UNDER this title, W. Linnik, of the Leningrad Optical Institute, described in a recent issue of NATURE (April 20) an effect obtained with copper K radiation and a mica crystal. A narrow beam of X-rays was passed through a cleavage plate of mica, which had previously been heated to redness and cooled. A photograph of the diffracted beam showed a series of spots, the arrangement of which is attributed by Linnik to the action of very thin layers of the crystal as independent two-dimensional gratings. It is supposed that “the space effect will be destroyed by the incoherence of waves produced by scattering from incorrectly spaced layers” the heat treatment having broken up the mica into very thin plates without disturbing their orientation. The arrangement of the diffracted beams corresponds to the pseudo-hexagonal network of points at intervals of 57.2 A. in the cleavage plane (001), which is so striking a feature of the mica structure.
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BRAGG, W. Diffraction of X-rays by Two-Dimensional Crystal Lattice. Nature 124, 125 (1929). https://doi.org/10.1038/124125a0