MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
415 
The direction is behind the normal, towards the vertical axis c. It is the second 
median line and corresponds to the a index. As the angle between the normal to 
c {001} and the axis c is 14° 57' (the angle ac = (100) : (001) = 7 5° 3'), this second 
median line is inclined 13° 37' to (in front of) the axis c. The first median line is 
consequently inclined 1° 20' to (above) the axis a, and lies 13° 37' below the normal 
to a (100). Both median lines are in the obtuse angle of the crystal axes ac. The 
conditions are clearly shown in the diagram given in fig. 5. 
n 
Optic Axial Angle .—Three pairs of section-plates were ground, perpendicular 
to the first and second median lines, and afforded the values for 2E, 2H a and 2H 0 
recorded in the two following tables. The angle in air 2E is very large, and only 
Plates 1 and 3 of the three perpendicular to the first median line afforded it 
satisfactorily clearly. Monobromonaphthalene was the immersion liquid employed for 
the measurements of 2H a and 2H 0 , for which very clear and suitable interference 
figures were obtained. 
Apparent Optic Axial Angle in Air, 2E, of RbFe Selenate. 
Light. 
Plate 1. 
Plate 3. 
Mean 2E. 
Li. 
o f 
130 18 
o / 
130 35 
130 27 
C. 
130 23 
130 40 
130 32 
Na. 
131 3 
131 7 
131 5 
T1. 
131 34 
131 41 
131 38 
Cd. 
131 59 
132 1 
132 0 
F. 
132 36 
132 30 
132 33 
