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PEISMATIC SYSTEM. 39 tances between two adjacent poles equally and unequally distant from the pole ill. Then sin|v = sin60°sinT, w = 180° — v. 162. In the forms ohkl, alkh, let v be the distance between adjacent poles equally distant from the pole ill; u, w the dis tances between adjacent poles unequally distant from the pole m. Then sin-J, v = sin 60° sin t, u = 180° — h, w = 180° — k. 163. The cleavages, in crystals belonging to the rhombo- hedral system, are parallel to the faces of the forms oil, 111, or to those of forms which have two of their indices equal. PEISMATIC SYSTEM. 164. In the prismatic system the axes make right angles with each other. 165. The form Ml is bounded by all the faces having for their symbols the different combinations of ± /(, ± k, ± I, each index having always the same place. When h, k, I are all finite, the form hkl will have the eight faces. h k I hkl hkl hkl hkl hkl hkl hkl When one of the indices is zero, the number of faces will bo four. When two of the indices are zero, the number of faces will be two. 166. The arrangement of the poles of hkl on the sur face of the sphere of projec tion is shown in the annexed figure. 167. The form bounded by all the faces of hkl, which have either an odd number of positive indices, or an odd number of negative indices, is said to be hemihedral with inclined faces, and will be denoted by kItkl, where hkl is the symbol of any one of its faces. The symbols of the faces of kMI are contained in the upper lino, those of Mel in the lower line of the above table. _The form «.■/ikl consists of four alternate faces of the form hkl, nhkl of the other four alternate faces. FIO. 65. 010 hkl >100 hkl hkl okl oto
