CHAPTER III 



THE OPTICAL EQUIPMENT : OBJECTIVES AND OCULARS 



THE most important part of the apparatus required for photo- 

 micrography is the optical system. It may conveniently be 

 divided and treated under four headings : 



1. The objective, or magnifying system. 



2. The ocular, or eye-piece. 



8. The sub-stage condensing apparatus for illuminating the 

 object. 



4. The collecting or condensing lens or lenses. 



As a matter of convenience the last two are separated in this 

 manner to distinguish between the actual sub-stage condenser, 

 as fitted to all modern microscopes, and any optical system 

 that may intervene between the source of light and the sub- 

 stage condenser for the purpose of concentrating or diffusing 

 the light. The latter optical system will be referred to 

 throughout as the collecting-lens or lenses. 



The Objective. It does not come within the scope of this 

 work to deal with the theory of microscopic optics, but an effort 

 will be made to give some simple directions so that a student, 

 or microscopist of limited experience, may be able to determine 

 within certain limits whether the objectives he already has, or 

 may contemplate purchasing, are satisfactory for the purpose 

 in view. The novice must of necessity rely on the judgment 

 of others, either depending on those from whom the apparatus 

 is purchased, or perhaps preferably seeking the assistance of 

 a friendly expert. 



There are two well-defined types of microscope objectives 

 now to be obtained achromatic and apochromatic the differ- 

 ence between them depending on their degree of correction for 



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