XII. ELECTRON IVI I C R O S C O P Y 



387 



magnifications are desired. Similarly, the substagc condenser, unlike 

 its light-optical counterpart, is not usually provided with an iris 

 diaphragm. Nevertheless, the angular aperture of the illumination 

 can be controlled over a wide range of values by varying its focal 

 length. The magnification of the objective of an electron micro- 

 scope is a fixed quantity determined by the positions of the specimen, 

 objective lens, and projection lens. On the other hand, the mag- 



ELECTRON GUN |" "| 



4>:x= 



no V. AC. 



CONDENSER LENS 



SUPPLY 



20-110 mA. 



STABILITY 0.1% 



(ILLUMINATION CONTROL) 



SPECIMEN 

 OBJECTIVE LENS 



Z|>0c= 



110 V. AC. 



PROJECTION LENS 



SUPPLY 



(MAGNIFICATION 



CONTROL) 



10-120 mA. 



STABILITY 0.068% 



PHOTOGRAPHIC PLATE 



30-60 KV. NEGATIVE fZX(1h 

 ACCELERATING ^^ 



POTENTIAL SUPPLY 



0.5mA. 

 STABILITY 0,015% 

 R.F. FILAMENT SUPPLY 



no V. AC. 



FILAMENT 

 ANODE 



CONDENSER LENS 

 WITH APERTURE 



OBJECTIVE LENS 



SUPPLY 

 (FOCUSING CONTROL) 



10-130 mA. 

 STABILITY 0.0075 % 



=^ 



110 V AC 



INTERMEDIATE 

 IMAGE SCREEN 



VACUUM CHAMBER 



PROJECTION LENS 



FLUORESCENT SCREEN 

 (HINGED) 

 PHOTOGRAPHIC PLATE 



Fig. 3. Simplified cross section of magnetic type of electron microscope. 



nification of the projection lens can be varied over a wide range by 

 adjusting the current in the exciting coil. The available range may 

 be as much as 4 to 1. If a greater range of magnification is desired, 

 they can be achieved by replacing the polepieces of the projection 

 lens or through the provision of a third lens, which — when varied 

 appropriately in conjunction with the normal projection lens — can 

 provide variations in magnification over a range of 500 to 1. 



In the light microscope all of the useful magnification is usually 

 obtained directly in the instrument. In the electron microscope this 

 is seldom done for a number of purely technical reasons. The first 

 of these is that at the highest magnifications the field of view is 



