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 
rig. 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 
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. 
106 
