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On some new forms of Quartz-wedge and their uses

Published online by Cambridge University Press:  14 March 2018

John W. Evans
John W. Evans*
Affiliation:
Imperial Institute
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Extract

The determination of the amount and sign of the relative retardation of the wave-fronts of the two rays, into which a plane-polarized ray of light is resolved in passing through an anisotropic crystal, furnishes us with the readiest means of identifying transparent minerals occurring in thin sections or as minute fragments without definite crystalline form. In sections which are of practically the same thickness the individual crystals of each mineral show a certain range of colours resulting from the interference of the rays after resolution parallel to the principal plane of the analyser, and these colours are usually sufficient, with the help of other characters, for the identification of the mineral species.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 14 , Issue 64 , May 1905 , pp. 87 - 92
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1905

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References

Page 88 note 1 Strictly speaking the wave-fronts will be parallel only if the ray meets a plane surface of the anisotropic medium at right angles.

Page 89 note 1 See Mallard, Traité de Cristallographie, Paris, 1884, vol. ii, p. 899.

Page 89 note 2 For this purpose it is best placed in the focus of the eye-plece, a microscope being employed in which the analyser is above the eye-piece.

Page 90 note 1 The graduations of the ‘birefractometer’ represent the thickness in decimal parts : of a millimetre, not the relative retardation.

Page 90 note 2 The focus is best determined by using a strong monochromatic light passing through a small aperture six inches at least from the mirror of the microscope. Interference rings are formed unless the focus is very exact (see Lévy and Lacroix, ‘Les Minéraux des Roches,’ Paris, 1888, p. 60). The thickness may also be measured through the mineral and multiplied by its approximate index of refraction as estimated by Becke's method. This usually gives a sufficiently approximate value for the index of relative retardation to enable the mineral to be identified.

Page 91 note 1 I find that somewhat the same principle has been applied in a different manner in the twin-compensator.

Page 92 note 1 loc. cit.