It has been noted that the family of plane electromagnetic waves and the "electromagnetic universe" of Bertotti-Robinson exhaust the entire class of conformally flat Einsteirr-Maxwell spaces. In the formalism of Newman-Penrose a family of exact solutions of the Einstein-Maxwell equations of the type of Bertotti-Robinson is obtained with a cosmological term belonging to the degenerate type D in the algebraic classification of Petrov and describing the space-time generated by a covariantly constant, nonisotropic electromagnetic field.

2.3. SCATTERING BY GROUPS COMPOSED OF SEVERAJ, TYPES OF PARTICLES 65 2.3.1. General Theory 2.3.2. Widely Separated Particles 2.3.3. Influence of the Closer Packing of Particles 2.4. GENERAL CASE 2.4.1. Limiting Value of the Scattered Intensity at Very Small Angles 71 ix x CONTENTS 2.4.2. Calculation of the Scattered Intensity as a Function of p(X) 75 2.4.3. Matter of Uniform Density and Random Distribution 78 3. EXPERIMENT AL EQUIPMENT 83 3.1. GENERAL CONSIDERATIONS .

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Many crystals produce a diffuse scattering of X-rays which is localized in a series of relplanes. It is shown that the corresponding linear disorder in the crystal may have various origins. In silicon, the scattering is due to thermal vibrations and is well explained by the elastic properties of the crystal. In neutron irradiated quartz the radiation damage is responsible for the major part of the scattering. The case of BaTiO3 and KNbO3 is discussed in detail. A linear disorder is proposed which accounts better for the different distributions of the scattering in the 4 allotropic phases than the alternative explanation of the soft mode. In spite of some neutron inelastic scattering results it is not yet possible to distinguish between static and dynamic disorder.