686 REPORT OF NATIONAL MUSEUM, 1897. 



(letermiiiiug the system in which a mineral crystalHzes. All minerals 

 may be classed as either isotropic or anisotropic with reference to their 

 behavior m light. Anisotropic minerals may be further divided in 

 accordance with their behavior in polarized light, into uniaxial and 

 biaxial. 



Isotropic class. — This class includes all amorphous and isometric min- 

 erals. If a section of such a mineral be examined in polarized light 

 under given conditions, the field of view remains dark and a revolution 

 of -the section in any plane i)roduces no change in appearance. 



Anisotropic class, uniaxial minerals. — Includes all tetragonal and hex- 

 agonal minerals. If a section of such a mineral, cut perpendicular to 

 the vertical axis, be examined in polarized light under given conditions, 

 the field of view remains dark in the center, but round this center is 

 seen a series of concentric colored rings intersected by a dark cross. 

 No alteration takes place when the section is rotated around the normal 

 to its faces. (See models.) 



Anisotropic class, biaxial minerals. — This includes all orthorhombic, 

 monoclinic and triclinic minerals. If a section of such a mineral, cut at 

 right angles to a line bisecting the angle of the optic axes, be examined 

 in polarized light under certain conditions, the field of view remains 

 dark at two points, each of which is the center of a series of colored 

 rings which in one position are intersected by a dark cross. In another 

 position the dark cross is replaced by two dark curved bauds, or 

 brushes, each of which passes through the center of one of the sets of 

 rings. (See models.) 



DISPERSION OF THE OPTIC AXES. 



In all biaxial crystals there are three directions in which light ether 

 is propagated with its maximum, minimum, and mean velocities. These 

 directions are known as the axes of elasticity and are found in directions 

 at right angles to each other. 



The plane of the optic axes must include the axis of greatest and of 

 least elasticity, these two serving as bisectrices for the acute and 

 obtuse angles formed by the intersection of the optic axes. When the 

 acute bisectrix is the angle of least elasticity the crystal is called posi- 

 tive, and when the axis is that of greatest elasticity it is called negative. 



Crystals possessing three axes of elasticity have also three indices 

 of refraction. Furthermore, the direction of the optic axes changes for 

 rays of different colored light, giving a larger optic axial angle for some 

 rays than for others. Such a change in direction is known as the 

 dispersion of the optic axes. 



The two cases possible are distinguished l>y writing p> v when the 

 angle for the red rays (p) is greater than for the violet (f), and /j<j' 

 when the converse is true. 



Orthorhombic dispersion. — In orthorhombic crystals the position of 

 the three axes of elasticity corresponds with that of the crystallographic 



