. Biophysical research methods. Biophysics -- Research. 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&g
. Biophysical research methods. Biophysics -- Research. 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= noV. AC. CONDENSER LENS SUPPLY 20-110 mA. STABILITY (ILLUMINATION CONTROL) SPECIMEN OBJECTIVE LENS Z|>0c= 110 V. AC. PROJECTION LENS SUPPLY (MAGNIFICATION CONTROL) 10-120 mA. STABILITY PHOTOGRAPHIC PLATE. 30-60 KV. NEGATIVE fZX(1h ACCELERATING ^^ POTENTIAL SUPPLY STABILITY 0,015% FILAMENT SUPPLY no V. AC. FILAMENT ANODE CONDENSER LENS WITH APERTURE OBJECTIVE LENS SUPPLY (FOCUSING CONTROL) 10-130 mA. STABILITY % =^ 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. Please note t
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