256 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK m. 



The value of the magnification is expressed in another way when the 

 eye- piece is composed of a system of lenses. 



Fig. 194 shows the inner arrangement of the inverting telescope. 



The eye-piece is generally formed of two plano-convex lenses 

 separated by a diaphragm and adapted by a sliding tube inside 

 the hirger tube which holds the object-glass. 



FIG. 194. Inverting telescope ; section or inner view. 



By an external milled head V, the tube of the eye-piece is drawn 

 in or out, in order to focus it, that is to say, to bring it into the position 

 where the image is formed perfectly distinctly. This depends both on 

 the magnifying power used and on the observer's eye, and for objects 

 comparatively near, on the distance of these objects themselves. For 

 celestial objects, as their distance may be regarded as infinite, the 



FIG. 1U5.- Astronomical refractor with tinder mounted on ordinary stand. 



change of focus is only required by change of magnifying power, and 

 the observer's eye which may be normal, near, or far-sighted. 



The applications of this kind of telescope are very various, as by its 

 construction it is possible to insert in the eye-piece cross wires, by 

 which it can be very accurately directed to distant objects. Hence, 



