Electron diffraction patterns of etched tungsten surfaces apparently indicate a preferred orientation of the crystallites, with [100] ⊥ surface. Actually, the etching leaves, on grains of a certain orientation, a structure exceptionally favorable for diffraction, and these selected grains dominate in the scattering. Analogous examples of weighted sampling, rather than random sampling, appear in studies of clays and of graphite. We suggest that crystal habit may frequently replace preferred crystal orientation as the cause of arcing in reflection patterns.

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