SPECIFYING QUART/ CRYSTAL ORIENTATION 



whence we solve for the values in this table 



convergence ^ = 5° 10° 20° 30° 



253 



thickness 



/' = 7.8 1.94 0.48 0.21 



This shows that if we wish to examine thin plates in a conoscope the lenses 

 must be strongly convergent. The conoscope used in the Western Electric 

 has a convergence corresponding to about the 20° entry of the table so it 



Fig. 2.35 — Ring eccentricity 



can be used on crystals down to a half millimeter thick — that is for orien- 

 tation studies. In determining handedness we remember that this is a ques- 

 tion of the rotatory power of quartz causing the rings to expand or contract 

 on rotating the analyzer. Also we said that this rotatory power effectively 

 disappears at 15° from the axis. If no ring is found within 15° of the axis 

 there is no ring capable of expanding or contracting and we cannot test the 

 handedness of such a thin crystal no matter how strong a lens we employ. 

 We can then fall back on the succession of colors shown when we rotate the 



