DESCRIPTION OP PLATE 2. 
FIG. i . Improved petrographic microscope (made after specifications by the writer 
by R. Fuess & Co., Germany) equipped with device for simultaneous rotation of the 
nicols and with ocular support for different mounted wedges and plates which serve in 
the accurate determination of the optical constants of mineral grains in the thin section. 
FIG. 2. Double-screw micrometer ocular for use in the measurement of optic axial 
angles; // and V, horizontal and vertical micrometer screws respectively; 5, small stop 
in the eye-circle of the Ramsden ocular O to reduce parallax of rays from interference figure. 
FIG. 3. Interference figure of muscovite in sodium light. Shows also fineo.i mm. 
division lines of coordinate scale of new cross-line micrometer eyepiece for use in measur- 
ing optic axial angles. 
