218 MINERALOGY 



axis, as determined in the section by the cleavage, outline, or elon- 

 gation. Whether it is parallel or inclined is noted and the angle 

 of extinction measured, also the twinning of the section may be 

 seen in relation to the extinction angles. 



8. Pleochroism. This is caused by the light being absorbed 

 along one plane of vibration in the section differently from that along 

 the other. The section is turned on the microscopic stage until 

 extinction, when the vibration planes of the section will be parallel 

 to those of the nicols. The analyzer is pushed out of the line of 

 view, when the section will be illuminated by the rays vibrating 

 parallel to the one plane of vibration in the section only, that of the 

 polarizer, usually running from right to left. The color of the 

 section and the degree of illumination are both noted, as is also the 

 relief of the section ; then the section is revolved on the stage 90, 

 when the light will be vibrating parallel to the second plane of vi- 

 bration of the section, and any change in color, shade, .or relief is 

 noted. 



9. In crossed nicols and converging light. All sections of the 

 mineral under observation in the specimen are carefully examined, 

 and one selected as nearly perpendicular as possible to the optic 

 axis, in uniaxial crystals, and to the acute bisectrix, in biaxial crys- 

 tals, is chosen. The interference figure is observed and the uniaxial 

 or biaxial character of the crystal noted, as well as the approximate 

 axial angle in the latter. 



10. When uniaxial, the optic sign is determined with the mica 

 plate ; and when biaxial, with the quartz wedge. Where the inter- 

 ference figure is well formed, the character of the dispersion may 

 also be noted. 



