Microscopical Essays. 1C3 



concave glafs, which are thus preferved from being crufhed, and 

 prevented from wandering out of the iield of view. 



The. Screw Barrel, or Wilson's single Pocket 

 Microscope, Fig. 1 and 2, Plate II. B. 



This microfcope of Mr. Wiifon's is an invention of many years 

 Handing, and was in fome meafure laid afide, till Dr. Lieberkuhn 

 introduced the foiar apparatus, to which he applied it, there 

 being no other inftrument at that time which would anfvver his 

 purpofe fo well ; fmce which time it has been revived, and much 

 efteemed, though very troublefome in many cafes. 



The body of the microfcope is reprefented by AB, Fig. 1, 

 and is made either of filver, brafs, or ivory. 



C C is a long fine-threaded male fcrew that turns into the 

 body of the microfcope. 



D, a convex glafs at the end of the faid fcrew, on which may 

 be placed, as occafion requires, one of the two concave pieces of 

 thin brafs, with holes of different diameters in the center of them, 

 to cover the faid glafs, and thereby diminifh the aperture when 

 the greater! magnifiers are ufed. 



E, three thin plates of brafs within the body of the microfcope, 

 one whereof is bent femicircularly in the middle, fo as to forms 

 an arched cavity for the reception of a tube of glafs. 



F,. a 



