150 



MICROSCOPIC TECHNIQUES 



The diaphragm (D) has an aperture of 25 mm. over which are 

 placed a few fine wires to serve as fiduciary marks in focusing the 

 lens Lo. The image is formed with lens Li (Fig. 30) on the aperture 

 of the diaphragm in order to reduce spherical aberration. The half- 

 wave rectifier and filter system (Fig. 29) supplies the potential 

 difference required for accelerating the electrons. The current pass- 

 ing through the 10^ ohms resistance placed across the high-voltage 

 leads is measured by the microammeter from which the magnitude 

 of the accelerating voltage can be obtained. The rectifier is supplied 

 by the secondary of a transformer that is insulated from the primary 

 for 15,000 volts. A variable resistance in the primary circuit of this 

 transformer permits the choice of the high D.C. voltage employed. 



Fig. 30. Diagrammatic representation of the electron path and consequent 

 image formation. A, cathode and support; C, fluorescent screen; D, diaphragm; 

 E, batteries for heating filament of cathode; F, schema of magnetic field of 

 the lenses Li and La; V, high-voltage source. From Scott and Packer (1939a) 



Constancy is maintained in the accelerating voltage in order to 

 prevent a distortion, similar to chromatic aberration in optical 

 systems, by the use of a voltage regulator, of the saturated-core 

 transformer type, in the primary circuit of the transformer. Another 

 precaution taken to avoid distortion of the image is the placing of 

 the batteries, high-voltage supply, and all iron objects at quite a 

 distance from the microscope. 



In order to compensate, at the magnification used (< 150x), 

 for the deflection of the electron stream by the earth's magnetic 

 field, the first lens (Li) is tilted. 



Water-cooling coils of copper tubing are employed to remove heat 

 from the microscope tube. One coil is wound around the tube on the 

 image side of lens Li and another at the junction of the tube and the 



