Fig. 3) obtained3 through the use of this hypothesis is also wrong. The distance 1.440 =*= 0.015 Á. determined by us is not completely outside of the range 1.45 to 1.58 Á. for A to 0, which has been calculated to be necessary in order to account for the observed birefringence of ruby.4 SummaryThe crystal structures of hematite and corundum have been determined through the use of Laue and spectral photographs, interpreted with the aid of the theory of space groups. The unit of structure is a rhombohedron with a = 55°17' and a = 5.420 ± 0.010 Á. for hematite, and with a = 55°17' and a = 5.120 ± 0.010 Á. for corundum. The space group underlying the atomic arrangement is Dgd.The data require that atoms of iron or aluminum be located at www, www, § -wf -w § -w, w + J w + J w + |, and atoms of oxygen at uüO, üOu, Ouü, f-u u + |í, u + ií |-u, | u u + |, with u = 0.292 ± 0.007 and w = 0.1050 ± 0.0010 for hematite and u = 0.303 ± 0.003 and w = 0.1050 ± 0.0010 for corundum. These structures show that crystals of hematite and corundum consist of a compact arrangement of approximately but not exactly spherical ions, as shown in Fig. 3. The derived inter-atomic distances in Angstrom units are as follows.

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