(2) ▄Biaxial▄, or those in which the optical characters are no longer symmetrical to an optic axis but to three planes at right angles to each other (for monochromatic light). These crystals have, however (for light of each wave-length and for each temperature), _two_ directions parallel to which there is a single value only for the light velocity and hence no double refraction. These directions are called “_optic axes_.”[14] An investigation of these biaxial crystals shows that of all the rays traversing these crystals there are three rays which advance with maximum, minimum and some intermediate velocity. The vibration directions of these three rays are called the _principal vibration directions_ and are at right angles to each other (being the intersections of the three planes above referred to). The direction of ether vibration of the fastest ray is denoted by a, of the slowest ray by c, and of the ray advancing with intermediate velocity by b.[15] Each of the three planes, containing two principal vibration directions, is called an _optical principal section_. The index of refraction of the a ray is denoted by α, of the b ray by β, and of the c ray by γ. To this group belong all crystals in the _Orthorhombic_, _Monoclinic_ and _Triclinic_ systems. In the _Orthorhombic_ system, the principal vibration directions are parallel to the crystallographic axes; hence all pinacoidal sections contain two of these principal vibration directions. In all sections parallel to the three crystallographic axes _ă_, _ƃ_ and _ć_, the vibration directions are parallel or symmetrical to cleavage cracks, crystal edges, etc. In the _Monoclinic_ system, one principal vibration direction is parallel to the ortho axis _ƃ_, the other principal vibration directions are in the plane of symmetry, at right angles to _ƃ_, but are not parallel with either the vertical axis _ć_ or the clino axis _á_. In clino pinacoid (010) sections the principal vibration directions will make definite angles with crystallographic lines, such as cleavages or crystal outlines. These angles are called _extinction angles_. They will vary, in this system, with reference to the direction of the _ć_ axis from a maximum on the clino pinacoid (010) to 0° on the ortho pinacoid (100), when the vibration directions of the two doubly refracted rays will be parallel and at right angles to the plane of symmetry. Hence the vibration directions are parallel or symmetrical to cleavages, edges, etc., _only_ in sections parallel to the ortho axis _ƃ_; but in all other sections are unsymmetrical. In the _Triclinic_ system, the principal vibration directions are not parallel to the crystallographic axes, and there is no definite relation between these directions and the crystallographic axes; hence in all possible sections there will be extinction angles.