526 



THE TERRESTRIAL TELESCOPE. 



by the object glass c d\ a 2 b 2 the virtual image seen by 

 the eye looking through the eye-piece eg; /i is the 

 focus of the object lens, / 2 the focus of the eye-lens. 

 In the figure the distance of the object from the tele- 

 scope has been taken very small, otherwise the instru- 

 ment would have to be drawn on too small a scale. It 

 will be at once seen from the figure that the image a. 2 l. 2 

 is in reality smaller than the object, but nevertheless it 

 appears to an eye at o larger, because it is much nearer. 

 The image a-^ ^ becomes the larger the farther it is 

 from the lens c d in othSr words, the greater the focal 

 length of the object glass. The lens eg magnifies the 

 more the smaller its focal length (compare page 519). 

 The magnifying power of an astronomical telescope is 

 accordingly the greater the greater the focal length of the 

 object glass and the smaller the focal length of the eye- 

 piece. In the astronomical telescope an inverted image 

 of the object is seen. This matters little in the obser- 



Fio. 287. 



vation of celestial bodies, but would be a great incon- 

 venience in the use of the telescope for viewing distant 

 terrestrial objects. 



The terrestrial telescope consequently differs from the 

 astronomical in producing images in the same position 

 as the objects. The object glass cd (fig. 287) of the 

 terrestrial telescope produces, as in the astronomical, an 



