Nomograms are presented herewith for the solution of several well-known petrographic formulae that are used in the measurement of optic angles. To solve for any one of the four variables α, β,γ, and V, when three of them are known, a grid type nomogram has been prepared; and a similar chart has been drawn to show the relationships between the variables α, β,γ, and E. Two nomograms are also presented for the solution of the equations sin E = β sin V and D = K sin E (Mallard's formula). The method of preparing these charts is outlined, and their uses given. The topic of errors in observations and in computations is also considered.

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