428 Wright and Larsen — Quartz as a Geologic Thermometer. 



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Fig. 1. Curve I of this figure indicates the change in birefringence of 

 quartz with increasing temperature. The results of a number of measure- 

 ments of the birefringence in white light with Babinet compensator and 

 thermal microscope are plotted and curve I represents average values for 

 the series. The change at the inversion point is clearly shown. Curve II 

 indicates the combined birefringence observations on two quartz plates in 

 sodium light. The curve is exceedingly well defined and sharply cut, not- 



