Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . the stage canyingthe section is rotated, f * In the Seibert microscope use No. V. objective, in Fuess microscope No. 7 objec-tive, and in English microscopes a \ or \ objective. t Each convergent ray will have its vibration direction either in or at 90° to the planethrough the ray and the optic axis. Hence all rays vibrating parallel to the vibrationplanes of both nicols will be completely cut out. As the section i
Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . the stage canyingthe section is rotated, f * In the Seibert microscope use No. V. objective, in Fuess microscope No. 7 objec-tive, and in English microscopes a \ or \ objective. t Each convergent ray will have its vibration direction either in or at 90° to the planethrough the ray and the optic axis. Hence all rays vibrating parallel to the vibrationplanes of both nicols will be completely cut out. As the section is rotated new rays suc-cessively come into these positions, so the same effect is maintained. 40 IXVESTIGATIOX OF CHARACTERS OF MINERALS. [p) Sections oblique to the optic axis show a portion of a dark-cross, with or without colored rings, Fig. 25. The centre of the cross is not in the axis of rotation, and as thestage bearing the section is revolved, the centre of the cross de-scribes a circle, the arms always maintaining parallel positions. If the section is still more oblique to the optic axis the centreof the interference cross may be outside the field of view, and only. Fig. 25. the dark arms will be seen swinging past, when the section is ro-tated, thus making the figure rather indefinite. Sections parallel to an optic axis show hyperbolic curves, whichmight be confused with a biaxial interference figure with axial angleof 180°. Sections which are thick and have strong double refraction willshow the cross and rings clearly and sharply defined, there beingquite a number of rings crowded close together. Sections whichare very thin and have weak double refraction show only a broaddark cross and no rings. The interference figures will vary be-tween these extremes, depending on the thickness of the sectionand the strength of the double refraction. To obtain the most characteristic figures, observations must bemade on sections about perpendicular to the optic axis, that issec
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