. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . Fig. 3. Focused (a) and overfocused (b) diffraction pattern as well as micrograph (c) of an individual MgO crystal. ( = 80 kV.) ponding to the defocusing of the intermediate lens. Those of the overfocused pattern are rotated with respect to the focused one by 180\ Experimental results.—We have examined the dif- fraction patterns of MgO and ZnO crystals. The specimens were prepared by burning magnesium or zinc ribbon and exposing platinum specimen carriers to the smoke. Although the holes of the carriers were
. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . Fig. 3. Focused (a) and overfocused (b) diffraction pattern as well as micrograph (c) of an individual MgO crystal. ( = 80 kV.) ponding to the defocusing of the intermediate lens. Those of the overfocused pattern are rotated with respect to the focused one by 180\ Experimental results.—We have examined the dif- fraction patterns of MgO and ZnO crystals. The specimens were prepared by burning magnesium or zinc ribbon and exposing platinum specimen carriers to the smoke. Although the holes of the carriers were not covered by the usual collodion film, in order to avoid additional scattering, a great number of perfectly grown crystals were found to adhere to the rim of the holes. First, we looked for those types of fine structures that are predicted by the dynamical theory of elec- tron diffraction. In the diffraction patterns, that had been published hitherto, the fine structures in the undiftYacted beam were masked by the well-known extensive spot of intense halation, which is caused by the superposition of the scattered intensities of a great number of crystals. This effect was eliminated with our method by producing diflVaction patterns of only one crystal. In the undiffracted beam of the patterns of individual MgO crystals we have ob- tained fine structures., which are caused by interference double refraction, provided that strong Bragg reflec- tions occur (fig. 2, b). This has been predicted by Moliere and Niehrs (7). At longer exposure times, even more weak spots are found on straight lines drawn through the doublets corresponding to each crystal wedge. A great number of spots is obtained with relatively large crystals some 1000 A in size (fig. 2, d). By using the selecting aperture to screen the first-stage image of the crystal, with the exception of a single wedge-shaped part at an edge of the MgO cube, the figure is reduced to (i) the double- refraction doublet, and (ii) so
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