O. N. Rood’s Stauroscopic Observations. 391 
5. TRICLINIC SYSTEM. 
The cross appears through every plane inclined at a certain 
angle, when any one of the planes or corresponding edges of the / 
erystal is placed horizontal or vertical on the engraved stage. 
Examples: Kyanite, albite, sulphate of copper. 
Art, XLIV.—Stauroscopice and other Optical Experiments; by 
rof. OapEN N. Roop.—Part I. 
1. Stauroscopic Observations on Cooled Glasses. 
d 
1. Through the centre of the equilateral triangle the cross ar- 
Tanges itself in three positions, at right angles to its three sid 8; 
€ cross is of course not stationary when the triangular plate is 
revolved. The same is true when the polarized beam is trans- 
iuitted through three spaces near the middle of its sides. 
Through the angles of the triangle the cross is inclined to the 
above mentioned planes, os ES ay 
T found no spot in the triangle where the cross remained unal- ee 
tered when the glass plate was revolved. OY taal 
hie rough the centre of a cit- A. 
ar pla glass the cross re- 
oe tolerably unaltered by rev- 
Suton, this position of the plate 
oy 8 ae to the basal 
; & crysta i e 
dimettic « ‘lag belonging to th 
ugh the spaces 0, 0, 0, 0, the 
on are altered by revolution 
ais arange themselves in the po- 
ons of the dotted lines. Seen 
Tough the spaces marked with 
8 the crosses arranged them- 
“ives at these angles to Se dotted lines. 
wacee-------4 
60° 
- Bat 
