watchmakers’ and jewellers’ magnifiers. 
25. Very convenient pocket magnifiers are mounted in tortoise¬ 
shell or horn cases, in the form shown in fig. 9. Lenses of 
Fig. 9. 
different powers are provided which may he used separately or 
together; when they are used together, however, the interposi¬ 
tion of a diaphragm is necessary to diminish the effects of 
spherical aberration by cutting off the lateral rays. 
Lenses thus mounted are well fitted for medical use, and for 
certain researches in natural history. 
26. When a higher power is required than that which these 
common magnifiers afford, a magnifying glass, called from its 
inventor a Coddington lens, is 
used with much advantage. To 
produce such a lens, a solid ball 
or sphere of glass, about \ an inch 
in diameter, is cut round its 
equator, so as to form round it 
an angular groove, leaving two 
spherical surfaces on opposite 
sides uncut. The angular groove 
is then filled up with opaque 
matter, the circular edge of the 
groove serving as a diaphragm 
between the two spherical sur¬ 
faces. A section of such a lens 
is shown in fig. 10, where A B and 
A' b' are the two spherical sur¬ 
faces left uncut, and a a a' and 
B c b' the section of the angular 
groove filled with opaque matter. 
The course of the rays passing 
through it from any point such 
as o, is shown by the lines o o, and it will be evident from the 
107 
