34 FRED EUGENE WRIGHT 
showing green of the second order, in which, however, the ray 
vibrating parallel to the long direction has the lesser velocity 
(c). These two plates are cemented one above the other 
between glass plates (Fig. 1, cross section. Vertical scale 
exaggerated. ) 
With this arrangement the central part of the wedge (Fig. 
1, A) appears dark between crossed Nicols as the effect of the 
quartz wedge on the light passing through at that point is 
exactly compensated by the selenite plate. To the right and 
left of A, however, the interference colors rise from dark to 
TGS ie 
blue of the second order at the two ends. This wedge has 
therefore no noticeable effect on the rays passing through it at 
the center (Fig."1,,.2)).) The interference color of) the mineral 
seen through the wedge at this point will be the same as though 
no wedge were there. If, however, the wedge is pulled out or 
pushed in, the interference color of the mineral either rises or 
falls, but with a gradual transition from one color to another 
without an abrupt rise or fall of the color at the start. 
Near the center of the wedge is a point for which the differ- 
ence between the two rays isid. This part of the wedge can 
therefore take the place of the quarter undulation plate. 
To make the combination wedge as useful as possible, it was 
fitted in a metal frame of the same outer dimensions as the 
ordinary wedge, and with it in the same frame a short selenite 
plate, red first order, is placed at one end. A space is left free, 
and is thrown into the field when the wedge is not in use. To 
steady the motion of the wedge, and also to mark the position 
of the open space, a small steel spring is screwed onto the tube 
