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. 0. 



different powers are provided which may be 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 Fi 10 



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 AHA' and 

 BOB' 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 



