OPTIC AXIAL ANGLE. 
165 
through 90 also produced a slight shift of the axial bars on mounted plates, 
as in muscovite. 
Both theory and observations show, therefore, that, as a general rule, a 
uniradial, plane-polarized light-wave, after transmission through a bare 
crystal plate (preferably a fresh cleavage plate so that the disturbing effects 
of surface films caused by polishing are not serious), is still plane-polarized, 
but its plane of polarization has suffered a slight rotation, depending on the 
direction of transmission, with the result that the plate, when examined 
under crossed nicols, 
does not appear per- 
fectly dark. In the 
thin crystal plates the 
two refracted waves W\ 
and Wt overlap to a 
large extent and there 
exists no position of 
total extinction for the 
tilted crystal plate or 
for obliquely incident 
waves, even should the 
upper nicol be rotated 
alone. In general it 
may be stated that 
from an incident plane 
polarized wave enter- 
ing a birefracting plate 
two refracted waves 
are formed which, on 
emergence from the 
plate, are still plane- 
polarized, but their planes of polarization are not precisely 90 apart. 
The resultant light, as observed through the analyzer, is consequently 
elliptically polarized and there is no possible position of total extinction of 
the plate, but rather a region of minimum illumination which may extend 
over several degrees. 
These relations have an important bearing on methods based on the 
determination of the positions of extinction of obliquely transmitted waves 
and preclude at once a high order of accuracy in the measurements. If 
the observed crystal plates are mounted in Canada balsam, as in ordinary 
thin sections, the rotatory influence of the glass and Canada-balsam mount 
enter the problem and tend to complicate the phenomena still further. 
The rotatory effect of the glass surfaces of the lens system is still another 
factor which modifies the phenomena to a certain extent. 
If settings be made at the apparently darkest positions of a tilted crystal 
plate during the rotation of the microscope stage these positions are often 
several degrees from 90 apart, and, if the observed azimuths of the plane 
of polarization be taken as the azimuths of the refracted waves within the 
crystal, errors of several degrees are easily possible. This reasoning and 
similar observations apply to the zero isogyres in the interference figure. A 
j? IG 
