THE MICROSCOPE. 



25 



achromatic object-glasses now supplied by good opticians 

 leave nothing to be desired. 



Object-glasses. A general view of an achromatic object- 

 glass is given in Fig. 3. It is a system of three pairs of 

 lenses, 1, 2, 3, each composed of a double convex of crown 

 glass and a plano-concave of flint, a, 6, c, represents the 

 angle of aperture, or the cone of rays admitted. It is 

 unnecessary to consider the optical principles which un- 

 derlie this construction. Different opticians have different 

 formulae and propose various arrangements of lenses, and 

 there is room for choice among the multitude of micro- 

 scopes presented for sale. For high powers, the German 



Flu. 3. 



FIG. 4. 



Achromatic Object-glass. 



Huygenian Eye-piece. 



and French opticians have lately proposed a princip'e of 

 construction which is known as the immersion system. 

 It consists in the interposition of a drop of water between 

 the front lens of the objective and the covering glass over 

 the object. This form of object-glass is corning into gen- 

 eral use. For the more perfect performance of an objec- 

 tive, it is necessary that it should be arranged for correct- 

 ing the effect of different thicknesses of covering glass. 

 This is accomplished by a fine screw movement, which 

 brings the front pair of lenses (1, Fig. 3) nearer or further 

 from the object. In this way the most distinct and accu- 

 rate view of an object may be obtained. 



