140 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
inserted or drawn out from this point of zero rotation the angle of rotation 
increases proportionately in a positive sense on one side of the junction 
line of the combination and in a negative sense on the opposite half. This 
combination wedge, which is introduced at the focal plane of the ocular 
(Plate 6, Fig. 3), divides the field under crossed nicols into two halves, whose 
intensities of illumination at any instant are equal, provided no intervening 
crystal plate is present or is rendered inactive by the parallelism of its ellip- 
soidal axis with the principal planes of the nicols. As soon as the crystal is 
turned even a very small angle out of this position, the intensity of illumina- 
tion of the two fields is no longer equal. By inserting or withdrawing the 
combination wedge, the most advantageous angle of rotation in the two fields 
can be procured, so that the difference in intensity between the two halves 
is most apparent. In effect this wedge is identical with that of the bi-nicol 
ocular noted above, but is much simpler in construction, and requires no 
adjustment; the one condition which must be fulfilled for satisfactory re- 
sults is that the wedge be not tilted on insertion ; the optic axis must remain 
parallel with the optic axis of the microscope, otherwise disturbing bire- 
Fic. 82. Bi-quartz wedge plate. Consists of two quartz plates with superimposed 
quartz wedges, all cut normal to the axis, the right-handed and left-handed elements 
arranged as indicated. Cementing material is Canada balsam whose refractive index 
is 1.54 while <" for quartz is 1.544,3 difference so slight as to render inappreciable the 
exceedingly slight deviation of the waves caused by the slight wedge surface of the wedge. 
This inclined surface is mounted next the Canada balsam and care is taken (by inserting 
a thin glass strip at the thin end) to make the upper and under surfaces of the completed 
wedge parallel. In preparing the wedge it is necessary that the edges be ground and 
polished in order that the central division line be as sharp as possible. The two halves 
are eventually cemented side by side with Canada balsam and any disturbing influence 
thus eliminated which might arise from total reflection on the sides. At the point where 
wedge and plate have the same thickness the rotation is zero and a dark band traverses 
the wedge under crossed nicols. On the wedges which have been constructed, the 
rotation for sodium light at the one end has usually been about == i , while at the other 
end it has been either ^.a or 10 or 15. Specifications for a wedge of any angle of 
rotation at the thick end can readily be given. 
fringence phenomena appear. The wedge carriage should, therefore, slide 
in an accurately fitting holder, such as that shown in Plate 6, Fig. 3. If 
the wedge be inserted horizontally to the point where wedge and plate have 
the same thickness, the effect of the combination on the plane of polarizal i< >n 
of transmitted light is nil and a straight black vertical band appears in each 
half of the field similar to the zero band of the Babinet compensator, \ : \^. 
83. By means of this band the position of total extinction of an inserted 
plate can be found with great accuracy, for the setting is thus made to depend 
upon the exact alinement of two black bands and the photometric principle 
