§4 



Microscopical Essays. 



this will be in the fame proportion that the limits of natural fight 

 bear to the focus of the lens. If, for inftance, the convex lens is, 

 of one inch focus, and the natural fight of eight inches, an ob- 

 ject feen through that lens will" have it's diameter apparently 

 increafed eight times ; but as the object is increafed in every 

 direction, we muft fquare this apparent diameter, to know how 

 much the object is really magnified ; and thus multiplying 8 by 8 t 

 we find the fuperficies is magnified 64 times. 



From thefe principles, the following general rule for afcertain- 

 i-ng the magnifying pov/er of (ingle lenfes, is deduced. Place a 

 fmall thin tranfparent object on the ftage of the microfcope, 

 adjuft the lens till the object appears perfectly diftinct, then mea- 

 fure the diftance accurately between the lens and the object, 

 reduce the meafure thus found to the hundredths of an inch, and 

 calculate how many times this meafure is contained in eight 

 inches, firft reducing the eight inches into hundredths, which 

 will give you the number of times the diameter of the object is 

 magnified ; which number multiplied into itfelf, or fquared, 

 gives the apparent fuperficial magnitude of the object. 



As only one fide of an object can be viewed at a time, it is 

 fufficient, in general, to know how much the furface thereof is 

 magnified : but when it is neceffary to know how many minute 

 objects are contained in a larger, as for inftance, how many given 

 animalcula are contained in the bulk of a grain of fand, then we 

 muft cube the firft number, by which means we mail obtain the 

 folidity or magnified bulk. 



'The 



