320 



COMPOUND AND ELECTRON MICROSCOPES 



slightly changed in the low-numerical-aperture objective by a slight 

 vertical displacement. 



Front lens of an 

 oil-immersion objective 



Front lens of dry objective 



Upper surface of 

 condense> N 

 n=1.520 \ 



Fig. VIII-4. The rel- 

 ative size of a cone of 

 light originating at the 

 virtual source C when 

 oil is used as compared 

 with the virtual source 

 C a when air is used be- 

 tween all mountings. 

 The numerical aperture 

 of the oil cone is larger 

 than that of the air cone. 

 Note how much larger 

 the angular aperture of 

 the lens becomes as the 

 result of its oil immer- 

 sion. 



Microscope Objectives 



Objectives (Fig. VIII-6) may be designated by their equivalent focal 

 lengths. These numbers (0.25 N.A., 16 mm) indicate that the objective 

 produces a real image of approximately the same size as that produced 

 by a simple convex lens whose principal focus lies at the distance marked 

 upon the objective. 



Fig. VIII-5. The circles of diffusion at 

 equal distances from the focal point F are 

 shown as black disks. 1. Lens with large 

 aperture. 2. Lens with small aperture. In 

 (2) circles of diffusion as large as in (1) can 

 be found further to the right and left of F. 

 Depth of focus (2) > (1). 



Microscope Objective Corrections 



The purpose of the microscope is to give the best possible image of the 

 detail in the object, and faithfully reproduce that detail in shape and 

 color in the image. The designer of a microscope objective tries to 

 reduce spherical and zonal aberration as well as chromatic aberration 

 and coma to a minimum. As a result of the application of these cor- 

 rections to an objective, the image is substantially free from haze and 

 color. 



