CHAP. III.] 



THE MICROSCOPE. 



235 



under which the eye sees either one or the other supposed to be 

 placed at the distance of distinct vision. 



In the case of the magnifying glass, as the distance from the eye 

 to the lens may be neglected, the magnification is equal to the ratio of 

 the angles A'OB' and aOb, or sensibly to that of the dimensions A'B', 

 AB, which again is equal to the ratio of the distances 00' and 00. 



The distance 00' being that of distinct vision, the magnification 

 only depended, as we see, on the distance 00 between the object and 



FIG. 171. Magnifying glasses of different kinds. 



1. 2. Watchmaker's and engravers' magnifying glass. 3, 4, 5. Achromatic magnifying glasses. 6. 

 Stanhope lens. 7. Magni lying glass with cylindrical surface 8. Brewster's (or Coddington's). 

 9 and 13. Other forms. 11, 12, 14, and 15. Naturalist's pocket magnifying glasses, with one, two, 

 or three lenses of different powers. 



the lens, that is from the principal focal distance which differs only 

 slightly from it. 



Therefore, the sharper the curves of the magnifying glass, and the 

 longer the distinct vision of the observer, the more considerable will 

 be the magnifying power. 



The mounted magnifying glass, shown in section and perspective 



