Optic axial angle

The optic axial angle 2F is related to the three refractive indices by the expression... 

If three of these quantities are known, the fourth can be calculated. Thus, when it is possible to measure all three principal refractive indices, the measurement of the optic axial angle is, strictly speaking, superfluous. But in some eases it may be possible to measure only two of... 

The value of measurements of the magnitude of the optic axial angle has been urged by Bryant (1932) and where dispersion of the optic axes occurs, the variation of the optic axial angle with light frequency is a highly characteristic feature which is valuable evidence for identity. (See Bryant, 1941, 1943.)... 

The crystals have density 3-60 at 19° C.4 According to Krenner,5 the sublimed crystals have axial ratios a b c =0-58787 1 0-88258 the optic axial angle 2Ha = 108° 46 and the optical character is positive. 

Figure 4. A practical distinction between optically uniaxial and optically biaxial drawn (or extruded) material. For optically uniaxial material, the area fraction exhibiting extinction between crossed circular polars is greatest when the normal to the plane of the thin section is parallel to the draw axis. For optically biaxial material, the greatest area fraction is observed in a section cut so that the angle between its normal and the draw axis is equal to half the optic axial angle of a monodomain.

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