MAGNIFYING GLASSES. 



sionally used for reading small type, by persons of very weak 

 sight ; they consist of double convex lenses of 5 or 6 inches focal 

 length, and having consequently a magnifying power no greater 

 than two; they are usually double convex lenses, from 2 to 3 

 inches in diameter, mounted in tortoise-shell or horn, with conve- 

 nient handles. 



22. Magnifiers of somewhat shorter focal length and less 

 diameter, similarly mounted, are used by miniature-painters and 

 engravers. 



23. Lenses having a focal length of about one inch, set in a 

 horn cell, enlarged at one end like the wide end of a trumpet, the 



magnitude being made to correspond with the 

 Fig. 7. socket of the eye, as represented in fig. 7, 



are used by watch-makers. The wide end 

 being inserted under the eyebrow, is held in 

 its position by the contraction of the muscles 

 surrounding the eye-ball, and the minute 

 work to be examined, is held within an inch 

 of the lens set in the smaller end of the horn 

 case ; if the focal length be an inch, the mag- 

 nifying power of such a glass, for average 



eyes, will be ten. 



Glasses somewhat similarly mounted are used by jewellers, gem- 



.sculptors, and other artists. 



24. To relieve the artisan from the fatigue of holding the mag- 



nifier in the eye-socket 

 j^ or in the hand, a stand 



with a moveable socket 

 is sometimes resorted 

 to, such as that repre- 

 sented in fig. 8. A hori- 

 zontal arm slides upon a 

 vertical rod, upon which 

 it can be fixed at any 

 desired height by a 

 tightening screw. This 

 arm consists of two 

 joints, connected to- 

 gether by a ball and 

 socket, by which they 

 can be placed at any 

 desired inclination; at 

 the extremity of the 



lower arm a fork supports a ring-shaped socket, made to receive 



the magnifier. 

 10G 



