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BELL SYSTEM TECHNICAL JOURNAL 



The back face is larger than the front face so that outside the borders of the 

 front face the thickness tapers ofif very rapidly. Since the dark contours 

 here show thickness of the section (as the lines of a contour map show height 

 above sea level) one may easily determine the shape of the wedge. The 

 inner contour AA is near the greatest thickness, contour BB intermediate 

 thickness, and outer contour A A near the thinnest portions at the edges. 

 It might be added that the thickness-contours do not exactly indicate a 

 region of equal thickness unless the eye is distant from the stone, and unless 

 the stone is viewed exactly along its optic axis. 



Figure 4.7 is a polarized light view of an inch-thick basal section (i.e., 

 Z-cut) with the parallel ground surfaces perpendicular to the line of sight 



Fig. 4.6 — A wedge-shaped basal section of quartz viewed along the optic axis in polar- 

 ized light. The THICKNESS-CONTOURS locate regions of equal thickness, A-A 

 thickest to C-C thinnest. 



(i.e., the stone is viewed along its optic axis). Neglecting for the moment 

 the ring pattern C, one observes a wide, diffuse vertical thickness-contour 

 at B. With parallel surfaced stones a thickness-contour should cover the 

 whole stone, since the thickness of the stone is uniform. Here the stone is 

 not viewed exactly along the optic axis. If the eye be placed close these con- 

 tours become circular. 



Further, this view shows the effect of placing a lens between the tank 

 window and polarizer at E, Fig. 4.5. The result is a ring pattern, the real 

 image of which is at the focal distance of the lens on the eye side. The 

 image may also be obtained on a ground glass at this point and its location 

 is independent of the distance between quartz and lens (no rings will appear 



