OPTICAL INSTRUMENTS. 



363 



WTiat is I 

 Ttlescope ? 



How many 

 kinds of tele- 

 scopes are 

 there? 



T21. A Telescope is any I"ig- 296. 



instrument which magni- 

 fies and renders visible to the eye the 



images of distant objects. This result 



is efiected in the same manner as in 



the microscope, viz., by enlarging the 



\isual angle under which the objects 



are seen. 



Telescopes are of two 

 kinds, refracting telescopes 

 and reflecting telescopes; 



the principle of construction in both 



beins: the same as that of the com- 



pound microscope. 



What is a Re- '^^- The Ecfractiug 



Icope?" ^^^^' Telescope consists essen- 

 tially of two convex lenses, 



the object-glass and the eye-glass. 



An inverted image of an object, as a 



star, is produced by the object-glass, 



and magnified by the eye-glass. 



Fig. 297 represents the principle of construction 

 of the astronomical refracting telescope. is an 



object-glass placed at the end of a tube, which collects the rays proceedinfr 

 from a distant object and forms an inverted imago of tlic s:ime at o o', in tho 

 focus of the eye-glass, G. By this the image is magnified and viewed by tb« 

 eye at K 



Fig. 297. 



"Wliat is 

 Equatorial 

 Telescope ? 



723. When a telescope is mounted on an 



axis inclined to the latitude of a place, so that 



it can follow a star, or planet, in its diurnal 



revolution, by a single motion, it is called an Equato^ 



RIAL Telescope. 



Such an instrument is generally moved by clock-work, an4 i^ accurately 



