Abstract
The optical dispersion phenomena of felspars at the alkalic and calic ends of the plagioclase series have been investigated by C. Viola and S. Kôzu, but for those of intermediate composition the data have been lacking. In view of the usefulness of the dispersion data of felspars for their determination in cleavage-flakes by the method to be described in the next paper, the present writer has made a study of the dispersion of certain intermediate plagioelases.

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