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"A biaxial interference figure showing the emergence of an optic axis. The arrow indicates the movement taking place upon inserting a quartz wedge above the section. The mineral is augite. (+)." -Johannsen, 1908

Biaxial Interference

"A biaxial interference figure showing the emergence of an optic axis. The arrow indicates the movement…

"Determination of the optical character of biaxial crystals in sections showing the emergence of an optic axis by means of the gypsum plate." -Johannsen, 1908

Gypsum Plate

"Determination of the optical character of biaxial crystals in sections showing the emergence of an…