29 



during observation. They are made in a large 

 variety of forms, the difference appearing in their 

 optical as well as mechanical construction. The 

 most common are those with one or several double 

 convex lenses and mounted in hard rubber or vul- 

 canite, as shown in Fig. 12 and Fig. 13. Those 

 containing several lenses are preferable, since they 

 offer a variety of magnifying powers and in com- 

 bination give the greatest magnification admissible 

 with single lenses. 



Coddington Lens. A lens of greater efficiency 

 than the ordinary magnifier is the so-called Cod- 

 dington lens, Fig. 14. While this is also a single 



Fig. 14. 



double convex lens it will be noticed that it has con- 

 siderable thickness, being really the central portion 

 of a sphere and provided with a circular incision 

 at the middle, which is blackened and thus acts as 

 a diaphragm, shutting out the marginal rays and 

 correcting the spherical aberration, at the same 

 time however, limiting the size of field. 



