Figure 15
The simplified `stereographic projection' of Gadolin for the homohedral forms of the cubic system: the small triangles and quadrangles symbolize rotation axes of order 3 and 4, respectively, the ellipses mirror planes (source: Gadolin, 1871BB9). Each of the 32 crystal `classes' has such a specific `stereogram' of its own to characterize its outward symmetry.  [article HTML]