Axial Angle Apparatus click images for more information Axial angle apparatus Instrument for the determination of the optic axial angle (2V). The instrument was used to measure the apparent angle made by the optic axes of a biaxial crystal in air (2E) or in oil (2H). The true angle between the optic axes (2V) may be obtained by correcting the measurement for the refractive index (β) of the crystal. The measurement is done on a crystal section cut at right angles to the bisectrix. Lacquered brass, nickel plated brass and oxidized brass. Graduated circle diameter: 16 cm, height: 32 cm, total length: 33 cm. Signed: "R. Fuess Berlin". First half 20th century. Axial angle apparatus Axial angle apparatus, model II, signed: "R.Fuess Berlin-Steglitz". Lacquered brass, nickel plated brass and oxidized brass. Graduated circle diameter: 16 cm, height: 32 cm. Circa 1900.