312 COMPOUND AND ELECTRON MICROSCOPES 



material under examination appear, but in how much of the actual 

 structure he can render clearly visible. This is determined by the 

 resolving power of his instrument, not by its magnifying power. 



Magnifying Power 



The total magnification of a microscope must be sufficient to make 

 the closest detail one hopes to see in the virtual image appear separated 

 at the distance of distinct vision, at least 0.074 mm. By examining 

 Fig. VIII-1 it is seen how this magnification is obtained. 



The magnifying power of a compound microscope is the relative size 

 of the virtual image to object. 



There are five ways of varying the magnifying power of a compound 

 microscope : 



1. By using a higher- or lower-power objective. 



2. By using a higher- or lower-power ocular. 



3. By lengthening or shortening the draw tube. (Normal length 

 160 mm.) 



4. By increasing or diminishing the distance at which the virtual 

 image is projected. (Normal or distance of distinct vision, 250 mm.) 



5. By the use of an optical amplifier, i.e., a diverging lens placed 

 between objective and ocular. 



Structure of the Compound Microscope 



In its simplest form (Fig. VIII-1) the microscope consists of an objec- 

 tive (11), which forms a real, magnified, and inverted image of an object 

 placed just below the principal focus of the objective. The magnified 

 image is viewed by the aid of the ocular (1). This real image lies in the 

 eyepiece aperture below the eyepoint and hence must be spread suffi- 

 ciently by the eye lens of the ocular so that the retinal image has two 

 adjoining structures fall on at least two retinal elements. It thus 

 becomes apparent that a compound microscope must comprise a short- 

 focus objective and a short-focus eye lens. 



Numerical Aperture or N.A. 



The term numerical aperture was introduced by Professor Abbe to 

 allow a precise comparison to be made of the resolving power of one lens 

 or train of lenses with another. 



It must be remembered that the numerical aperture is the measure 

 of the resolving power of a microscope objective. Numerical aperture 

 is not a direct expression of the resolving power, though from it one 

 may be able to calculate the resolving power. 



