426 Wright — New Ocular with Petrographic Microscope. 



taken, and for ordinary petrographic work the expensive 

 bi-quartz wedge might possibly be omitted altogether. The 

 time saved, however, by its use in accurate work is a factor 

 which would soon offset the expense involved. 



For the adjustment of the petrographic microscope the 

 bi-quartz wedge plate is also well adapted. * 



With the three plates, a, o, c (fig. 1), accompanying this 

 ocular, it is therefore possible to determine with sufficient 

 accuracy the birefringence, the optic axial angle and extinc- 

 tion angles of mineral plates in the thin section. There remains 

 still the determination of the refractive indices, and no satis- 

 factory method has yet been devised for the accurate deter- 

 mination of these on plates in the covered thin section. On 

 the polished thin sections the refractometer attachment of 

 of Wallerant has proved satisfactory, while for minerals in 

 fine grains or powdered sections the immersion method in 

 liquids of known refractive index is best adapted. By this 

 method, the refractive indices can readily be obtained on 

 favorable sections with a probable error of about ±'002.f 

 By such direct refractive index measurement, birefringence 

 determinations and optic axial angle measurement can be 

 checked and possible errors eliminated. 



Summary. 



With the ocular pictured in fig. 1, three fundamental optic 

 properties of minerals in the thin section can be measured. (1) 

 With the combination wedge "a" the birefringence ; (2) with 

 the ruled plate "b" (cross section ruling interval -l mm ) the 

 optic axial angle, provided one or both optic axes appear 

 within the field of vision ; (3) with the bi-quartz wedge plate "c" 

 the extinction angle. These three features, and particularly 

 the first two, are usually estimated only roughly and not meas- 

 ured accurately in petrographic work, chiefly because of the 

 complicated apparatus now required for the purpose. The 

 present ocular was constructed to serve as a simple but effective 

 substitute for such apparatus, and thus to facilitate the actual 

 measurement of these important properties. In the same 

 holder other wedges and plates can be introduced which may 

 serve for the determination of the above and other optical prop- 

 erties of a crystal plate. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C, January 31, 1910. 



*F. E. Wright, this Journal (4), xxvi, 386-388, 1908. 



f Compare O. Maschke, Pogg. Ann., cxlv, 568, 1872 ; Wiedemann's Ann., ii, 

 722-734, 1880 ; J. Thoulet, Bull. Soc. Min.,Fr., iii, 62-68, 1880 ; J. L. C. 

 Schroeder van der Kolk, Zeitschr. f. Wiss. Mikroskr., viii, 458, 1892; F. E. 

 Wright, this Journal (4), xxvii, 385-387, 1907; Tscherm. Mm. Petr. Mitteil., 

 xx, 239, 1901. 



