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On the determination of the optic axial angle and crystal-forms from observations by the Becke method in thin sections

Published online by Cambridge University Press:  14 March 2018

Extract

Some years ago the writer read a paper on the determination of the optic axial angle of a crystal by the Becke method. This proposed method was to record the trace of the optic axial plane when horizontal, the position of one axis, and the extinction-direction. From these data and the established constants of the microscope, the position of the second optic axis may be found, as the extinction bisects the angle between the centre of the pole of the section and the two optic axes. Hence, the optic axial angle may be determined.

Considerable errors arise in these determinations owing to defects in the optical system of the microscope, and also to the inability to determine the exact position when the trace of the optic axial plane is parallel to the E-W. lines in the microscope.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1928

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References

page 552 note 1 Collingridge, H., Min. Mag., 1918, vol. 16, p. 348 Google Scholar; 1914, vol. 17, p. 147.

page 553 note 1 This method of projecting crystals has been used by Buchanan, J. Y., On the use of the globe in the study of crystallography. Phil. Mag., 1895, ser. 5, vol. 40, pp. 153172 Google Scholar.