72 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
wedge is not very thin and usually shows gray of the first order between 
crossed nicols, while the sensitive tint and quarter undulation plates are 
still thicker and produce on insertion a sudden increase or decrease in path 
difference between the waves emerging from the crystal section, thus giving 
rise to the abrupt change or discontinuity observed in the interference 
color, which is often sufficient to render the determination uncertain. In 
FIG. 43. 
deeply colored minerals this is especially noticeable, as the natural color of 
the mineral veils the interference color to a great extent ; on inserting the 
plate or wedge one observes a change in the interference color of the mineral, 
but is often unable to distinguish whether the color has risen or fallen. 
This defect has been remedied in the combination wedge* which consists 
of (i) a quartz (or selenite) wedge showing I-III order interference colors 
and elongated parallel with the greatest ellipsoidal axis (ct), and super- 
FIG. 45. 
FIG. 44. 
FIG. 46. 
imposed on (2) a quartz (or selenite) plate showing an interference color 
of the first order and elongated parallel with the least ellipsoidal axis, C, 
or principal axis of quartz, as indicated in Fig. 44. With this arrangement 
the point a of the combination wedge, for which both wedge and plate are 
of the same thickness, appears dark under crossed nicols, the effect of the 
quartz wedge on the transmitted light-waves being exactly compensated 
F. E. Wright. Tschermak's Miner. Petro. Mittcil . 20. 375-376, 1901; Journ. Geology. 10.33-35, 1903. 
