. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . Fig. 10. Sodium faujasitc crystal viewed along [110] showing two sets of (11 I) planes intersecting at 70'. with the (402) reflection corresponding to ^ = A since a microphotometer trace across the image shows that the intensity distribution follows fairly closely a function of the form cos- 0. This would be expected from interference between two beams, the zero order and the first order (20T) indicating that although the (402) beam passes through a 50 /( aperture to the image it is making no useful con
. Electron microscopy; proceedings of the Stockholm Conference, September, 1956 . Fig. 10. Sodium faujasitc crystal viewed along [110] showing two sets of (11 I) planes intersecting at 70'. with the (402) reflection corresponding to ^ = A since a microphotometer trace across the image shows that the intensity distribution follows fairly closely a function of the form cos- 0. This would be expected from interference between two beams, the zero order and the first order (20T) indicating that although the (402) beam passes through a 50 /( aperture to the image it is making no useful contri- bution to the image of the planes. Resolution of image.âThe high resolution appar- ent in the image can be explained in terms of simple lens theory: neglecting effects of chromatic aberra- tion and astigmatism, the phase delay imposed on a ray passing through a lens at an angle -x is given by e = J Cja*, where Cj is the spherical aberration con- stant. The diffracted beams from the crystal lattice may be regarded as plane parallel beams approxi- mately equal in width to the width of the specimen (neglecting the finite divergence of the incident illu- mination). Thus provided a and Cg are small the distortion of the wave front of the narrow diffracted beam in passing through the lens may be very small (7). A simple calculation shows that the difference in phase between the two ends of a wavefront of width Ir is given by Gr-* Ar /\ where/is the focal length of the lens and /â the distance of the beam from the axis in the lens plane. Inserting values for the (20T) reflection from platinum phthalocyanine, /⢠10-3 cm, Cs cm,/ cm, Ar 10^ cm, we find that the phase difference across the wavefront Ae = 3 > 10 '^ cm, ^e< /, since A =4 lO"'" cm. Thus the wavefront remains virtually undistorted in passing through the lens and is able to form an image of the (20T) planes by interference with the
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