liberality and under their own immediate 

 direction and superintendence, accomplished the completion 



