208 



It is not necessary that the angle between the acute bisectrix 

 and the vertical axis should be constant, and it is constant 

 only for individual species when chemically pure. The value of 

 this angle will change with the composition of the mineral, and 

 with the wave length of light for the same composition. In 

 common hornblende this angle is 19 53' in the obtuse angle P. 

 Expressed, Bx aA C = 19 53' in front; this is also a measure of 

 the extinction angle, which is inclined. If monochromatic light 

 of different wave lengths is used, it will be found that this angle 

 will vary with the color of light used. The interference figure as 

 a whole is displaced, and that of one color will not be superimposed 

 on that of another, yet the trace of the plane of the optic axes 



will divide them all 

 symmetrically. This 

 is termed inclined dis- 

 persion, Fig. 358. 



2. When the ortho- 

 axis is the obtuse bi- 

 sectrix, the plane of 

 the optic axis may ro- 

 tate around it as an 

 axis of revolution; and 

 the interference figure 

 for each wave length 

 of light which is in 

 the section perpendic- 

 ular to the acute bi- 

 sectrix may be dis- 

 placed sidewise through an arc measured in the plane of symmetry. 

 The trace of the plane of the optic axes will also be displaced 

 through this same angle for each wave length. The traces of 

 the planes for each color will lie parallel on the section, but the 

 planes will all intersect in the obtuse bisectrix or orthoaxis, which 

 is fixed, Fig. 359. This is termed horizontal dispersion. 



3. When the acute bisectrix coincides with the orthoaxis. 

 Now the interference figure will lie in the clinopinacoidal section, 

 and will revolve around the acute bisectrix as a center. The 

 traces of the plane of the optic axes for light of the various colors 

 will all pass through the fixed point, the center or acute bisec- 

 trix, Fig. 360. This is termed crossed dispersion. 



Dispersion in the triclinic system. In the triclinic system, 



FIG. 359. Horizontal Dispersion. 



