EXTINCTION ANGLES. 141 
of comparing two dimly lighted fields is for the most part eliminated. The 
same line can be used to advantage for the adjustment of the nicols.* 
MBTHODS INVOLVING ROTATION OP THB UPPER NICOL. 
In all of the preceding methods the nicols have been considered crossed 
and the crystal plate has been turned. The intensity formula shows, how- 
ever, that the relative intensity is dependent not only on the angle 6 of the 
crystal plate but also on <f>, the angle between the principal planes of the 
nicols. It was shown in the mathematical treatment that this method is 
in general more sensitive than the method based on the rotation of the 
crystal plate under crossed nicols. The mode of application of this method 
to any particular crystal plate is obvious and consists simply in placing the 
crystal under crossed nicols in its position of apparent total extinction and 
then observing, either in white or monochromatic light, the changes which 
occur on rotating the upper or lower nicol through small angles from its 
normal position. In case the crystal is actually in its position of total 
extinction, the crystal and field attain their position of maximum darkness 
simultaneously and show the same increase in its intensity of illumination; 
if, however, the crystal be not in position of total extinction, but a small 
-f- angle, as 30' distant, then for a position of the nicol +2 from its normal 
position the crystal plate will appear lighter than the field; and, vice versa, 
for the nicol 2 from its normal position the crystal plate will appear 
darker than the field. This methodf is extremely simple in manipulation 
A c B 
FIG. 83. 
and does not require special apparatus. Weinschenk.J in describing the 
adjustment of the nicols in the microscope, uses the interference phenomena 
which occur under these conditions for the accurate adjustment of the 
nicols, but does not appear to have applied, conversely, the principle to the 
practical determination of the optic ellipsoidal axis in a given crystal plate. 
CONVERGENT POLARIZED LIGHT. 
Several different methods have been proposed which require convergent 
polarized light and are based on the change in aspect of symmetrical inter- 
ference figures caused by the intervening crystal plate when it is not pre- 
cisely in the position of zero extinction. The idea underlying these methods 
is that the eye can detect more readily slight changes in the shape of a 
*In a review of a paper by the writer on the " Application of the bi-quartz wedge plate to polarimeters and 
saccharimeters " O. Schdnrock (Zeitschr. Instrumentenkunde. 30, 1910) has criticized the writer's descrip- 
tion on the basis that this wedge had been described in 1900 by Mace de Lcpinay (Jour. Phys. (j). 9, 585. 
1900). A comparison of the two descriptions is sufficient, however, to disprove this statement of the reviewer, 
as Mace de Lepinay 's wedge consists simply of a thin bi-quartz wedge without the quartz plates and does not 
therefore, show zero rotation, nor the black line noted above. Schonrock suggested in a review of Mace de 
Lcpinay 's paper (Zeitschr. Instrumentenkunde. 21, 90. 1901) the use of two bi-quartz wedges, but even with 
that arrangement failed to obtain the sensitive black line of zero rotation, which is one of the features of 
the bi-quartz wedge plate. If the construction suggested by Schdnrock were carried out the fields would 
be uniformly lighted, if strictly parallel polarized light were used; to obtain different angles of rotation with 
this arrangement it would be necessary, moreover, to move the upper half over the lower half and this, 
mechanically, is not advantageous. 
tF. E. Wright. Amer. Jour. Sci. (4). 26, 379. 1910. 
JZeitschr. Krystall.. 24, 581-583, 1895. 
