of Minerals in the Thin Section. 361 



Measurement of the optic axial angle on the total refraclorneter. 



Pulfrich,* Soret,f Viola4 Corim§ and Wallerant|| have shown 

 that it is possible on a single section of a biaxial or uniaxial 

 mineral to determine not only the three principal refractive 

 indices a, /S and 7, but also, by observing the planes of polari- 

 zation of each wave corresponding respectively to a(3,/3<y and 

 7a, to determine accurately the relative position of the princi- 

 pal planes of the ellipsoid to the given section ; and from the 

 accurate refractive indices thus ascertained to figure the optic 

 axial angle with great exactness. These methods, however, 

 require specially ground and polished sections and are not, in 

 general, microscopic methods, although the total refractometer 

 of Wallerant is attached directly to the microscope and is em- 

 ployed on thin uncovered and polished sections of rocks. 

 Unfortunately, the writer has had practically no opportunity 

 to work with the total refractometer of Wallerant, and is, there- 

 fore, not in a position to judge personally of its fitness for optic 

 axial angle determinations. Viola and others have shown that 

 on the Abbe total refractometer results of great accuracy and 

 certitude can be obtained rapidly and without difficulty. The 

 mineral plates should measure then l sq mm or over to furnish 

 sufficiently intense reflexion signals for nice adjustment and 

 measurement. 



Measurements. 



So much space has been devoted above to the theoretical con- 

 siderations and descriptions of methods that in this section only 

 a part of the available observational data can be enumerated 

 and a brief resume of the results presented. Enough data will 

 be offered, however, to indicate certain inferences bearing on 

 the relative accuracy and applicability of the different methods 

 under test. 



Different minerals, as.aragonite, topaz, muscovite, etc., were 

 first chosen and oriented sections cut to show the different 

 phenomena required by the several methods. The correct 

 optic axial angle for each mineral was then measured in sodium 

 light on a Wulfmg-Fuess axial angle apparatus, the angle 

 obtained thereby being adopted as the standard of comparison 

 for all methods. For each mineral a series of measurements 

 of the optic axial angle for different sections and by the differ- 

 ent methods was taken and the relative degree of accuracy of 

 each method judged, not only by the results obtained, but also 



* Pulfrich, C, Das Total reflektometer, Leipzig, 1890. 



fSoret, Zeitschr. Kryst., xv, 43, 1899. 



% Viola, C, Zeitschr. Kryst., xxxi, 40-48, 1889 ; xxxvi, 245-251, 1902. 



§ Cornu, Compt. Rend., cxxxiii, 125 ; Bull. Soc. Min., xxv, 7. 



|| Wallerant, F., Bull. Soc. Min., xx, 12-26, 1898. 



