136 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
FIG. 81. 
TWINNED SELENITB PLATE. 
The use of a twinned sclenite plate has been recommended recently by 
E. Sommerfeldt* for the accurate adjustment of the ocular cross-hairs to 
the planes of the nicols. But the same twins can be made to serve admira- 
bly in the measurements of extinction angles. 
The extinction angle which the ellipsoidal 
axis makes in each plate with the twinning 
plane is 37.5, and if the twinning line on 
such a plate be turned to the diagonal posi- 
tion with the crossed nicols, the extinction 
angle on each side of the nicol measures 
45 37.5 = 7.5, but in the opposite halves 
different ellipsoidal axes are adjacent to the 
principal nicol plane. The result of this 
arrangement is a change in intensity depen- 
dent not only on the angle, but also on the 
different compensations of the path-differ- 
ences in the two plates. If white light be 
used, this results in a rapid change in the interference color of the two 
halves of the selenite plate if the crystal be only a small angular distance 
from its position of total extinction. 
The writer has had such a plate cut, showing the sensitive tint and also 
a wedge, so that on insertion different interference colors, or intensities in 
monochromatic light, can be used for which the eye under given conditions 
of observation is most sensitive. These plates, as well as the preceding, 
are viewed by a low-power objective when used for the adjustment of the 
cross-hairs of the ocular, the junction line serving for the vertical cross-hair. 
For the determination of the positions of total extinction the Ramsden 
positive ocular has been found by experience to be best suited and a specially 
constructed holder to be convenient (Plate 6, Fig. 3). 
ARTIFICIALLY TWINNED QUARTZ PLATE. t 
Still another advantageous arrangement can be had by cutting, on a 
polished quartz plate parallel to the principal axis, a vertical edge making 
an angle of from 3 to 6 with the principal axis. The quartz plate is then 
divided transversely to the polished edge and the polished edges cemented 
together, thus producing an artificial twin whose two halves extinguish at 
equal and opposite angles from the common line of junction. Such bi- 
quartz plates may then be ground to a thin plate showing either the sensi- 
tive-tint or dull gray of the first order or to wedge form, thus increasing 
the range and usefulness of the device. Still lower interference figures can 
be obtained by superposing two polished plates of quartz (the one with the 
ellipsoidal axis c parallel with the elongation and the other with the ellip- 
soidal axis a parallel with this direction) after the manner of the com- 
bination wedge and then preparing from this a combination wedge divided 
*Ztcbr. f. wuseosch. Mikroskopic, 34, 24-25, 1907. 
tF. E. Wright, Amcr. Jour. Scl. (4). 26, 374, 1908. 
