THE PROPERTIES OF MINERALS. 687 



axes; consequently the optic axes must lie in one of the three piuacoidal 

 phiues, and syniinetrical dispersion takes phice in this i)lane. The 

 model shows the dispersion in this case. The position of the axes of 

 elasticity is indicated by the white threads, that of the optic axes by 

 the red and blue threads. The vertical axis is the acute bisectrix, and 

 red (/j) is greater than violet (p); hence /j>r. 



Monorlinic flispersion — In monoclinic crystals one of the axes of elas- 

 ticity corresponds in position to the crystallograi)hic axis h, and the 

 other two lie in a plane of symmetry at right angles to it. Conse- 

 quently there are three cases of dispersion possible, depending upon 

 which two of the three axes lie in the plane of symmetry. These 

 kinds of dispersion are: inclined, when the plane of the optic axes is 

 the symmetry plane of crystal, in which case unsymmetrical dispersion 

 of the axes and bisectrices takes j)lace; horizontal, when the plane of 

 the optic axes is at right angles to the plane of symmetry and the 

 acute bisectrix and axis of mean elasticity lie in this plane; crossed, 

 when the plane of the optic axes is at right angles to the plane of sym- 

 metry and the acute bisectrix corresponds to the crystallographic axis 

 6, so that the obtuse bisectrix and the axis of mean elasticity lie in 

 the idane of symmetry. The three models following show the several 

 kinds of monoclinic dispersion, and are described as follows: 



Inclined. — The dispersion in this case is unsymmetrical. The posi- 

 tion of the optic axes is indicated by red and blue threads. (Ireenish- 

 yellow is the acute bisectrix for red and orange-yellow for blue. The 

 dark-green thread is the obtuse bisectrix for red and grass-green for 

 blue. The crystallographic axes are in white, axis h corresponding 

 to that of mean elasticity. 



Horizontal. — In this case the dispersion is symmetrical. The posi- 

 tion of the optic axes is shown by the red and blue threads. Orange 

 yellow is the acute bisectrix for red and greenish-yellow for blue. The 

 crystallographic axes are in white, with axis h the obtuse bisectrix and 

 not dispersed. 



Crossed. — The dispersion in this case is symmetrical. The position 

 of the optic axis is shown by the red and blue threads. Grass-green 

 is the obtuse bisectrix for red and dark-green for blue. The crystallo- 

 graphic axes are in white, with axis h as the acute bisectrix and not 

 dispersed, 



Triclinic dispersion. — In triclinic crystals none of the axes of elas- 

 ticity correspond to the crystallographic axes; consequently they have 

 no fixed i)osition. Complete unsymmetrical dispersion of the optic 

 axes, their plane, and of the bisectrices takes place. 



The model shows the unsymmetrical dispersion. The position of the 

 optic axes is indicated by the red and blue threads. Orange-yellow is 

 the acute bisectrix for red and greenish-yellow for blue. The grass- 

 green thread is the obtuse bisectrix for red, dark-green for blue. The 

 crystallographic axes are in white. 



