170 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
consideration the indistinctness and width of the axial bars, it is easily 
possible to make an error of three or four divisions of the micrometer scale 
in these readings, so that a greater accuracy than 2 to 3 can not be 
FIG. 102. 
FIG. 103. 
FIG. 104. 
FIG. 105. 
FIGS. 102-105. In Figs. 102 to 105 the axial curves (zero isogyres) are constructed 
under the assumption that the mean refractive indices of both the mineral and the medium 
in which its interference phenomena are observed, are identical; in short, an orthographic 
projection of the phenomena as they appear within the crystal is given. In Fig. 102 
the section is normal to an optic axis; in Fig. 103 the section makes angles, X -5, n =6 
(small circle coordinates) with the optic binomial; in Fig. 104 the angles are X = i2; 
/*" 13. while in Fig. 105 the section makes angles X-2o, n**2o with the optic axis; 
the angle of rotation of the stage with consequent new position of the trace of principal 
plane of lower nicol in each case is indicated by the arrows. The area included by the 
inner circle of Fig. 102 indicates the relative field of view of ordinary microscopes. 
