ASTRONOMY OF EIGHTEENTH CENTURY. 261- 



dence near Windsor, where he constructed a splendid 2o-feet tele- 

 scope in 1783, and for two years continued to employ it in a series 

 of admirable observations. At the end of 1785 the construction of a 

 telescope of 30 feet focal length was entered upon; but the King 

 afterwards induced Herschel to extend his design to one of 40 feet, 

 which was erected at Slough, and used for the first time in February, 

 1787 ; but it was not until the 27th of August, 1789, that Herschel had 

 completely satisfied himself of its perfect completion. On the follow- 

 ing day he discovered the sixth satellite of Saturn by its means. The 

 speculum in this telescope was 4 feet in diameter and 31 inches thick, 

 and weighed 2,118 Ibs. The metal in which it was cast was an alloy 

 of one hundred parts of tin to three of copper. Its tube was 4 feet 

 i o inches in diameter, made of sheet iron, and was supported by a 

 massive framework of wood, the dimensions and machinery of which 

 may be inferred from the fact that the construction employed thirty men 

 for six months. Herschel in his large telescope suppressed the second 

 small mirror used in the Newtonian reflector (p. 217), and received the 

 rays reflected from the speculum at once into the eye-piece placed at 

 one side of the mouth of the tube. The great advantage of this arrange- 

 ment consisted in the increased distinctness and brilliancy cf the image, 

 as a second reflector necessarily entails some loss of light and of defini- 

 tion. 



The general method of mounting the great Slough telescope will be 

 understood from Fig. 126, page 262, which represents, however, another 

 gigantic telescope erected by Mr. Ramage, of Aberdeen, in the begin- 

 ning of the present century. 



With the close cluster of stars called the Pleiades the reader is no 

 doubt familiar. By the naked eye few persons can see more than six 

 stars in this group, but when it is viewed under very favourable cir- 

 cumstances even by the naked eye, or still better by an opera-glass or 

 common telescope, it is seen to consist of many more stars. A good 

 astronomical telescope reveals fifty or sixty, and some observers have 

 counted double that number. To persons of weak eyesight the 

 Pleiades present the appearance of a light cloud ; and there is in the 

 constellation Cancer, an object (Prcesepe, or the "Beehive") which to 

 the best unassisted eyesight also presents the appearance of aluminous 

 cloud, but which a very ordinary telescope resolves into a cluster of 

 stars. Of these Galileo counted twenty-eight in the nebula here referred 

 to. Every person who has seen the skies of the Southern Hemisphere 

 has probably noticed the two luminous patches called the Magellanic 

 clouds. Several other similar nebulce were observed and catalogued 

 by Ptolemy and other astronomers, and La Caille and others added 

 to the list of nebulas belonging to the Southern Hemisphere. But 

 Herschel immensely increased the numbers, for in 1786 and 1789 

 he published two catalogues of them, giving an account of no fewer 

 than 2,300. Nor was this all, for Herschel's telescopes resolved very 



