348 



COMPOUND AND ELECTRON MICROSCOPES 



condenser coils, the virtual focal point A\ moves down owing to the 

 constriction of the electron beam by the magnetic field. This lowering 



of A\ continues until a c reaches a maximal 



value a' such that 



<x r 



a 



Objective 



coil 

 pole pieces 



electron image 



Fig. VIII-24. A geomet- 

 rical analysis of an electron 

 beam leaving a small aperture 

 d with electron condenser 

 lens converging a beam at P. 

 Specimen in the object plane 

 at POP. The objective elec- 

 tron lens deviates the beam, it 

 crosses over and comes to a 

 focus in the electron image 

 plane. (Borries and Ruska 

 [1939].) 



Then the focal point coincides with the 

 source d. A further increase in the field 

 intensity reduces this angular aperture, with 

 an accompanying decrease in the electron 

 density impinging on the object POP. A 

 change in current through the condenser coil 

 can therefore regulate the density of the 

 electron stream " illuminating " the specimen 

 in the aperture POP of the object stage. 



If the values of a and b in the electron 

 microscope are 5 and 15 cm, respectively, 

 the maximum angular aperture available 

 for illuminating the point P on the speci- 

 men stage would be 0.007. This very con- 

 stricted beam subtends an angle of 25 

 seconds of arc and, therefore, fulfills the 

 requirement that the electrons incident on 

 this point make small angles with the axis 

 of the lens. 



Object Stage 



The object stage, which supports the 

 specimen in the very small aperture POP, 

 may be centered above the equatorial plane 

 of the pole pieces of the object coil, or it 

 may be placed between the pole pieces, in which event it is designated 

 as an immersion electron objective. 



The specimen may be placed on very fine-meshed wire gauze or sup- 

 ported on a very thin collodion film suspended in a single mesh of the 

 gauze. These collodion films may have densities as great as 1.6 

 grams/cc (Ruska [1934]), and they should not be more than 3 X 10 -! 



mm thick. 



The aperture in which the specimen is mounted is about 0.1 mm in 

 radius. It should be made of a gold-platinum alloy to prevent geo- 

 metrical distortions by oxide particles which may form along its highly 



