58 



Microscopical Essays,. 



If you advance towards M, half the focal diftance, the apparent 

 diameter will be only increafed one-third. If, on the contrary, 

 the point of fight is lengthened to double the diftance of it's 

 focus, then the magnified diameter will appear to be three times 

 that of the real object. Mr. Magny concludes from hence, that 

 there is an impropriety in eftimating the magnifying power of the 

 eye glafs of compound micro fcopes, by feeing how often it's focus, 

 is contained in eight or ten inches ; and to obviate thefe defects, 

 he recommends two methods to be ufed, which reciprocally con- 

 firm each other,. 



The firft and molt fimple method to find how much any com- 

 pound microfcope magnifies an objecl, is the fame which is. 

 defcribed by Dr. Hooke in his Micrographia, and is as follows : 

 place an accurate fcale, which is divided into very minute parts, 

 of an inch, on the ftage of your microfcope; adjuft the micro- 

 fcope, till thefe divifions appear diftinft ; then obferve with the 

 other eye how many divifions of a rule, fimilarly divided and 

 held at the ftage, are included in one of the- magnified divifions : 

 for if one divifion, as feen with one eye through the microfcope, 

 extends to thirty divifions on the rule, which is feen by the naked 

 eye, it is evident, that the diameter of the objea is increafed or, 

 magnified thirty times. 



For this purpofe, we often ufe a fmall black ebony rule, (fee 

 Fig. 4, Plate II. A.) three or four tenths of an inch broad, and 

 about feven inches long ; at each inch is fixed a piece of ivory, 

 the firft inch is entirely of ivory, and fubdivided into ten equal: 



