of Minerals in the Thin Section. 341 



accuracy of more than =fc 2° cannot be claimed for this 

 method. With the drawing table, this probable error increases 

 about ± 5° under the same circumstances. 



In place of the expensive double screw micrometer ocular, 

 the writer has had constructed a simpler, although slightly less 

 accurate ocular, consisting of a Ramsden ocular with finely 

 divided cross section scale in its focal plane (arrangement simi- 

 lar to that in ocular of Czapski). (Ocular of fig. 5.) After 

 insertion of the Amici-Bertrand lens, the secondary image of 

 the interference figure is brought to focus in the focal plane of 

 the ocular, where the location of any point can be read oif 

 directly in coordinates, which in turn are to be reduced, just 

 as the readings of the double micrometer screw ocular, to angle 

 directions within the crystal, and then plotted in suitable 

 projection. The coordinate scale employed in this ocular is 

 a photographic reproduction on thin glass of a greatly reduced 

 drawing. 



Klein's lens, which was first described by Becke, can also be 

 changed to fit the new conditions by simply introducing the 

 above fine cross-grating or coordinate micrometer scale in place 

 of the single micrometer scale. 



By the use of such oculars with fine coordinate scales, one 

 has the entire field of the interference figure under control, 

 and, by use of projection plats, can readily measure 'optic axial 

 angles on all sections which are so cut that one optic axis at 

 least is in the field. If two optic axes appear within the field 

 of vision, their positions can be read at once from the coor- 

 dinate scale of the ocular and after proper reduction plotted in 

 stereographic or orthographic projection where their angular 

 distance can be determined directly. 



Michel-Levy Method. — For sections normal to an acute 

 bisectrix of a mineral with large optic axial angles, Michel- 

 Levy has suggested a method which, although interesting 

 theoretically, is not of great practical value, owing to the 

 indistinctness of the phenomena to be observed. His method 

 consists in reading the angle of revolution of the stage necessary 

 to bring the interference figure from the crossed position to 

 that in which the emerging axial bars of the interference figure 

 are tangent to a given circle.* Actual practice with this 

 method has shown that it is not sufficiently accurate and of 

 such general application as to warrant a more detailed descrip- 

 tion at this point. 



Method with Universal Stage. — In practice, it frequently 

 happens that a given section is not favorably cut to show the 



* Michel-Levy, et Lacroix, Les Mineraux d. Koehes, 94-95, 1888. For a 

 modification and simplification of his formula, see F. E. Wright, this Jour- 

 nal, xxii, 289, 1905. 



