12 



rays, or the lenses may be made of a smaller diam- 

 eter. An incision may also be cut into the glass 

 equally between the two surfaces, when from the name 

 of the inventor, it is called a Coddington. Fig. 2 shows 

 a section of a Coddington, while Fig. 3 shows it in 

 its mounting. 



Achromatism. The most approved method, 

 however, for eliminating these appearances, is by the 

 use of one or two con cave flint glass lenses in connec- 

 tion with the double convex crown glass lens. When 

 the color or chromatic aberration is thus removed, the 

 lenses are said to be achromatic, and when both the 

 chromatic and spherical aberrations are avoided, the 

 lens is called aplanatic, and is then said to be cor- 

 rected. An achromatic lens, composed of one flint 

 and one crown glass lens, is called a 

 doublet ( Fig. 4 ) ; one with two flint 

 glass lenses and one crown glass is 

 Fig. 4. called a triplet (Fig. 5). The latter is 

 the best form, as it gives the highest cor- 

 rection ; such a lens (it is thus called from 

 the the fact that the lenses are cemented 

 together and act like one) may be held 

 with either side toward the object with 

 equally good results, and may also be held Fig. 5. 

 at quite an obliquity, without loss of definition ; this 

 feature is important, as it is almost impossible to give 

 a lens a theoretically correct position to both the eye 

 and object with the unaided hand. 



