When the polarizer of a normal petrological microscope is rotated at between 2.25 and, 6.25 revolutions per second with the analyzer in position and a biabsorbant section on the stage, positions of minimum flicker are observed through the eyepiece as the stage is rotated. If at these positions, θ is the angle subtended between the principal vibration axis with maximum transmission in the section and the privileged direction of the analyzer, the biabsorption coefficient of the section is given either by the function λ0 ln tan θ/2πt (if positive) or λ0 ln cot θ/2πt (if negative), where λ0 is the wavelength in a vacuum of the light source used and t the thickness of the section.

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