CHAPTER III. 
BIREFRINGENCE. 
The standard methods for the determination of birefringence involve the 
use either of the Babinet* compensator, the Chrustschoff compensator,! the 
Bravais-Biot compensator,! the Michel-Le'vy comparator, the graduated 
wedge, or the Michel-LeVy color chart. The methods of application are 
treated at length in the standard text-books and need not be presented here. 
Their relative sensitiveness, however, and the degree of accuracy obtainable 
by the use of such methods in actual work merits consideration, since bire- 
fringence is one of the characteristic features of birefracting minerals. 
One of the most accurate methods for determining, in monochromatic 
light, the path-difference between plane polarized light-waves emerging from 
a crystal plate is by use of the Babinet compensator. After calibration of 
the compensator in sodium or other monochromatic light, the displacement 
of the bands produced by a birefracting plate in the same monochromatic 
light is measured directly on the scale of the Babinet compensator. This 
displacement is directly reducible to path-differences in millimeters and, if 
the thickness of the plate itself in millimeters be known, its birefringence 
can be figured directly from the formula 
l\ 
in which X = wave-length of light used expressed in millimeters; 8 = distance 
in scale divisions between two successive bands on the Babinet compen- 
sator for the wave-length X ; I = displacement (in scale divisions) caused by 
inserted plate ; d = thickness of plate. If the birefringence for different wave- 
lengths be desired, it is necessary either to calibrate the compensator for 
each wave-length used or to calculate the required constants from the known 
dispersion of quartz. 
As the determination of the thickness of the plate is much less accurate 
than that of the path-difference, graduated wedges|| have been substituted 
for the expensive Babinet compensator. These quartz wedges are usually 
so cut that the path-difference of the emergent waves can be read directly 
from the scale engraved on their upper surface. In the wedges of Ce'saro, 
Amman, Leiss, and Evans an arbitrary scale is used and afterwards cali- 
brated, while in the wedge of Siedentopf and that of the writer the pitch of 
the wedge is so calculated that, from the scale divisions, the path-difference 
in fji/j. is read off directly. 
*Jamin. Ann. dc Chim. etPhys., 29, 263, 1850; see also K. E. P. Schmidt. Zeitsch. Inst., II, 439, 1891. 
Wied. Ann. 35, 360, 1888; J. Macg de Lcpinay. Journ. de Phys. (2), 10, 204, 1891. 
tZeitschr. Kryst., 30, 389, 1899. 
iAnn.Chim. Phys. (3), 43, 139. 1855; Biot. Trait<5 de Phys. ; seeE. Mascart. Trait* d'Optique. 2, 61. 1892. 
{See Michel-Levy, Bull.Soc. Miner. Fr , 7, 57. 1884: Les Mincraux des Roches. 57. 1888. F. Becke, T. 
M. P. M., 22, 378, 1903. V. de Souza-Brandao, Centralblatt f. Miner.. 23-29. 1905. C. Hlawatsch. 
T. M. P. M. 23, 415-450. 1904; 21, 107-155. 1902. 
||G. Clsaro, Bull. Acad. Roy. Belg.Cl. Sciences, 208, 1893. J. Amman. Zeitschr. wiss. Mikrosk.. 11,440- 
454, 1894. C. Leiss, Neues Jahrb. B. B.. 10, 426-428, 1896. J. W. Evans, Min. Mag., 14, 87-92,1905. 
H. Siedentopf, Centralblatt f. Min.. 745-746, 1906. F. E. Wright, Amer. Jour. Sci. (4). 29, 417, 1910. 
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