86 MICROSCOPIC ILLUSTRATIONS. 



tical department, particularly in the eye-pieces, one 

 could hardly imagine that their usefulness would be 

 abridged through their inefficiency in other respects. 



By a microscope suited for general observations, is to 

 be understood one of such construction as will allow us 

 to examine both opaque and transparent bodies with 

 such facility that we may ascertain their true forms and 

 structure. In order to compass this, both the^ instru- 

 ment and the illumination must admit of being turned 

 in all directions. The instrument must be further ca- 

 pable of carrying a wide range of magnifying powers, 

 so that whether the object to be investigated be of mo- 

 derate dimensions or a mere atom, it may come fully 

 within its scope. It is not an uncommon case to find a 

 microscope excellently constructed for a given power, 

 and totally unfitted for all others. For instance, that 

 beautiful little instrument for carrying Wollaston's 

 doublets, can only be employed with effect for viewing 

 very minute transparent tissues, as the scales of the 

 Podura, &c. It is not suited for examining the living 

 objects described in the previous chapters, nor most 

 animal or vegetable structures ; and that splendid and 

 extensive class of bodies, opaque objects which require 

 strong reflected light and moderate shallow powers to 

 render their beautiful structures appreciable is entirely 

 beyond its reach. In either of the two microscopes I 

 shall describe in this work, every class of bodies that 

 I am acquainted with can be correctly judged of, whilst 

 they command a very extensive range of powers. Dr. 



I 



