Suppose a crystal has two good cleavages, and that a thin section is cut in a random direction. Cleavage-cracks are seen in the section which consist of two sets of parallel lines inclined at an acute angle ϕ, which may have any value whatever, depending on the direction in which the section is cut. The question suggests itself: what is the chance that ϕ may lie between assigned limits? This question proves to be incapable of a direct mathematical solution, but it can be solved graphically.

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... is worked out here for two typical cases, and the general method of performing the graphical solution in every case is indicated.