36 



Allen, etc. — Diopside and its Relations 



the refractive indices were measured directly on the total refrac- 

 tometer, experience having shown that with polished plates of 

 the crystallized mass of substance the 7 and a limiting refrac- 

 tive index lines can usually be distinguished if the reducing 

 attachment be used. In such cases where the maximal and. 

 minimal indices are determined from aggregates of crystals 

 rather than from one crystal, it is of course not possible to 

 determine also the refractive index /3. Optic axial angles 

 were determined by use of the two-screw micrometer ocular 

 and also by the modified universal stage* of Feclorow. Extinc- 

 tion angle measurements were made with the aid of the bi- 

 quartz wedge plate,f care being taken to select favorable 

 sections in each case. Direct determinations of the birefrin- 

 gence in the thin section or in flat crystal grains were accom- 

 plished by measuring the thickness of the plate or grain with 

 the micrometer screw of the microscope (model Fuess) and 

 then ascertaining the path difference of the emerging waves 

 by means of a Babinet compensator. By this method only a 

 fair degree of accuracy can be obtained because of the 



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* This Journal, xxiv, 317-369, 1907. f This Journal, xxvi, 377-379, 1908. 



