258 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



When the largest telescopes are used other arrangements are 

 adopted. 



To the telescope another small telescope,, called a finder, is fixed 

 parallel to the principal one, and its field of view is furnished with 

 two threads or spider lines crossed at right angles. The magnification 

 of the large refractor being considerable, the. field of view embraces 

 only a small part of the sky ; therefore, on using this to find an object 

 it is brought into the. field with much difficulty. The field of the 



Fid. 197. Theodolite (another form). 



finder being comparatively large, the object is easily found, and by 

 bringing the object to the point where the threads cross, the object 

 observed, or at any rate its central part, is then in the field of the 

 principal refractor. The field of view of the large telescope itself 

 has a system of movable threads at the common focus of the object- 

 glass and eye-piece, i.e., where the image of the object falls, so that 



