Petrographic Microscope. 



423 



that the birefringence of a plate, inclined at an angle of 5-10° 

 with the true direction (optic normal or bisectrix), will be in 

 error about 2 per cent of the true value (y-a), (7-/?) or (/3-a) ; 

 an inclination of 10-15°, about 5 per cent, while for 15°-20° 

 inclination the error may be as much as 10 per cent of the 

 true value desired. By means of the optic axial angle grating 

 device described below, the angular inclination of the section 

 can be ascertained and the probable error due to this cause 

 thus eliminated. 



In actual practice, therefore, the method of procedure in the 

 determination of the birefringence of a mineral plate in the 

 thin section or a mineral grain is to measure first the thickness 

 by one of the methods noted above and then to insert the 

 wedge "a" and determine under crossed nicols and in homo- 

 geneous (e. g. sodium) light, the path difference between the 

 interfering light waves. ' For less accurate work the direct 

 determination of the interference color and equivalent path 

 difference indicated on standard color charts like that of 

 Michel Levy is sufficient. — The actual error of such a deter- 

 mination should not exceed 10 per cent of the true value of 

 the birefringence of the section. The probability of finding a 

 section making an angle within 10° of a particular direction 

 (optic normal) is about 1 in 66 ; and a section within 20° about 

 1 in 16. Since wedge "a" is merely a refined combination 

 wedge, it can be used for all purposes for which the latter 

 serves. 













Fig. 



9. 









T1 1 



^-L^U 







4fH ■ m 



-+— 





















; ' 1 ! : 







, 









< : — 1 — ■ — 





















J — 











— — 

























- 





■ 







1 — 



■ — 









■ — 



■ ■ 























Fig. 9. Top view showing cross grating ruling on plate b used in the 

 measurement of optic axial angles of mineral plates in the thin section. In 

 this figure, the 0*o mm divisions are indicated but not the 0'l mm . 



(b) Plate b (fig. 1 and fig. 9) and the measurement of the 

 optic axial angle. — By the use of this device, which is simply 

 a thin glass plate l-5 mm wide, on which fine coordinate lines 

 0*l mm apart have been ruled, the optic axial angle of a mineral 

 can be measured, provided one or both optic axes appear 

 within the field of vision. The principles on which the 

 method is based are considered in detail in a former paper.* 



* The Measurement of the Optic Axial Angle of Minerals in the Thin Sec- 

 tion, this Journal (4) xxiv, 317-369, 1907 ; also, Das Doppel-Schrauben- 

 Mikrometer-Okular und seine Anwendung zur Messung des Winkels der 

 optischen Achsen von Kristalldurchschnitten unter dem Mikroskop, 

 Tscherm. Min. Petr. Mitteil., xxvii, 293-314, 1908. 



