Reed Ke 
390 Prof. von Kobell’s Stauroscopic Observations. 
(8.) Through the faces of the scalenohedron the cross arran 
itself according to the lines of altitude of the sides of its holo- 
hedral dihexagonal pyramid, or at right angles to the sides of 
its horizontal twelve-sided transverse section. 
(4.) Through all the prismatic sides the cross is seen normal 
in the direction of the principal axis. 
(5.) Through the basal plane the cross is seen normal and un 
changed by the revolution of the crystal. 
Examples: Apatite, quartz, calcite, chabazite, emerald. 
Systems with two optic ames. 
In these systems there occur no planes through which the nor- 
mal cross remains unchanged during the revolution of the crystal. 
8. TRIMETRIC SYSTEM, 
(1.) Seen through a plane of the rhombic pyramid the cross 
makes three angles with the three sides, corresponding to the in- 
equality of the sides of the triangle. 
(2.) Through the prismatic faces and also through the macro- 
and brachy-diagonal planes the cross is in the direction of the 
‘principal axis, also through the domes it is in the direction 
the dome edge. : 
(3.) Through the basal plane when it is rhombic the cross 
stands in the direction of the diagonals, when it is rectangular in 
the direction of its sides. WR 
__ By revolving the crystal the cross becomes pale or is alte 
s . 
_Hxamples: Heavy spar, topaz, sulphate of magnesia, atago 
_hite, chrysolite. 
4, CLINO-RHOMBIC OR MONOCLINIC SYSTEM. 
clinodiagonal chief section to the right or left with equal angi 
when seen respectively through the back or front sides 
stal 
(2.) Through the orthodiagonal plane the cross is normal 19 
the direction of the principal axis. - «actined to 
(3.) Through the clinodiagonal plane the cross is , 
the principal axis. pages PP 
_ (4) Through the basal plane of the rhombic prism the 
stands according to the dia ea 
Zor ate . 
Examples: Diopside, selenite, orthoclase, epidote, borax- 
