Subsurface Laboratory Methods 



203 



limited by the wave length of visible light. Theoretically, the electron 

 microscope should be capable of resolving powers as small as atomic di- 

 mensions. In actual practice, however, the microscope has not been per- 

 fected to that extent. Nevertheless magnifications of over 100,000 diame- 

 ters are practical with the electron microscope, as compared with a useful 

 limit of 2,000 diameters for the light microscope. 



Description of the Microscope 



Figure 85 is a comparison of the optical microscope with the electron 

 microscope showing equivalent parts: magnetic fields are equivalent to 

 lenses; both have specimen levels; and both have photographic plates for 



OPTICAL 

 MICROSCOPE 



ELECTRON 

 MICROSCOPE 



Photographic Piote 



Eyepiece or 



Photo -projector 



Obiective Lens System 



Substoge Condenser 



Reflector 



Projected » 

 Innage 



Phologrophic Plate 



Magnetic Coil 

 Projector 



''^ZZZ>,' 



Mognetic Coil 

 Serving as 

 Objective 



Magnetic Coil 



Concenlrotmg 

 Electron Beom 



- • C Source 

 Light' ^E 



A, 



Figure 85. Comparison of light microscope and magnetic electron microscope. 

 (From Burton and Kohl.) 



pictures. For comparison, the electron microscope is diagrammed upside 

 down. 



A simplified drawing of the R.C.A. compound magnetic electron mi- 

 croscope, type EMB, is shown in figure 86. Focusing is accomplished by 

 varying the lens power. The specimen mount is the movable stage. As the 

 stage is inside the vacuum portion of the microscope, it is moved by means 

 of fine screws and a metal flexible bellows. 



The electron beam is concentrated on the specimen by the magnetic 

 field produced in the condenser-lens coil. After passing through the speci- 

 men, the electrons are focused by the objective-lens coil into an intermedi- 



