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Published online by Cambridge University Press: 01 May 2009
In some cases it is of practical importance to petrologists to estimate the strength of the double-refraction exhibited by doubly-refracting minerals in thin slices of rock under the microscope.
Babinet's compensator, a description of which will be found in Rosenbusch's Microskopische Physiographic der Mineralien und Gesteine, affords a means of accurately calculating the refractive indices of the rays into which light is divided in its passage through doublyrefracting crystals. Michael-Lévy also devised another exact method based on observations made on the tints presented by slices of doublyrefracting minerals, an account of which appeared in the Bulletin de la Societe Mineralogique de France, and which is discussed in a work recently published by Michael-Lévy and Lacroix.
page 549 note 1 The wedge should be a tolerably flat one so as to give a wide spread of colour.
page 550 note 1 The eye-piece of my microscope is furnished with two slots, one above the other, so I can either use two wedges at once or combine one wedge with an eye-piece micrometer.
page 551 note 1 Basalts, for example, are usually sliced thinner than granites.
page 551 note 2 When testing for double-refraction the stage must be revolved until the dark line appears in the mineral under examination. The azimuth in which this appears affords another valuable means of distinguishing between different species.
page 552 note 1 I use the word weak to indicate a slightly higher double-refraction than feeble. Thus, γ–α for chlorite is 0·001, but for sanidine is 0·008.
page 552 note 2 The dark line will not appear in any azimuth in a section quite normal to an optic axis.