JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. V FEBRUARY 19, 1915 No. 4 



PHYSICS. — The accurate measurement of the refractive indices 

 of minute crystal grains under the petrographic microscope. 

 Fred. E. Wright, Geophysical Laboratory. 



The principal refractive indices of fine crystal particles, meas- 

 uring 0.01 mm. in diameter or larger and of medium refrin- 

 gence and birefringence can be determined by the immersion 

 method with an accuracy of ± 0.001.^ In strongly birefracting 

 minerals and in deeply colored substances of high refractive 

 index this degree of accuracy is rarely attainable by the im- 



1 For a discussion of the principles and details of practical application of 

 the immersion method to refractive index determinations under the petro- 

 graphic microscope see: Tschermak's Miner, petrogr. Mitteil., 20:239. 1901; 

 Am. Jour. Sci., (4), 17:385-387. 1904; (4), 35:63-82. 1913; Carnegie Insti- 

 tution of Washington, Pub. 158: 93. 1911; Jour. Wash. Acad. Sci., 4: 270, 389. 

 1914. Since the publication of these papers a new procedure for the measure- 

 ment of the refractive indices of immersion liquids on the total refractometer 

 has been adopted by the writer and has proved so satisfactory that it may be 

 described briefly. Plane parallel disks (2 cm. in diameter, 2 to 3 mm. thick) 

 of high refracting glass (n > 1.74) serve in place of ordinary object glasses. The 

 refringence tests between mineral grain and immersion liquid are made in the 

 usual way under the microscope. The glass disk is then mounted directly on the 

 glass hemisphere of the total refractometer with a drop of highly refractive 

 liquid intervening (methylene iodide or Rohrbach solution), and the refractive 

 index of the liquid is measured through the glass disk. By this method, it is 

 possible to measure the three principal refractive indices of a biaxial mineral 

 grain without removing it from the liquid, the refractive index of the liquid 

 itself being laised or lowered the proper amount to match the different refrac- 

 tive indices of the mineral by the addition of a smaU drop of a suitable re- 

 fractive liquid. The danger of scratching the surface of the glass hemisphere 

 by mineral grains embedded in the refractive liquid is also eliminated. 



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