236 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



in Fig. 171, 1, 2, is that most used by watchmakers and engrav'ers. 

 It is held in the hand or even by the eye, where. the observer retains 

 it by an effort of the muscles of the eyebrows and the cheek ; in this 

 way the hands remain free ; but it is best to adapt it to a support or 

 upright stand (Figs. 172 and 173). 



The magnifying power of these lenses rarely exceeds five times ; 

 they possess, moreover, a serious defect : that is, the spherical aberra- 

 tion is very great. The proof of this is easy. If you look at an 

 object of a certain size with one of these lenses, it will be seen that 

 the image is only sharp in the centre : at the edge it is deformed and 



FIG 172. Support for lens. 



Vic,. 173. Another kind of stand. 



ill-defined. Moreover, it is coloured, which shows another defect 

 that simple lenses lack achromatism. But they have an advantage 

 which partly compensates for these inconveniences : that of a large 

 field ; the great focal distance leaves space for the movement of the 

 hands and the objects below the lens, and work may be carried on 

 without inconvenience. 



Spherical aberration is diminished by applying a diaphragm or 

 opaque annular plate to the -edges of the lens ; this stops the rays 

 from this part of the lens, bat the field is thereby diminished. 



The magnifying glasses represented at Fig. 171, 11, 12, 14, and 

 15, are used by naturalists. The same mounting encloses two or three 



