6 



Microscopical 



Essays, 



vex, and confequently the magnifying power very great, the 

 field of view is fo fmall, and it is fo difficult to adjuft with accu- 

 racy their focal diftance, that it requires fome practice to render 

 the life thereof familiar ; at the fame time, the fmallnefs of the 

 aperture to thefe lenfes has been found injurious to the eyes of 

 fome obfervers : notwithstanding, however, thefe defects, the 

 .great magnifying power, as well as the diftinct vifion which is 

 obtained by the ufe of a deep fmgle lens, more. than counter- 

 ballances every difficulty and difadvantage. It was with this in- 

 ftrument that Leeuwenhoek and Swammerdam, Lyonet and Ellis 

 ■examined the minima of nature, laid open fome of her, hidden 

 recefles, and by their example ftimulated others to the fame 

 purfuit. 



The conftruction of the fmgle microfcope is fo fimple, that it 

 is fufceptible of but little improvement, and has therefore under- 

 gone but few alterations ; and thefe have been chiefly confined to 

 the mode of mounting it, or the additions to it's apparatus. The 

 greateft improvement this inftrument has received, was made by 

 Dr. Lieberkuhn, about the year 1740; it confifted in placing the 

 fmall lens in the center of a highly-polifhed concave fpeculum of 

 filver, by which means he was enabled to reflect a ft rong light 

 upon the upper furface of an object:, and thus examine it with 

 great eafe and pleafure. Before this contrivance, it was almoft 

 impofhble to examine fmall opake objects with any degree of 

 exactnefs and fatisfaction ; for the dark fide of the object being 

 next the eye, and alfo overfhadowed by the proximity of 

 the inftrument, it's appearance was neceffarily obfcure ^and 

 indiftinct. 



Dr. 



