Prof. von Kobell’s Stauroscopic Observations. 389 
If the cross is seen unchanged in its position then the polar- 
ized rays of the crystal vibrate in the direction of the side of its 
plane and at right angles to it, but if no cross should appear, or 
if it should be changed in its position, it is a proof that the rays 
do not vibrate in the direction of the edge of the crystal Big 
under observation, and it is necessary to turn the cylinder to 
which the crystal is attached a certain fixed number of degrees 
till this happens and the cross again appears in its upright nor- 
mal position; the angle is read off by means of the vernier. In 
this manner it is possible to obtain through the stauroscope cer- 
tain characteristic optical distinctions for the different crystalline 
systems, by which they often may be determined when other 
means fail, — 
I. System of simply refracting crystals. 
MONOMETRIC SYSTEM. 
Monometric crystals show, in every position in which they can 
put on the stage, the cross normal; however the stage may 
be turned it remains unchanged. ; : 
Examples: Rock salt, alum, spinel, fluor. Amorphous pieces 
behave in the same manner. “a 
II. Systems of doubly refracting crystals. 
Systems with one optic axis. : 
1. DIMETRIC SYSTEM. : 
(l.) Seen through a plane of the quadratic pyramid the cross 
aranges itself according to the verticals of the triangle or at 
nght angles to each of its sides. ee oy 
(2) Through the prismatic faces the cross has the position 
Principal axis, jet 
(@.) Through the basal plane the cross appears normal ‘a 
Temains unchanged when the crystal is revolved. sai 
Examples : Apophyllite, idocrase, zircon, scapolite. 
2, HEXAGONAL SYSTEM. 
(l.) Through a plane of the hexagonal pyramid the cross 
Pong in the directions of the verticals of the triangle or at 
“eit angles to each of its sides. C 
iste Through the sides of the rhombohedron the CTOSS ‘jipecmared 
. 
In the directions of the diagonals. ; 
