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HANDBOOK OF PHOTOGRAPHY 



Classes of Objectives. — Objectives are divided into four general classes: achromatic, 

 semiapochromatic, apochromatic, and monochromatic. These objectives do not 

 consist of single lenses but are composed of two or more lenses very accurately centered 

 and permanently mounted in a metal holder. The component parts of the lens system 

 are selected so as to correct or compensate certain errors which are always characteris- 

 tic of a simple lens. The value of the objective depends on the degree to which these 



Recr/ image 

 inverfea 



> Eye piece 



Magnified 

 viriual 

 image 



Mirror 



Fig. 1. — Optical system of compound microscope. 



imperfections have been overcome. The difference in quality between the first three 

 classes of objectives is primarily a matter of the degree to which corrections for chro- 

 matic and spherical aberrations have been applied. The achromatic objectives are 

 intended primarilj' for visual observations and the principal optical defects are cor- 

 rected or approximately so for the yellow-green of the visible spectrum since the eye 

 can see best with light of this color. In these objectives the correction becomes less 



