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THE universal stage is beginning to receive in America the attention which is due so powerful an instrument. This growing interest of American petrographers has inspired the discussion offered here since at the time of writing there is no unified account in English of the uses and varied applications of the instrument. I am limiting my subject matter to the bare essentials of theory and I am emphasizing the practical application of the universal stage to everyday petrographic problems. Further I am treating only the universal stage with five instead of four axes of rotation in recognition of the evident reluctance of the average American petrographer to use the stage with four axes of rotation except in the limited study of uniaxial crystals.

Although the better modern petrographic microscopes do not contain Nicol prisms, the term has become virtually synonymous with polarizing prism. Accordingly, the polarizing prisms are referred to throughout as “nicols”—“polarizer” or “analyzer.”

I assume a knowledge such as the geologist possesses after an elementary course in petrography. However, since the principles of crystal optics as ordinarily taught do not stress those aspects of the subject on which universal stage procedure is mainly based, there is included here a brief review (Chapter 1) of those principles most needed in this work.

Search for liquids to be used in the double variation procedure has been spread over 12 years. Many liquids have been tried for a year or two in the student mineral laboratory at the University of

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