306 optics. 



We shall here state the dimensions of one of 

 Mr. Short's reflecting telescopes, as described in 

 Dr. Smith's optics. 



The focal distance of the great mirror is 9.6 

 inches, its breadth 2.3 ; the focal distance of the 

 small mirror 1.5, its breadth 0.6 : the breadth of 

 the hole in the great mirror 0.5 ; the distance 

 between the small mirror and the next eye-glass 

 14.2 j the distance between the two eye-glasses 

 2.4 ; the focal distance of the eye-glass next the 

 metal .3.8 ; and the focal distance of the eye-glass 

 next the eye 1.1. 



One great advantage of the reflecting telescope 

 is, that it will admit of an eye-glass of a much 

 shorter focal distance than a refracting telescope 

 will ; and, consequently, it will magnify so much 

 the more ; for the rays are not coloured by a re- 

 flection from a concave mirror, if it be ground to 

 a true figure, as they are by passing through a 

 convex glass, let it be ground ever so true. 



The difficulties which attend this instrument 

 are, the tarnishing of the metallic speculum, and 

 the very great accuracy required in giving it the 

 true figure ; for an error in a reflecting surface 

 affects the direction of the rays much more than a 

 like error in a refracting surface. An alloy of 

 metals is made for forming the speculum ; but 

 it has been found that platina is the best substance 

 known for this purpose, as it never tarnishes. It 

 is, however, extremely difficult to work, and, con- 

 sequently, has been hitherto very little used. 



The adjusting screw on the outside of the great 

 tube fits this telescope to all sorts of eyes, by 

 bringing the small mirror either nearer to the eye, 

 or removing it farther ; by which means the rays 



