OPTICAL PROPERTIES "I < RYSTALS 



207 



-uppre--ed along I he concave .-ide of the hyperbola-, the -ide far- 

 thest away from the acute bi-ectrix, and the color appearing will be 

 \\hite light minu- red, or blue. ( )n the convex side of the hyper- 

 bola- will be white light minus violet, or red. 



The convex side is red when p^-u, and the convex side is blue 

 when u>p. In the orthorhombic system the dispersion of the optic 

 acee take- place in one of the planes of symmetry of the crystal, 

 and the acute bisectrix holds the same position for light of all 

 color-, except when 2 E is nearly 90. 2E for red may be less 

 than 1)0 with the vertical axis c the acute bisectrix, and greater 

 than 90 for violet when one of the lateral axes would be the acute 

 bisectrix for the violet wave. Again the plane of the optic axis 

 may change from one pinacoidal plane to another with the wave 

 length, as is the case in brookite, in which the plane of the optic 

 for waves including red to yellow is parallel to the base, 

 with the hrachyaxis as the acute bisectrix; for waves shorter 

 than yellow the plane of the optic axis is parallel to oio, or macro- 

 pinacoid, with the brachyaxis still the acute bisectrix. In such 

 a case the plane of the optic axis is said to be crossed, or the dis- 

 persion is crossed, and there must be a wave length of light a little 

 shorter than yellow for 

 which the angle between 

 the optic axes is 90, or 

 for which the mineral 

 brookite would be uni- 

 axial. 



Dispersion in the mono- 

 clinic system. In the 

 monoclinic system, where 

 hut one of -the axes of the 

 indicatrix is fixed in rela- 

 tion to the crystallograph- 

 ical axes, that in the di- 

 rection of the orthoaxis, 

 three kinds of dispersion 

 are possible, as the three 

 axes of the indicatrix, a, |J, 



y, each in turn may be parallel to the orthoaxis. 1. When p 

 is parallel to the orthoaxis, the plane of the optic axes will 

 lie in the plane of symmetry of the crystal and will be fixed at 

 90 to the orthoaxis, but may revolve around it as an axis. 



FKI. 358. Inclined Dispersion of the Optic 



