390 F. E. Wright — Measurement of Extinction Angles. 



An application of the bi-quartz wedge plate, together with 

 an artificially twinned quartz-plate or wedge, to the accurate 

 adjustment of the petrographic microscope is considered in 

 outline in the second part above, and a method of procedure 

 for accomplishing the same indicated. 



In part 3 a simple device for holding and rotating small 

 crystals for the purpose of determining extinction angles or for 

 measuring optic angles directly, is described briefly, such an 

 apparatus having been found peculiarly useful in work with 

 artificial crystals. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 



Washington,' D. C, June, 1908. 



