534 WRIGHT: MEASUREMENTS OF REFRACTIVE INDICES 



that the angle a within the crystal is known. This angle cannot 

 be obtained from microscopic measurements unless one of the 

 indices of refraction is known. 



For measuring the dark rings a standardized microscope 

 with screw micrometer ocular is most satisfactory, but a cross- 

 grating ocular can be used effectively, especially if, when the 

 rings are closely spaced, the readings are made along the diago- 

 nals of the squares. The average of readings in the four quad- 

 rants should be taken. If this is done, errors due to distortion 

 by the tube nicol and to lack of precise perpendicularity of the 

 plate to the optic axis are largely compensated. If the plate 

 lacks 5° of being normal to the optic axis no appreciable error 

 will result. 



As shown by measurements on several plates of arsenic tri io- 

 dide and calcite the angle (3 can be found within about 0.2°. 

 The corresponding calculated value of w — e is in error 2 per 

 cent when /S = 10°, 1 per cent when fS = 20°, and tV per cent 

 when /3 =35°. 



PHYSICS. — Measurernents of refractive indices on the principal 

 optical sections of hirefracting minerals in convergent polarized 

 light. Fred. Eugene Wright, Geophysical Laboratory. 



The principal optical sections of birefracting minerals play 

 an important role in petrographic microscope work. They are 

 parallel to the planes of symmetry of the optic ellipsoid and 

 exhibit, in convergent polarized light, characteristic interference 

 figures. On any given principal optical section two of the three 

 principal refractive indices of the mineral can be measured 

 directly by the immersion method. The third principal re- 

 fractive index of the mineral can also be determined by use of 

 the phenomena observed on the same section in convergent 

 polarized light. The methods available for this determination 

 involve some computation and have not been used to any extent 

 by petrologists; but, in exceptional cases, especially on thin 

 tabular crystals, the usual methods may prove inadequate and 

 the methods outlined below may then be used to advantage to 

 obtain accurate results. The usefulness of such formulas has 



