in the linear, it is magnified sixteen times in area. The 

 commonly accepted term to express magnifying power 

 of simple as well as compound microscopes is in 

 diameters* (linear). 



To Determine Magnifying Power. For simple 

 lenses the magnifying power may be determined by 

 dividing ten by the focal length in inches. Thus a 

 single lens of i inch focus magnifies about ten dia- 

 meters; one of 2 inch focus, about five diameters; 

 one of \ inch focus, twenty diameters, and so on. In 

 a lens of high magnifying power, the focus is ordinarily 

 made about twice the diameter, so that if a lens is 

 \ inch diameter its focus is about i inch. 



While the determination of focus in single lenses 

 gives approximate magnifying power, it will not do so 

 in some of the forms which have been described. The 

 following method, if carefully followed, will give very 

 accurate results and is withal simple and interesting: 

 Place a sheet of white paper on the table. With the 

 source of light at the right or left hand, arrange a pile 

 of books to such height that when the magnifier to be 

 tested is placed upon the pile with the lens projecting 

 toward the observer the distance between the upper 

 surface of the lens and the paper \\ill be exactly ten 

 inches. The magnifier can be held in place by 

 another book placed upon it. Place a pocket ruler 

 between the leaves of the upper book so that when the 

 edge is close to the magnifier the divisions on the ruler 



25 



