ASTRONOMY OF THE NINETEENTH CENTURY. 433 



1835) was a native of Cumberland, and was originally brought up to 

 farming, but at seventeen years of age he went to London, and under 

 his brother John learned the optician's art. The brothers commenced 

 in London a business which soon proved a success, and, upon John 

 Troughton's death, Edward continued to carry it on for many years, 

 and at length acquired a world-wide reputation as a maker of accurately- 

 divided astronomical instruments. Troughton constructed several mag- 

 nificent instruments for the Royal Observatory at Greenwich, among 

 which may be mentioned a mural circle and a transit instrument with a 

 lo-feet telescope. Among improvements in instruments may be named 

 the method of measuring reflected angles devised by John Pound. This 

 method depends upon the lawof the equality of incidence and reflection, 

 and was put into practice by observing the angular points of an object 

 by direct vision, and also by reflection from the surface of a trough of 

 mercury. The position of the bisection of the angle between the two 

 directions is that of the horizon. In transits Pound found it neces- 

 sary to have these observations made simultaneously with two different 

 instruments. The present Astronomer Royal has devised a plan of 

 obtaining the required observation with only one circle. Mr. Simms, 

 the successor of Troughton, has constructed more recently for the 

 Greenwich Observatory some fine instruments of large dimensions; 

 but as a detailed description of graomiometrical instruments would ne- 

 cessarily be of too intricate a nature to interest the general reader, we 

 will pass on to mention some of the recent improvements of astro- 

 nomical telescopes. 



In producing specula of a very perfect figure, Mr. Lassell of Liver- 

 pool has especially distinguished himself. It was with a reflector of 

 2 feet diameter made by his own hands that he discovered the satellite 

 of Neptune and the eighth satellite of Saturn, and made some im- 

 portant observations of the planets. But all other reflecting telescopes 

 yet constructed have been surpassed by the gigantic instruments made 

 by the Earl of Rosse at his seat, 'Birr Castle, at Parsonstown, about 

 fifty miles from Dublin. In 1840 Lord Rosse completed a reflecting 

 telescope having a speculum of 3 feet diameter, which had been 

 ground to a true form by machinery which he had devised for that 

 purpose. The focal length of this telescope is 26 feet. In 1845 Lord 

 Rosse had finished and erected the largest telescope in the world. 

 This has a speculum of 6 feet diameter, and 54 feet focal length. The 

 speculum weighs 4 tons, and in its casting and mounting special and 

 ingenious arrangements were required to overcome the difficulties 

 entailed by its magnitude, and the nicety of the operations involved 

 in shaping to a mathematically perfect parabolic curve a piece of 

 metal weighing 4 tons, or seven times as much as the 4-feet speculum 

 of Sir W. Herschel,. may be inferred from the fact that between the 

 spherical and parabolic figures the difference is so small, that if two 

 surfaces having sections coincided at the centre of the 6-feet speculum, 



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