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The use of the gnomonic projection in the determination of the optical indicatrix of crystals

Published online by Cambridge University Press:  14 March 2018

N. Joel
N. Joel*
Affiliation:
Birkbeck College Research Laboratory, London
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Extract

In a previous paper a description of a method was given which had been developed to determine the indicatrix of small crystals, both in orientation and magnitude, making use of the simple one-circle stage goniometer described by Bernal and Carlisle.

As was explained there, when a crystal is viewed in parallel polarized light, between crossed nicols, the two observed extinction directions are the axes of the ellipse determined by the intersection of the indicatrix with a plane through its centre parallel to the plane of the microscope stage.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 29 , Issue 213 , June 1951 , pp. 602 - 608
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1951

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References

1 Joel, N., Min. Mag., 1950, vol. 29, p. 206.Google Scholar

2 Bernal, J. D. and Carlisle, C. H., Journ. Sci. Instruments, 1947, vol. 24, p. 107.CrossRefGoogle Scholar

1 It should be noted that whatever the direction chosen for the plane of the gnomonic projection, the constant refractive index curves will always be conic sections. Their equations may be completely different, but the problem remains essentially the same.

1 If many sections of the same crystal have to be studied, it is worth while to start drawing a complete chart for the given values A, B, and C at suitable intervals of N, either in the stereographic or in the gnomonic projection. ]f many sections of different crystals have to be examined, one single chart (in gnomonic projection) drawn for any arbitrary values of A, B, and C (for instance A—B = B– — C, that is 2V: 90) may be used by altering the scale of the co–ordinates and choosing the and two similar equations with N2 and N2. But if only a small number of sections are examined it is still more practical to draw only the pair of curves for each case.