254 BELL SYSTEM TECHNICAL JOURNAL 



analyzer using white light. Red, yellow, blue are observed for clockwise 

 rotation with IRE right-hand quartz. 



If the z section is not a true one, Eq. 2.9 will be replaced by one allowing for 

 this error 8: 



Hs - % = .00917 sin2 (^-5) 



This will make the rings non-concentric and although the "cross" inter- 

 section is still the true optic axis the ring centers are not. 



Hence if we are tempted to find the optic axis by centering a large sharp 

 ring in preference to a small fuzzy one we find this eccentricity error must be 

 allowed for. 



2.8 Rotation of the Plane of Polarization 



If we have a polarizer and an analyzer set for extinction (Fig. 2.36), then 

 place a thin z section of quartz between them, the field brightens up but 

 can be extinguished again by rotating the analyzer, Fig. 2.37. For the min- 

 eralogist's right-hand quartz the analyzer must be rotated 21.7° (yellow 

 light assumed) clockwise to re-establish extinction, counter-clockwise 21.7° 

 for left-hand quartz. The rotation is more for blue light, less for red. If 

 the section is not a perfect z section the rotation is less than this, efifectively 

 disappearing at about 15° from the optic axis. 



A thick slab can be examined in this way and, due to the color difference 

 in rotation, "rainbows" will be seen in the quartz when held at just the 

 right orientation. These rainbows will follow the contours of the specimen 

 unless both right and left quartz are present in one piece. When this is 

 the case the one kind generally occurs as spike- or blade-like intrusions in 

 the other. It will then cause the rainbows to have sharp, jagged outlines 

 bearing no relation to the specimen contour. 



Also, since red, yellow, blue, are here in the order of increasing rotation, 

 if we rotate the analyzer clockwise for right-hand quartz (I.R.E. RHQ) we 

 will pass through best transmission for red, best for yellow and best for 

 blue in that order so that the field will assume these colors in this order. 



With uncut stones this examination is best made under an immersion fluid. 

 The inspectoscope is made for this work. We spoke of the rotation of the 

 plane of polarization and its complicating of the issue for the conoscope. 

 Due to this the field at the center is not dark when the analyzer and polar- 

 izer are crossed. Also if we rotate the analyzer clockwise the rings of the 

 pattern either expand or contract according to whether the crystal is right- 

 hand quartz or left-hand quartz (IRE definition). 



A different kind of pattern is visible in the conoscope when viewed per- 

 pendicular to z, a double set of hyperbolae as shown in Fig. 2.38. This pat- 



