134 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
BRAVAIS*-STOBERt PLATB. 
This device is cut to show the sensitive violet interference color and con- 
sists of two plates in combination instead of a single one. A single sensitive- 
tint plate of mica or quartz is cut along a line at 45 with the directions 
of extinction; one half is then turned through a vertical angle of 180 and 
cemented to the other as indicated in the figure. By this combination 
plate, which is placed in the focal plane of the ocular, the interference color 
is made to fall in the one half and to rise an equal amount in the second, 
thus doubling the sensitiveness of a single plate (Fig. 79). This plate is 
intended for use only in white light, but under certain conditions it may 
serve to good advantage in monochromatic light. 
THE KOENIGSBERGER PLATE-t 
The Koenigsberger plate is a modification of the Bravais plate and con- 
sists of two very thin mica plates arranged as in the Bravais plate and placed 
in the focal plane of the ocular ; it shows, however, a very low interference 
color instead of the sensitive tint. This plate was originally constructed for 
the detection of weak anistropy, but under certain favorable conditions it 
can be used for the determination of the position of total extinction. As the 
shift in path-difference on insertion of the plate is very slight, this method 
can only be applied satisfactorily to crystal plates whose interference color 
is changed perceptibly by a slight change in path-difference between the 
emergent waves. 
THE DOUBLE COMBINATION WEDGE. 
On the principle of the Bravais-Stober plate, the writer has had a com- 
bination wedge prepared in which the interference colors range from total 
darkness to green of the second order. This wedge (Fig. 80) was made by 
taking an ordinary combination wedge] I showing the zero interference band 
exactly in the center and green of the second order on each end, and cutting 
the wedge in half longitudinally parallel to the ellipsoidal axes; the edges 
were then polished and the halves again recemented the one half, however, 
having been rotated first through 180, so that in the resultant combination 
wedge the phase difference of the adjacent half at any point of insertion is 
always equal and opposite when no crystal plate is in the field. By this 
method the principle of the Bravais-Stober plate is extended to cover inter- 
ference colors from total darkness to blue-green of the second order, and to 
allow the observer to select an interference color which, in combination with 
that of the mineral plate examined, is most sensitive. The low gray tints 
Comptes Rendus. U, 113, 1851; Ann. de Chira. et de Phys. (3) 41, 129. 1855; Pogg. Ann., 96, 397. 414 
1835. 
fZeitschr. Kry*t., 3, 23-34. 1898. 
ICentralblatt fur Miner, 739-730, 1908; 349-330, 746-747, 1909. 
{This wedge WM prepared with great care by Voigt & Hochgesang. Gottingen, Germany, and the writer 
desires to eiptess bu appreciation of the interest taken by the firm in the same. The compensation on, 
different ends of the wedge, however, proved to be of slightly different value, with the result thut. although 
the dark zero interference bands were precisely adjacent, the interference colors near the ends of the wedge 
did not coincide exactly. This slight defect can be eliminated by combining two quartz plates (45 mm. 
long by 3 mm. wide and of such a thickness as to show interference color green- yellow second order, the 
ellipsoidal axis c of the one to be parallel to the long direction, while in the second a is parallel) with two 
wedges of the same pitch (45 mm. long by 3 mm. wide and ranging in interference colors from yellow of the 
first order to violet-gray of the third order, and likewise the ellipsoidal axis r parallel to the direction of elon- 
gation in one and a to the same direction in the second) ; the wedge of long direction c to be combined with 
toe plate of long direction a. In this manner the plate and wedge compensate in the center of the wedge 
and the interference colors rise to about blue-green of the second order at both ends. See Amer. Jour. Sci. (4). 
3*, 371. 1908. 
HCompare P. B. Wright, Tschermak's min. petr. Mitth.. 20. 333-306, 1901 ; also Jour. Geol., 33-35, 1903. 
