DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 141 



SECT. 18. Method of viewing Opaque Bodies by oblique 

 reflected 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. 



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 t, 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. 



