The derivation of the optical axial angle from the three chief refractive indices is a task often carried out graphically with the aid of nomograms. Of these several have been described in the course of time. A good example is the diagram constructed by H. Waldmann (1945) who also quotes and discusses the work of previous authors. A more recent paper on the same subject is that by C. P. Gravenor (1951). The purpose of the nomogram in all these cases is to supply a solution to a problem which in essence can be stated as follows: Given an ellipse, the main axes of which are proportional in length to nx(nα) and nz(nγ) of a given optically biaxial crystal; required the position within the ellipse of the two radius vectors having a length proportional to ny(nβ). These radius-vectors of the ellipse are also radii of one of the circular sections through the optical indicatrix and hence are perpendicular to one of the optical axes. The position of the latter within the ellipse and the angle between them therefore follows from that of the radius vectors ny.

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