MICROSCOPICAL PETROGRAPHY 499 
halves of the wedge show the same degree of illumination. If the 
mineral be not in the position of total extinction the halves are 
not equally illuminated. Experience has shown that with this 
device the probable error of a single setting on a favorable section 
is about +10’. The bi-quartz wedge plate is mounted in a metal 
carriage which in turn fits into the ocular holder described above. 
This bi-quartz wedge plate may also serve on dark days for the 
adjustment of the nicols in the petrographic microscope. 
Optical axial angles are most readily measured by means of 
the o.1 mm. co-ordinate micrometer disk' which in its metal- 
mounting fits in the ocular holder noted above. On favorable 
-sections (0.025 mm. and over in diameter) the probable error of 
measurements with this plate is about +1° in case both optic 
axes appear in the field of vision and +3° in case only one optic 
axis is visible. For measurements on mineral grains, the particles 
should be immersed in a liquid of the refractive index f to eliminate 
errors due to refraction on the uneven surfaces of the grains. In 
weakly birefracting substances and interrupted sections the axial 
bars are less sharply defined and the values obtained thereon are 
correspondingly less accurate. 
The divisions of the co-ordinate micrometer disk serve to locate 
the position of any point in the field. The interference figure, 
observed, is practically an orthographic projection of the inter- 
ference figure formed in the upper focal plane of the objective and 
the use of co-ordinates to locate points in the field is therefore 
permissible. The angular values represented by these co-ordinates 
are determined, once for all, by means of an Abbe apertometer, or 
a graduated sphere or a scale in the lower focal plane of the sub- 
stage condenser.? In case both optic axes, A; and A,, appear in the 
field the course of procedure is simple. The crystal plate is turned 
until the plane of the optic axes, A,;A., in the interference figure 
(observed, together with the co-ordinate micrometer scale after 
insertion of the Bertrand lens, in the lower focal plane of the 
tF, E. Wright, Amer. Jour. Sci. (4), XXIV, 316-69; XXIX, 423, 1910; XXXI, 
TS —Ditaey Ione, 
2 These are discussed at length in Amer. Jour. Sci. (4), XXIV, 317-69, 19073 
also in Carnegie Inst. Washington, Pub. 158, 1911. 
