MAGNETIC CONDENSER 347 



reprojection, with its accompanying magnification, into the plane of the 

 photographic plate placed at P. The port P 2 is used with the aid of a 

 movable fluorescent screen to view the composition of the final image. 

 The height of the electron microprojector, from the high-potential 

 voltage terminal V to the base B where the vacuum pump connection 

 enters, is about 5 feet. 



Resolving Power of an Electron Microscope 



According to Ruska [1934], magnetic field lenses can be constructed 

 to possess larger apertures than electrostatic electron lenses. The 

 practical limit is reached when a magnetic-coil electron lens has attained 

 a numerical aperture equal to 0.02. With a 50-kilovolt electron beam 



o 



(X = 0.055 A) and such an aperture it is possible to attain a theoretical 

 resolution of 1.4 X 10 -7 cm. Ardene [1938], in a detailed analysis of 

 the contributing factors which may reduce this theoretical value, showed 

 that with a 50-kilovolt electron beam the practical limit of resolution is 

 10~ 7 cm. 



Electron Image Defects 



The first-stage electron image may possess defects due to: (1) the 

 diffraction of the associated electron waves; (2) the influence of space 

 charge on the electron path; (3) chromatic aberration due to the fact 

 that not all the emission electrons have the same velocity and that 

 electron scattering by the specimen and its support are unavoidable; 

 (4) insufficient magnetic shielding against the earth's magnetic field and 

 other outside influences; (5) fluctuations in the accelerating potential 

 which should be reduced to at least 5 volts in 50 kilovolts. 



Magnetic Condenser 



A typical electron lens train, and the effects of the magnetic fields on 

 the electron beams passing through it, are shown in Fig. VIII-24. The 

 electron beam entering the lens from the top is shown filling the aperture 

 of the condenser lens. This beam has a minimum diameter d at a dis- 

 tance a above the equatorial plane of the field of the condenser lens. Its 

 virtual source A\ has an angular aperture 2a s . When no field has been 

 established, a point P in the plane of the object, (a + b) below d, sub- 

 tends an angular aperture 



d 



Ci c = 



2(a + b) 



When a magnetic field is established by a current flowing through the 



