434 HISTORY OF SCIENCE. 



separation at the edge would not amount to one icxoooth part of an 

 inch. Lord Rosse's great telescope is mounted between two piers 60 

 feet high. The speculum is mounted at one end of a wooden tube 

 formed of staves, which are hooped with iron. This tube is about 

 50 feet long, and 8 feet in diameter in the middle. The speculum, 

 the tube, and the parts connected with them, weigh about 12 tons, 

 and this heavy mass is so mounted and counterpoised that it can be 

 moved with the greatest ease and steadiness. The telescope com- 

 mands the southern sky from the horizon to the zenith, and a star on 

 the equator can be followed for an hour. The observer is stationed 

 in a gallery which is so arranged that it can be moved in altitude and 

 azimuth with great readiness. The cut Fig. 197, on page 441, gives a 

 distant view of both of Lord Rosse's great telescopes. The more 

 remote one is the 3-feet reflector, the nearer one on the right is the 

 6-feet reflector between its two piers of massive masonry. 



The advantages possessed by a reflecting telescope over a refracting 

 telescope are that the former will bear a much greater magnifying power 

 in proportion to its length, and that it gives images free from colour ; 

 for, as we have already seen (page 283), the best achromatic lenses 

 give necessarily some outstanding uncorrected coloration. Against 

 these advantages are to be set off the loss of light occasioned by the 

 two reflections, the liability of the surface of the speculum to become 

 tarnished, and the inconvenience arising from the weight of the mass 

 of metal if the speculum is of large size, and lastly the difficulty of giving 

 a parabolic curvature to the surface. From the time of the invention 

 of the achromatic lens by Dollond, down to quite recent times, reflect- 

 ing telescopes have not been much in use, always excepting those 

 famous large ones made by W. Herschel, Lassell, and Lord Rosse, and 

 others of large size. But during the last few years the use of reflect- 

 ing telescopes has been greatly extended by the happy application of 

 an easy chemical process by which a film of pure lustrous silver may be 

 deposited on the surface of polished glass. The mirrors for the new 

 reflecting telescopes are made not of speculum metal, but of glass. A 

 disc of glass is, in the first instance, ground spherically concave on one 

 side, and the spherically curved surface is then converted by careful 

 and skilful manipulation into a parabolic figure. Foucault of Paris, 

 Steinheil of Munich, and With of Hereford, have all acquired high 

 reputations for the excellent performance of their silvered glass specula. 

 With's specula are mounted in various ways by Browning of London. 

 Fig. 194 represents a 1 2-inch reflector mounted on an equatorial stand 

 of the most complete kind as constructed by Browning. The advan- 

 tages claimed for silvered glass specula over those made of speculum- 

 metal are their cheapness, lightness, greater reflecting power of the 

 silver, and the ease with which the brilliant surface can be renewed by 

 re.silvering when necessary. 



The cheapening and improvement of reflectors attending the adop- 



