262 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK m. 



images cannot be obtained. It is also necessary to subject them to 

 tests by experienced observers. Generally they are tested by certain 

 celestial objects difficult of observation certain double stars among 

 them, other objects may be seen moderately well through nearly all 

 instruments, and no glass is so bad that the moon cannot be looked 

 at with pleasure. But high powers must be avoided except for 

 stars. A medium power which gives clearness and sharpness is 

 preferable to extreme ones, which are too often applied to instruments 

 without any real use. 



Among the most noted and powerful astronomical refractors known 

 in the present day, we may notice those constructed by Messrs. Cooke 

 and Sons of York and by Alvan Clarke of America, of 25 and 26 

 inches aperture respectively. The former belongs to Mr. R. S. Newall 

 of Gateshead, the latter is at work in the Naval Observatory at 

 Washington. 



III. THE ERECTING TELESCOPE. 



Kepler made the theoretical discovery of the astronomical telescope, 

 with convergent eye-piece, but the great astronomer did not realize 



Fi ;. 201. Path .jf the luminous rays in the erecting telescope. 



his conception ; Father Schemer it was who first constructed a tele- 

 scope of this kind, which by degrees superseded Galileo's. A short 

 time alter, Reita invented the terrestrial telescope, which only differs 

 from the astronomical in the arrangement of the eye-piece. By 



