More than 100 measurements of 2V were made on forsteritic olivines from two picrite sills in Soay, Scotland, using both double and single axis conoscopic methods. The mean of double axis measurements agrees closely with the mean of single axis measurements, but a significant difference exists between the means of double axis measurements made over different bisectrices. Measurements of (–)2Vα are smaller than (–)2Vγ by about 2.5°. The difference can be explained if the recorded angles are smaller than the true angles when high radial angles are measured on the universal stage, and calculations show that refraction and displacement of light within the central layers of the sphere could produce this effect. Optical measurements of minerals with 2V near 90° may be unreliable unless corrections are made for the errors introduced by the high angles of tilt which are involved in the measurements.

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