OPTIC AXIAL ANGLE. 
-COORDINATED 
. SCALE. 
claimed for this method. With the drawing-table this probable error is 
about =*= 5 under the same circumstances. 
In place of the expensive double-screw micrometer ocular, a positive 
ocular with o.i mm. coordinate micrometer scale in its front focal plane 
(Fig. 1 06) has been used and found in practice to be equally satisfac- 
tory and nearly as accurate.* Plate 6, 
Fig. 3, shows another form of this ocular, 
in which the coordinate scale (Fig. 107, 
only millimeter and not o. i mm. divisions 
shown) is supported in a metal carriage, 
which in turn is inserted in a specially 
constructed holder. The same holder is 
used to support a number of other different 
plates and wedges and has been found 
so serviceable that in the microscope of 
Plate 2, Fig. i, it has been built directly 
into the microscope. The coordinate scale 
ocular, Fig. 106, is also useful in esti- 
mating the relative volumes of minerals 
present in a thin section (statistical method 
of rock analysis) and serves the purpose of 
an ordinary micrometer ocular as well. I?IG - Io6 - 
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 off 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 ocular of Czapski and also HtHIHmmilHIIIIH 
Klein's lens, which was first de- 
scribed by Becke, can be changed 
to fit the new conditions by sim- 
ply introducing the above fine co- 
ordinate micrometer scale in place FIG. I0 7- 
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 coordinate scale 
of the ocular and after proper reduction can be plotted in stereographic, 
orthographic, or angle projection where their angular distance can be deter- 
mined directly. In the measurement of optic axial angles by these methods 
the errors arise chiefly from the determination of the exact position of the 
dark axial bar rather than from the reading of the scales. 
^Constructed for the writer by R Fuess & Co.. Stcglitz, Germany; also by Bausch & Lomb Optical Co.. 
Rochester, N. Y. See Jour. Wash. Acad. Sci.. 1, 60-61, 1911. 
