344 COMPOUND AND ELECTRON MICROSCOPES 



of the microscope (maximum 60 kilovolts) is applied between the cathode 

 and anode, the anode being at ground potential, while the cathode is 

 negative. The electrons emitted from the filament are accelerated 

 towards the anode. A portion of these electrons passes through the 

 aperture in the anode, thence through the condenser, which concen- 

 trates them on the specimen. 



The magnetic condenser lens consists of an iron-clad coil with pole 

 pieces shaped to converge the electron beam on the specimen placed 

 just below it. The specimen is placed in position through an air lock. 



The electrons after passing through the specimen enter the magnetic 

 field of the objective. This lens deviates the emitted electrons, which 

 leave point sources at small angles, in such a way as to focus them in a 

 plane directly above the pole piece of the projection lens. In this focal 

 plane lies the first-stage magnified electron image of the specimen. 

 This first-stage image has a magnification of about 100 diameters. 



The final image is formed from that portion of the first-stage electron 

 image which fills the diaphragm stop of the projection electron lens. 

 The electron stream passing through the magnetic field of this lens is 

 deviated to form the second-stage image in the plane of the photographic 

 plate or fluorescent screen. The magnification at this stage can be 

 varied from 20 to 300 diameters by controlling the current in the pro- 

 jection coil. The total magnification varies from 2000 to 30,000 diam- 

 eters, Supermagnification can be obtained by enlarging the photo- 

 graphic image from 6 to 10 times, depending on the grain size of the 

 photographic emulsion. Under favorable conditions it is possible to 

 attain a magnification of 180,000 diameters. The limiting usable 

 magnification is about 100,000 diameters. 



The operating pressure in this instrument is about 10 mm of mercury. 

 No greased joints are used. Alignment is obtained by means of 

 flexible sylphon bellows. The photographic chamber is air locked and 

 is fitted for 2-by-10-inch photographic plates. Such a long plate permits 

 making a number of pictures in a row. Six ports are also provided for 

 viewing the definition of the final image on a removable fluorescent 

 screen. 



Figure VIII-22 is a 21 ,000 X photograph of a human tuberculosis 

 bacterium obtained with this electron microprojector. 



For the high-voltage power supply, Vance [1941] employed radio- 

 frequency power to actuate the step-up transformer unit, since stray 

 fields are more easily shielded when radiofrequency power is used. 

 The power supply can therefore be mounted as an integral part of the 

 unit. It is claimed that the measured voltage fluctuation in the power 

 supply over a period of half an hour was less than 0.002 per cent. The 



