336 F. E. Wright — Measurement of the Optic Axial Angh 



New Methods with Double Screw Micrometer Ocular. — 

 In seeking- for more accurate and at the same time simpler 

 methods than those of Professor Becke described above, the 

 writer lias substituted in place of the usual single screw 

 micrometer ocular, with a movement in one direction only, a 

 double screw micrometer ocular with movements in two direc- 

 tions normal to each other. By the use of this double screw 

 micrometer ocular, which was constructed in the workshop of 

 the Geophysical Laboratory (fig. 11)* it is possible to determine 



11 



the position of any point in the interference figure accurately 

 by means of two micrometer screw readings, which corre- 

 spond to rectilinear coordinates in the orthogonal projection 

 and small circle coordinates in the stereographic projection. 

 By means of the constant K of the microscope for each of 

 these micrometer movements, K v and K H , which must have 

 been determined previously by means of known angle values 

 (table 1, page 328), each of these readings is reduced as usual 

 to its angle value for the crystal by means of K and the aver- 

 age refractive index of the crystal, 



sin V = 



D_ 



Having given the interference figure from a section of a 

 biaxial mineral, cut so that one axial bar is visible, the course 



* The double screw micrometer ocular is fitted to microscope of fig. 7. 

 The two movements, H horizontal and V vertical, are effected by fine 

 micrometer screws, reading accurately to - 005 mm . The construction of this 

 ocular is similar to that of the single screw micromoter oculars with the 

 exception that here two screws with corresponding movements are used in 

 place of the single screw. = Ramsden ocular ; S, small stop aperture to 

 reduce errors of parallax. 



