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1873 J. Tyndall Six Lect. on Light iv. 133 This brings us at once to the part played by the analyzer, the sole function of which is to recompound the two vibrations emergent from the gypsum. It reduces them to a single plane, where, if one of them be retarded by the proper amount, extinction will occur.

1912 R. W. Clark tr. E. Weinschenk Petrogr. Methods 67 Maximum brightness is obtained when the vibration directions of the crystal are at 45° to those of the nicols. It diminishes upon further rotation and passes gradually over into complete darkness when these directions are respectively parallel. This latter position is also called the position of extinction, and the vibration directions in the crystal, the extinction directions.

1912 R. W. Clark tr. E. Weinschenk Petrogr. Methods 70 Extinction Curve for Diopside.

1912 R. W. Clark tr. E. Weinschenk Petrogr. Methods 70 In triclinic minerals the determination of the extinction angles is of value only when the orientation of the face upon which they are observed is accurately known.

1921 A. Holmes Petrogr. Methods 126 When a transparent mineral fragment is rotated between crossed nicols various phenomena may be observed... If the object remains dark, and is therefore isotropic, the total extinction indicates an amorphous or cubic (isometric) mineral in an unstrained condition, or a basal section of a uniaxial mineral (tetragonal, hexagonal, or trigonal).

1937 Discovery Sept. 283/2 A complete record of the colour variation and the angles of extinction can be permanently obtained.