Wright — N&w Ocular ivith Petrographic Microscope. 415 



Aet. XXXYII. — A JYeiv Ocular for Use with the Petro- 

 graphic Microscope ; by Fred. Eugene Weight. 



Minerals in the thin section are determined and recognized 

 chiefly by the effects they produce on transmitted light and 

 the relation of these effects to observed crystallographic 

 features, snch as cleavage, crystal form, etc. The usual optical 

 properties which are thus made use of in determinative work 

 are : crystal habit, cleavage, character of elongation, color, 

 pleochroism and absorption, refractive indices, birefringence, 

 extinction angles, optic axial angle, optical character, and rarely, 

 dispersion of the optic axes. 



These characters can be divided into two classes based on 

 the methods of their determination. Those of the first-class 

 (crystal habit, color, pleochroism and absorption, optical charac- 

 ter of elongation, optical character of the mineral, and dispersion 

 of the optic axes) are ascertained by direct observation without 

 measurement, while for the second class (cleavage angles, 

 extinction angles, optical axial angles, refractive indices and 

 birefringence) numerical values obtained by actual measure- 

 ment are required. 



The ordinary petrographic microscope is adequate and satis- 

 factory for the determination of the features included in the 

 first class, but not for the second, if accurate data are desired. 

 The result is that in petrographic determinative work and de- 

 scription these data are often only very roughly measured and are 

 then expressed in general terms, such as "optic axial angle large," 

 "birefringence strong," etc., without giving actual numerical 

 data. The importance of quantitative measurements in all 

 scientific work is obvious. The chief reason for the absence 

 of the quantitative element in the major part of petrographic 

 descriptions must, therefore, be sought in the cumbersome 

 methods now available for the purpose. Thus a Babinet com- 

 pensator, or other special device, is required for the measure- 

 ment of the birefringence ; a double screw micrometer ocular 

 or Becke drawing stage for the measurement of the optic axial 

 angle, etc. Believing that these various requirements can be 

 met with sufficient accuracy by the use of a single ocular, 

 which can be made to fit any ordinary microscope, the writer 

 has had the present ocular constructed, which has proved satis- 

 factory and convenient in actual use.* The ideas involved in 

 this ocular are not new,f but the assembling of the different 



*This ocular was constructed by Fuess & Co. of Steglitz, Germany, 

 (cost 200 mks.) and the writer desires to express his appreciation of the care 

 taken by that firm in carrying out his suggestions. 



•{•Compare F. E. Wright, this Journal (4) xxiy, 317-369, 1907; xxvi, 349- 

 399, 1908; Journal of Geology, x, 33-35, 1902; Tscherm. Min. Petr. Mitteil., 

 xx, 275, 1901. J. W. Evans, Min. Mag., xiv, 87-92, 1905. 



