342 COMPOUND AND ELECTRON MICROSCOPES 



Two-Stage Magnetic Electron Microscope Projector 



Electron microscopes may be roughly classified as those using elec- 

 trostatic- or magnetic-field electron lenses. Each of these two types 

 may be designed to use either a hot filament or a cold cathode immersed 

 in a low-pressure gas as a means of generating the electron stream. 



In both types the available electrons are accelerated by a controlled 

 electric field to give them the proper energy before they enter the 

 condenser lens. The condenser lens converges the stream of electrons 

 on the miniature aperture of the object stage in which the specimen is 

 supported. The electron lens acting as an objective converges the 

 beams of electrons that have succeeded in passing through the object, 

 and focuses them in the field of the image projector, which in turn takes 

 a small section of this electron pattern and focuses it on a photographic 

 plate or fluoroscopic screen. 



The two-stage electron microscope projector probably had its incep- 

 tion in the high-potential laboratory at the Neubabelsberg technical 

 school in Berlin. Various members of this laboratory had been experi- 

 menting with high-voltage cathode-ray oscillographs using cold cathodes 

 in low-pressure gas as the sources of uniform-velocity electron beams. 

 Knoll and Ruska by 1930 had successfully adapted one of these to a one- 

 stage magnetic lens microscope. By 1934 Ruska had developed a two- 

 stage, cold-cathode, magnetic lens microscope. The magnetic lenses 

 with their uniquely designed pole pieces made it possible for him to 

 attain exceptionally high resolution. His magnified electron images 

 reached 40,000 diameters. He succeeded in magnifying some organic 

 specimens from 8000 to 10,000 times. 



Mar ton [1934] at Brussels built a two-stage instrument having a hot- 

 filament cathode as source. With his subsequent adoption of a new his- 

 tological technique to prevent the destruction of organic cells by the 

 intense electron bombardment [Nature, 1934] he obtained excellent 

 microphotographs of Chromobacterium prodigiosum fixed in chromic acid. 



A two-stage cold-cathode electron microscope of very elaborate 

 design was completed by Martin, Whelpton, and Parnum [1937], at 

 London. The source of power was a 250-cycle, 20-kilovolt generator 

 with 0.1-microfarad condenser having a ripple voltage as small as 0.1 per 

 cent. 



By 1938, Borries and Ruska, at the laboratory of Siemens and Halske, 

 completed their " supermicroscope " and showed that their instrument 

 could produce recognizable structural details in Staphylococcus aureus 

 at 20,400 magnification. A later publication by Borries, Ruska, and 

 Ruska [1938] showed Bacillus enteritidis with internal nuclei at 13,500 X, 

 and some viruses appeared to have recognizable shapes at 20,000 X. 



