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one of .! inch focus, twenty diameters and so on. 

 In a lens of high magnifying- power, the focus is 

 ordinarily about twice the diameter, so that if 

 a lens is J inch diameter its focus is about 1 inch. 

 To Determine Magnifying Power. - -While 

 the determination of focus in single lenses gives 

 approximate magnifying power, it cannot be done 

 in some of the forms which have been described. 

 The following method, if carefully followed, will 

 give very accurate results and is withall simple 

 and interesting. Upon the upper or farther edge 

 of a sheet of white paper, which rests upon a table 

 with the light at the right or left hand, place a 

 series of books of such height that when the mag- 

 nifier is placed upon the inner edge or that nearest 

 the body, of th^ upper book with the lens project- 

 ing over it, the distance to the upper surface will 

 be exactly 10 inches and weigh down the mount- 

 ing with an additional book or weight. Now 

 place a pocket rule between the leaves of upper 

 book so that when the edge is close to the magni- 

 fier the divisions on the rule will be exactly in 

 focus, place the rule so that it shall not project 

 much over the lens. It is immaterial what the 

 divisions on the rule are, whether inches or milli- 

 meters, so long as they are reasonably fine. View 

 the divisions with the right eye and open the left 

 eye, when it will be found that the divisions are 

 apparently projected upon the paper. Take a 



