CHAPTER III 



OBJECTIVES AND EYEPIECES 



IN order to make this chapter as helpful as possible in 

 the selection of objectives and eyepieces, a brief account 

 is given of the main defects inherent in a simple lens, and 

 of the methods adopted to correct them. Several of the 

 more intricate errors are either entirely passed over or 

 merely mentioned, as a discussion of them, and of the 

 mathematical laws which govern their correction, is 

 neither essential to a realisation of the advantages of 



Fig. 19 



SPHERICAL ABERRATION 



one type of objective over another, nor to an intelligent 

 application of the principles underlying the various 

 methods and devices employed in photomicrography 

 to overcome them. Many of the better-known optical 

 terms are used without definition, and a knoAvledge is 

 assumed of the general principles of refraction and of 

 the formation of the solar spectrum. 



Spherical Aberration. Rays of monochromatic light 

 from a point P incident upon different parts of the 

 surface of a simple lens, LL, Fig. 19, are refracted in such 



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