of Minerals in the Thin Section. 



347 



In case one or botli optic binormals of a biaxial mineral sec- 

 tion can be brought bj revolution to coincidence with the axis 

 of the microscope, it is necessary to determine these angles of 

 revolution with the greatest possible accuracy. In all cases, 

 an approximate determination is first effected by revolving the 

 section about Y, and H 2 until it is dark and remains dark dur- 

 ing a complete revolution of the microscope stage H r In 

 weakly convergent polarized light the optic axis can be seen 

 in the center of the field. In ordinary microscopes, where 

 absolutely plane parallel polarized light cannot be obtained, the 

 section in such a position will not be perfectly dark, owing to 



21 



Fig. 21. In this figure, the method for locating the position of the optic 

 axes by means of optical curves is illustrated. The figures 0°, 20°. 30° and 

 45° opposite the curves indicate the angles which the plane of vibration of 

 the polarizer at the time of observation made with the plane of symmetry of 

 the microscope. 



internal conical refraction, but will preserve the same degree 

 of slight uniform illumination for all positions of the micro- 

 scope stage. 



More accurate determinations of the position of an optic 

 axis can then be made by means of extinction angles along 

 definite directions, which, when plotted in projection, give rise 

 to curves all of which pass through the optic axis. The aver- 

 age point of intersection of a set of such curves is then the 

 true position of the optic axis in projection. (Fig. 21.) 



Such curves have been called optical curves by Fedorow and 

 are obtained most readily by first placiug the crossed nicols in 



