46 Microscopical Essays, 



limits of diftincl vifion with the naked eye) to eight inches. For 

 example, if the femi-diameter of a lens, equally convex on both 

 fides, be half an inch, which is alfo equal to it's focal diftance, 

 we mail have as £ is to 8, fo is 1 to 16 ; that is, the diameter 

 of the object in the proportion of fix teen to one. 2. As the 

 diftance of eight inches is always the fame, it follows, that by 

 how much the focal diftance is fmaller, there will be a greater 

 difference between it and the eight inches ; and confequently, 

 the diameter of the objecl will be fo much the more magnified, 

 in proportion as the lenfes are fegments of fmaller fpheres. 3 . If 

 the objecl be placed in the focus of a glafs globule or fphere, and 

 the eye be behind it in the focus, the objea will be feen diftina 

 in an erea fituation, and magnified as to its diameter, in the pro- 

 portion of I of the diameter of the globule to eight inches ; 

 thus fuppofe the diameter of the fphere to be to of an inch 9 

 then 1- of this will be equal to -h; confequently, the real 

 diameter of the objea to the apparent one, as & to 8, or as. 

 3 to 3 20, or as 1 to 1 60 nearly. 



Of the Double or Compound Microscope-. 



In the compound microfcope, the image is viewed inftead of 

 the objea, which image is magnified by a fingle lens, as the ob- 

 jea is in a fingle microfcope. It confifts of an objea lens, L N, 

 Fig. 5, Plate I. and an eye glafs F G. The objea OB is placed 

 a little further from the lens than it's principal focal diftance, fo 

 that the pencils of rays proceeding from the different points of 

 the objea through the lens, may converge to their refpeftive 

 foci, and form an inverted image of the objea at P Q ; which 

 image is viewed by the eye through the eye glafs F G, which is 



