CHAP, iv.] THE TELESCOPE. 



in all levelling operations such a telescope is employed with a system 

 of levels and sometimes a horizontal circle. For surveying, a finely 

 divided vertical as well as a horizontal circle is attached, and we have 

 the Theodolite, Figs. 196 and 197, by which from any spot one can 

 determine the horizontal and vertical angles of distant points, and so 

 map a country. 



Fio. 196. Theodolite level. 



As the inversion of astronomical objects presents no disadvantage, 

 such an instrument is almost invariably used for all astronomical 

 purposes which do not require the maximum of light ; for these 

 reflectors are brought into play, as we shall see further on. 



Fig. 195 shows a telescope mounted on a stand for ordinary astro- 

 nomical observations. When it is required accurately to determine 

 the position of stars, the telescope is mounted so that it may 

 command either all the heavens, in which case we have the astro- 

 nomical equivalent of a theodolite called an alt-azimuth instrument ; or 

 so that its observations are confined to the plane of the meridian, in 

 which case we have the transit circle, Fig. 198. 



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