of Minerals in the Thin Section. 



543 



The methods for determination of the plagioclase feldspars, 

 rich in lime, which have been developed by F. Becke* by using 

 Klein's lens and Czapski ocular, also by means of revolving 

 drawing table, can be applied directly to the two screw microm- 

 eter ocular and more accurate data obtained by its use. 



Parallel Polarized Light. 



The introduction of the universal stage methods by Fedorow 

 in 1893+ and succeeding years placed a powerful instrument 



20 



Fig. 20. In its present form the universal stage comprises, when attached 

 to the microscope stage, five graduated circles ; Hi, the horizontal circle of 

 the microscope stage, H 2 , the large horizontal circle of the universal stage, 

 with H 3 , the inner and thin section bearing circle, Vj, the large vertical cir- 

 cle, and V 2 , an inner circle consisting of two segments V 2 « and V a &, and 

 placed to measure the angles of revolution of the inner disk H 3 about a hori- 

 zontal axis. Two glass hemispheres (Ai being the upper) are usually 

 employed with the stage to increase the angle of vision of the microscopic 

 field. 



of attack in the hands of penologists. With his methods, it 

 is now possible to obtain the optic properties of mineral sec- 

 tions which before were considered practically useless. The 

 universal stage (fig. 20) can be attached securely to any suitable 

 petrographic microscope ; parallel polarized light only is used. 

 By means of horizontal and vertical axes of revolution, a crys- 

 tal section can be brought to any given position and revolved 

 about any axis for optical examination. 



In plane polarized light an optic axis is recognized by the 

 fact that when placed parallel to the axis of the microscope it 

 remains uniformly dark during a complete revolution about 



*F. Becke, Tschermak's Min. petr. Mitth., xiv, 415-442, 1895. 

 fE. Fedorow. Zeitschr. Kryst., xxi, 574-678, 1893; xxii. 229-268, 1894; 

 xxv, 225-261, 1895 ; xxvii, 337-398, 1897 ; xxix, 604-658, 1898. 



