X-ray topographs of a new type have revealed fine fringes in the diffraction images of wedge-shaped parts of perfect and nearly perfect crystals. The fringes are analogous to those seen in electron microscope images of wedge-shaped parts of magnesium oxide crystals and can be interpreted according to the theory applicable to the electron case. Fringe spacing depends upon X-ray wavelength, wedge-angle, inclination of reflecting plane to the wedge surfaces, and the structure amplitude of the reflection. Discovery of these fringes shows that (a) parts of real crystals behave as ideally perfect from the X-ray diffraction standpoint (b) the dynamical theory of diffraction may be applied quantitatively under practical experimental conditions, and (c) structure amplitudes of low-order reflections may be determined by fringe-spacing measurements, without any need for measuring reflection intensities. Tests of the theory on prepared wedges of silicon and quartz indicate a slight systematic discrepancy of 4 to 5 % between calculated and observed values of structure amplitude, and suggest also that Wei's (1935) F-values for quartz 10T1 and 1T22 are too low by 9% and 4% respectivcly. Experiments suggest that to explain fully the observations some modification of the dynamical theory is required in the direction of allowing for a spherical wavefront of the incident beam.

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The visibility of Pendell6sung fringes is found to vary periodically with the fringe order. This variation of fringe visibility can be explained by the dynamical theory of X-ray diffraction taking into account the unpolarized nature of the incident X-ray beam. It has been experimentally demonstrated that there is no modulation in fringe visibility if the incident X-ray beam is approximately plane-polarized.

Images of the surface or interior of a single crystal formed by X-rays Bragg-reflected from its lattice planes provide information about lattice misorientations and defects in a unique way that was appreciated by the pioneers of topographic techniques: Berg, Barrett, Guinier, Ramachandran and Wooster. High-resolution images were achieved when use of fine-grain photographic emulsions was combined with diffraction geometries, providing micrometre-scale geometrical resolution. The detection of individual dislocations by X-ray diffraction contrast was reported in 1958 by three laboratories independently, employing quite different diffraction geometries. The value of X-ray diffraction contrast as a method of general application in detecting and identifying lattice defects (as opposed to special methods such as chemical etching) was demonstrated principally by the geometrically simply interpretable images of the projection topograph. Within a few months in 1958 this technique showed how dislocation Burgers vectors could be determined, how stereo-pairs of images could be formed, and how structure amplitudes could be measured absolutely from Pendellosung fringes in images of wedge-shaped crystals (though it was the hook-shaped Pendellosung fringe patterns appearing in section topographs that led Kato to develop his spherical-wave diffraction theory). The 'failure of Friedel's Law' revealed on early stereo-pairs of dislocation images suggested that sense as well as direction of Burgers vectors could be determined: confirmation came from the experiments of Hart applying the 'refraction of energy-flow' theory of Penning and Polder. X-ray topographic imaging by Polcarova and Lang of internal magnetic domain structures on the scale of a few micrometres in Fe-Si (magnetostriction constant 2.7*10-5) confirmed the high strain-sensitivity of the method and its ability to explore phenomena not accessible by other investigative techniques.