A theory of the formation of interference patterns due to X-ray spherical wave two-beam dynamical diffraction in a perfect crystal is presented. An asymmetrical Laue case is analyzed in detail, when a polychromatic focus is realized with different distances in front of and behind the crystal. Such a property is essential for high-energy X-rays produced by synchrotron radiation sources of the third generation because of the long distance between source and object. It is shown that a monochromatic X-ray spherical wave is focused due to dynamical diffraction when a definite relation between distances and crystal thickness is held. An X-ray beam of less than 10 mm width may be obtained. A two-dimensional intensity distribution (topograph) may be registered with a wedge-shaped crystal. It shows interference fringes of different kinds depending on crystal thickness and asymmetry rate. It is also discussed how a slit in front of the crystal influences the interference pattern. An example of an interference pattern is presented under the condition of highly asymmetrical diffraction which was obtained by a computer simulation technique. Fringes of a new kind are observed and their physical nature is discussed.

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