DESCRIPTION OF THE ACHROMATIC MICROSCOPE. \\\ 



Sect. 18. Method of viewing Opaque Bodies by oblique 

 refected Light condensed upon them by Mirrors. — It is often 

 necessary to examine an object under object-glasses, 

 which differ widely in their amplifying powers, or focal 

 lengths, when great difficulty is experienced in mounting 



Fig. 31. 



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it, so as to accommodate it to them. For instance — ■ 

 suppose the object to be mounted on a circular disc, 

 suitably for an object-glass of one-inch focus, it would 

 be barely possible to illuminate it with a silver cup, 

 adapted to an object-glass of l-4th of an inch. Deep 

 object-glasses necessarily require the silver cups to be 

 small, by which means it is impracticable to give an 

 oblique direction to the light. The method usually pur- 

 sued, is to employ a silver concave reflector of the form 

 shewn at /, fig. 31 ; by the aid of which light may be con- 

 densed upon the object, so as to exhibit it under object- 

 glasses of different focal lenghs. 



In order to obtain a greater condensation of light, Mr. 



