REFKACTING TELESCOPES. 



529 



in the position of the lenses and exclusion of nil light 

 which does not proceed from the observed objects. In 

 figures 286 to 288 the walls of the tubes are indicated 

 by somewhat thicker lines. The real form of the tube 

 of a telescope is however very different, and must admit 

 of altering the distance between the object glass and 

 the eye-piece. For since the real image produced by 

 the object glass is the farther from the lens the nearer 

 the object, and since the eye-piece must always have 



A 



FIG. 289 (A, S, % real size; C, % real size). 



the same position with reference to this image, whether 

 it is actually formed, as in the astronomical and terres- 

 trial telescope, or riot formed at all, as in the Galilean, 

 it follows that the tube of the telescope must be capable 

 of being lengthened for near objects and shortened for 

 more distant objects. The object glass and the eye- 

 piece are therefore each in separate tubes, which slide 

 one in the other with moderate friction. Fig. 289 

 shows the external appearance of three kinds of tele- 



M M 



