Electron microscopy; proceedings of the Electron microscopy; proceedings of the Stockholm Conference, September, 1956 electronmicrosco00euro Year: 1957 ijjjj' 'S*. J / = 3250 Amp., (J = 60 kV. Continuous variation of magnifi- cation between the three stage diffraction patterns and Amax = 28 000. Fig. 3. Cross section of a permanent magnetic microscope system. three-Stage diffraction patterns with different magni- fications. It is also possible to use the whole system as a diffraction condensor, and also to use it as an elec- tron shadow microscope. If crystalline matter is put into the causti
Electron microscopy; proceedings of the Electron microscopy; proceedings of the Stockholm Conference, September, 1956 electronmicrosco00euro Year: 1957 ijjjj' 'S*. J / = 3250 Amp., (J = 60 kV. Continuous variation of magnifi- cation between the three stage diffraction patterns and Amax = 28 000. Fig. 3. Cross section of a permanent magnetic microscope system. three-Stage diffraction patterns with different magni- fications. It is also possible to use the whole system as a diffraction condensor, and also to use it as an elec- tron shadow microscope. If crystalline matter is put into the caustic, dynamic diffraction patterns can be obtained which resemble Kikuchi lines. Such patterns from MgO crystals are shown in fig. 4. Another problem in the field of permanent mag- netic lenses which we have investigated is the so-called 'reflexion microscopy' which is really a dark field microscopy of surfaces hit by the electron beam in grazing incidence. In order to avoid immer- sion the focal length must be considerably greater than half the gap width in order to give space for the axial movement of the specimen. On the other hand the chromatic aberration constant C / C eU* cU* Fig. 4. Kikuchi lines of MgO crystals obtained in the caustic of a strong permanent magnetic lens. when using permanent magnetic lenses with the rela- tivistic accelerating voltage U*. The image brightness goes with the 2nd power of a, so using half the focal length one should get four times the brightness in the image at the same resolution and magnification. The requirement of non-immersion objective lenses sets a limit to the lens excitation. We have tried an objective lens with h = 1 mm bore, .v = mm gap-width,/ mm, C = 1-35 mm which needs / - 1450 Amp at 60 kV. In a two-stage permanent magnetic system this demands the same low excita- tion also for the projector which then is likely to cause distortion. This is a difficulty which arises when using permanent magnetic microscopes for 'reflex
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