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1837 Penny Cycl. VIII. 200/1 Let us suppose two diagonal lines to be drawn through opposite angles, and crossing each other on the faces of the Cube. It may be observed..that the solid angles at the extremities of all these diagonals are truncated to produce the octahedron; but it sometimes happens that the solid angles at the extremities of only one of those diagonals on one plane, and a transverse diagonal on a parallel plane, are truncated, producing a four instead of an eight-sided secondary figure; these are termed hemi forms, from their presenting only half the number of planes which might be expected from the symmetry of the primary crystal.