Microscopical Essays. 



The fccond obflacle to the perfection of" microfcopic glafles, is 

 the final 1 fize of great magnifiers ; by which means, notwithfland- 

 ing their near approach to the object, they receive a fmaller 

 quantity of light than might be expected. Thus, fuppofe a glafs of 

 only one-tenth of an inch focal diftance,fuch aglafs would increafe 

 the vifible diameter eighty times, and the furface 6400 times. If 

 the breadth of the glafs could at the fame time be preferved as 

 great as the pupil of the eye, which we fhall fuppofe one-tenth of 

 an inch, the object would appear magnified 6400 times, and every 

 part would be as bright as it appears to the naked eye. But if we 

 fuppofe the lens to be only 4s of an inch diameter, it will then 

 only receive £ of the light which would otherwife have fallen 

 upon it ; therefore, inftead of communicating to the magnified 

 objet: a quantity of light equal to 6400, it would communicate 

 an illumination fuited only to 1600, and the magnified object 

 would appear four times as dim as it does to the naked eye. This 

 inconvenience can, however, be in a great degree removed, by- 

 throwing a much larger quantity of light on the object Various 

 methods of ending this purpofe will be pointed out in the 

 courfe of the work. 



The third obflacle arifes from the fhortnefs of the focal 

 diftance in large magnifiers ; this inconvenience can, like the 

 former, be remedied in fome degree by artificial means of accu- 

 mulating light ; but frill the eye is fo (trained, as it muff be 

 brought nearer the glafs than it can well bear, which in fome mea- 

 fure fuperfedes the ufe of very deep lenfes, or fuch as are capable 

 of magnifying beyond a certain degree. 



The 



