The electron microscope, its development, present performance and future possibilities electronmicrosco00gabo Year: 1948 Supermicroscopes 35 Fig. 12. Design of magnetic objective (E. Ruska, 1934) If these electrons are ultimately accelerated by v volts 11,600 the root mean square divergence is 82 — \^ 11,600 ^ J In the case of a tungsten cathode with T = 2,600°K and V = 60,000 volts this is very nearly 2.\0r^. Whenever the cathode is imaged on the specimen, the di- vergence cannot be less than this value, though it can be more, as a consequence of lens errors and space charge effects. But
The electron microscope, its development, present performance and future possibilities electronmicrosco00gabo Year: 1948 Supermicroscopes 35 Fig. 12. Design of magnetic objective (E. Ruska, 1934) If these electrons are ultimately accelerated by v volts 11,600 the root mean square divergence is 82 — \^ 11,600 ^ J In the case of a tungsten cathode with T = 2,600°K and V = 60,000 volts this is very nearly 2.\0r^. Whenever the cathode is imaged on the specimen, the di- vergence cannot be less than this value, though it can be more, as a consequence of lens errors and space charge effects. But this divergence, small as it appears, is of the order of the aper- tures which can be allowed in uncorrected objectives. Numerous measurements, in particular those by R. F. Baker, , dem- onstrate that the resolving power and contrast improve very appreciably if the illuminating angle is reduced considerably below this limit. This, however, is possible only if the current density at the specimen is reduced by defocusing below the maxi- mum value which can be obtained by focusing the cathode or the cross-over directly on it, or if part of the beam is cut out by an aperture in the center of the condenser lens. Such arrange- ments have proved very successful, and the modern tendency in electron microscopy is to operate with almost parallel ilium-
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