6^ Microscopical Essays, 



micro fcope magnifies the diameter of the object ; fay, as 240, 

 the diftance of the eye from the image of the object, is to 800, 

 the diftance of the eye from the objea, fo is 1068, double the 

 meafure found on the micrometer, to 3563, or the number of 

 times the microfcope magnifies the diameter of the object. By 

 working in this manner, the magnifying power of each lens ufed 

 with the compound microfcope, may be eafily found, though the 

 refult will be different in different compound microfcopes, vary- 

 ing according to the combination of the lenfes, their diftance 

 From the objecl and one another, Sec. 



Having difcovered the magnifying power of the microfcope, 

 with the different objea lenfes that are ufed therewith, our next 

 fubjea is to find out the real fize of the objefts themfelves, and 

 their different parts ; this is eafily efeded, by finding how many 

 revolutions of the micrometer-fcrew anfwer to a known meafure 

 on the ieBoral fcale, or other objea placed on the ftage ; from 

 the number thus found, a table fhould be conftruaed, expreffmg 

 the value of the different revolutions of the micrometer with that 

 objea lens, by which the primary number was obtained. Similar 

 tables muff be conftruaed for each objea lens. By a fet of tables 

 of this kind, the obferver may readily find the meafure of any 

 objea he is examining ; for he has only to make the needle point 

 traverfe over this objea, and obferve the number of revolutions 

 the fcrew has made in it's paffage, and then look into his table 

 for the real meafure which correfponds to this number of revolu- 

 tions, which is the meafure required. 



C H A P. 



