100 



W. D. RIECKE 



Fig. 2. Fine structures in tlie undift'racted beam in the diffrac- 

 tion pattern from individual MgO crystals, (b) Pattern from 

 the crystal shown in (a), and (d) that of the crystal (c). 

 (H.T. = 80 kV.) 



this lens was used to form an image of the diflFraction 

 pattern, existing in the back focal plane of the 

 objective lens, within the object plane of the projector 

 lens, it had a focal length of 4.8 cm and an astigmatic 

 difference of focal lengths of 0.02 cm. This limits the 

 diffraction resolution to /? 1 120 d, which shall be 

 discussed in detail elsewhere, d is the diameter of the 

 selected specimen area in /(. As to the resolution of 

 fine structure in a reflection, the first-stage image of 

 the scattering crystal practically acts as the aperture 

 for the intermediate lens. The other rays, which pass 

 the selecting aperture, do not contribute to this 

 reflection. Even for a crystal size of I /*, an adequate 

 resolution R ^ 1100 may be expected. Under these 

 conditions, astigmatism and spherical aberration of 

 the objective lens have no detrimental influence on 

 the diffraction resolution. 



Apart from bright-held images or dark-field images 

 taken with definite reflections, defocused diffraction 

 patterns are useful for the interpretation of the 

 focused ones (fig. 1, B, D). The overfocused dif- 

 fraction pattern may be considered as a pin-hole 

 projection of the second-stage image of the specimen 

 onto the final screen. The "pin-holes" are formed by 

 the reflections in the second stage image of the 

 diffraction pattern (fig. 1 , B)_ At this, the undiffracted 

 beam produces a "bright-field" shadow image, and 

 each reflection a corresponding "dark-field" shadow 

 image. In a similar way, the underfocused diffrac- 

 tion pattern may be regarded as a point-projection 

 image of the second-stage image of the specimen. 

 The projecting rays emanate from the reflections 

 of the first-stage image of the diffraction pattern 

 (fig. I, D). The shadow images in the underfocused 

 diffraction pattern are correctly orientated to the 

 focused one, apart from a slight rotation corres- 



Fig. 3. Focused (a) and overfocused (b) diffraction pattern 

 as well as micrograph (c) of an individual MgO crystal. 

 (H.T. = 80 kV.) 



ponding to the defocusing of the intermediate lens. 

 Those of the overfocused pattern are rotated with 

 respect to the focused one by 1 80\ 



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) some weak spots on the 



