vauxite the range is from 1.551 for light travelling parallel to the a axis to 1.562 for light travelling parallel to the c axis. An optic axis is a direction in which light travels through a crystal such that the speed is the same for all directions of polarization for light of any given wavelength (i.e. color). Any direction in an isometric crystal has this property. Trigonal, tetragonal and hexagonal crystals have a single optic axis, parallel to the c crystal axis. They are said to be uniaxial. Triclinic, monoclinic and orthorhombic crystals have two optic axes, and are said to be biaxial. The angle between the two axes is denoted by 2V. Vauxite is biaxial. Optic sign Unpolarized light travels unchanged through an isometric crystal, whatever the direction of travel. In uniaxial and biaxial crystals, light travelling in any direction other than parallel to an optic axis is broken into two polarized rays, the ordinary ray and the extraordinary ray. The ordinary ray travels with the same speed no matter what the direction; this is a consequence of the plane in which it is polarized. The plane of polarization of the extraordinary ray is perpendicular to that of the ordinary ray, and in general its speed will be different. For rays travelling along an optic axis the speeds of the ordinary and extraordinary rays are equal. For all other directions in uniaxial and biaxial crystals the speeds are different. The crystal is said to be positive if the ordinary ray has a greater speed than the extraordinary ray, and negative if the reverse is true. Vauxite is biaxial (+). Dispersion of the optic axes The refractive index varies with the wavelength (color) of light, so the positions of the optic axes in biaxial crystals, and the angle 2V between them, will change when the color of the incident light is changed. This phenomenon is usually expressed in the form r > v, indicating that 2V is greater for red than for violet light, or vice versa. For vauxite r > v, 2V is greater for red light than for violet light. Pleochroism Pleochroism is the phenomenon of crystals appearing to change color as they are rotated in plane polarized light. This is due to differential absorption of light vibrating in different directions. Isometric crystals cannot be pleochroic. Uniaxial crystals (trigonal, tetragonal or hexagonal) may show two, but not three,