BIREFRINGENCE. 103 
of the one was parallel with the greatest ellipsoidal axis ft of the second, as 
indicated in Figs. 44, 45. At the point a of this wedge both plate and wedge 
have the same thickness and the dark band of exact compensation is observed. 
The interference color rises slowly on either side of this band, as in the 
ordinary combination wedge ; this part of the wedge especially is an exceed- 
ingly sensitive device for detecting anisotropy in very weakly birefracting 
substances. By combining this wedge with a Bravais-Biot compensator 
with isochromatically illuminated field it is possible to measure the path- 
difference of the emergent waves with great accuracy. 
Shortly after the appearance of the above paper and without knowledge S 
of the same, J. Koenigsberger* described a modification of the Bravate 
double plate showing the sensitive-tint, in which very thin mica plates were 
substituted for the thicker Bravais plates, and a low gray interference color 
was thus obtained. This combination is useful for the detection of weak 
anistropy. Following the example of Bravais, Koenigsberger has combined 
his plate with a Biot-Bravais compensator and is able, with the combination, 
to determine the path-difference with great accuracy.! With the sensitive- 
tint double plate Bravais was able to measure differences of slightly less 
than i ju/z, while Koenigsberger claims an accuracy of 0.0003 ^ or about 0.2 
Hfj. for the thinner double plate. The double combination wedge of the 
writer has the advantage of these plates because of its adjustable sensibility, 
by virtue of which the interference tint can be changed and the particular 
color selected which is the most sensitive under the given conditions of 
observation; although in the double combination wedge the field is not 
uniform throughout it is practically so for the small mineral grains in the 
thin section and no error is introduced thereby. 
The Fedorow mica comparator J consists of a set of 15 superimposed and 
overlapping thin mica plates, each successive plate being about 2 mm. shorter 
than the one immediately below it, the whole forming a step-like wedge of 
known optical intervals. The plates are of such a thickness that the path- 
difference of the emergent light-waves (of wave-length 510 up} is 127.5 ^M 
or one-quarter wave-length. This wedge is useful, but of less general appli- 
cation than the graduated quartz wedges and has not been generally adopted 
by petrologists. 
The Se'narmont-FriedelH method for measuring the birefringence is based 
on phenomena obtained by the use of elliptically polarized light and is a 
less direct method than the above methods. Just below the crystal plate 
whose birefringence is to be measured and whose ellipsoidal axes are placed 
in the diagonal position with respect to the nicols, a quarter undulation 
mica plate is inserted with its ellipsoidal axes parallel with the principal 
nicol planes. The lower nicol is then rotated until, in monochromatic light, 
the crystal plate becomes dark or until both field and crystal plate are 
equally illuminated. In the first case the angle of rotation of the lower 
*Centralblatt f. Miner., Dec. 729-730, 1908; 249-250, 746-747, 1909. 
tC. Kraft (Bull. International d. L'Acad. d. Sci. de Cracovie. Cl. Sciences, 310-353, 1902) also made use of 
this type of Biot-Bravais compensator in his work on the Newton color scale and in determining the path- 
cliff i-rcnces (for A" 550 MM) which produced certain observed interference tints. 
tZeitschr. Kryst., 25, 349-351, 1896; 26, 251-253, 1896; 29, 611-613. 1898. In this last paper Fedorow 
suggests the use .of the Bravais-Stober plate in white light and considers its accuracy twice that of the 
ordinary wedge. 
!|H. de Senarmont. Ann. Chim. Phys. (a) 73, 337, 1840; Pogg. Ann. Erg. Bd. I, 451. 1842; see also. H. 
Wiedemann, Pogg. Ann.. 151, i. 1874; Ber. d. Sachs. Gesell. d. Wiss., 1872; the same method was described 
by G. Friedel, Bull. Soc. Min. Kr., 16, 10-33. 1893; J. Walker, Phil. Mag. (6) 3, 541, 1902. 
