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An analysis of the movements of shadow-edges on the refractometer in the case of biaxial gemstones
Published online by Cambridge University Press: 14 March 2018
Extract
The study and identification of the gem minerals present difficulties which are not to be resolved by the ordinary methods of mineralogy. The commercial value of the material forbids recourse to chemical reagents and the blowpipe, and when faceted and mounted, the criteria of crystal form and specific gravity are also inapplicable. Optical phenomena then constitute the sole means of discrimination, and the refractometer, spectroscope, polariscope, and dichroscope become the only arbiters between gemstones of similar appearance but different species. The function of the polariscope is to narrow the field of inquiry by differentiating between isotropic and anisotropic gemstones; the verdict of the spectroscope is unequivocal in gemstones characterized by absorption spectra, which, unfortunately, form a minority ; dichroism, when present, can be regarded as supplementary evidence only; the refractometer, an instrument of wider application than the spectroscope, and capable of yielding information which is usually equally unambiguous, thus assumes a role of primary importance.
- Type
- Research Article
- Information
- Mineralogical magazine and journal of the Mineralogical Society , Volume 26 , Issue 178 , September 1942 , pp. 246 - 253
- Copyright
- Copyright © The Mineralogical Society of Great Britain and Ireland 1942
References
page 247 note 1 Herbert Smith, G. F., Min. Mag., 1905, vol. 14, pp. 83–86 CrossRefGoogle Scholar; 1907, vol. 14, pp. 354–359.
page 247 note 2 Tully, B. J., ibid., 1927, vol. 21, pp. 324–328.Google Scholar
page 247 note 3 Anderson, B. W., Payne, C. J., and Pike, J., ibid., 1940, vol. 25, pp. 579–583.Google Scholar
page 248 note 1 When algebraic methods are used to investigate the behaviour of anisotropic gemstones on the refractometer, the retention of the symbols a, b, and c of the ellipsoid is justitled on the ground of consistency, since those for the direction-cosines are lower-case members of the same alphabet. When the results are used to interpret biaxial phenomena, a, b, and c are equated to α, β, and γ respectively. Mathematically, an analysis of the properties of uniaxal minerals follows similar lines, the appropriate modification being introduced that two of the principal semi-axes are equal in length, and the final results are restated in terms of the omega and epsflon of mineralogy.
page 250 note 1 Spencer, L. J., Gemmologist, London, 1937, vol. 6, pp. 231–236.Google Scholar
page 250 note 2 This can be checked by rewriting equation (i) as a straightforward biquadratic, equating l to zero, and showing that the resultant equation has a root r 2 = a 2 by the factor theorem.