364 



WELLS S NATURAL PHILOSOPHY. 



counterbalanced by an arrangement of weights. A small telescope called tho 

 finder, is attached near the eye end of tho large one ; this is so adjusted that 

 when the object is seen through it, it appears in the field of the large tele- 

 scope, thus saving much trouble in directing the instrument toward any par- 

 ticular object. 



The mounting and attachments of an equatorial telescope are represented 

 in Fig. 298. 



Fic. 2D8. 



What is a Spy. 

 glass ? 



724, A spy-i^lass, or terrestrial telescope, 

 differs from an astronomical telescope only in 

 an adjustment of lenses, which enables the observer to see 

 the images of objects erect instead of inverted. This is 

 efiected by the addition of two lenses placed between tho 

 eye and the image. 



The arrangement of the lenses, and the course of the rays of light, in a 

 common spy-glass, are represented in Fig. 299. is the object-glass, and C 

 L M the eye-glasses, placed at distances from each other equal to double their 

 focal length. The progress of the rays through the object-glass, 0, and tho 

 first eye-glass, C, is tho same as in tho astronomical telescope, and an inverted 



