MONOCLINIC DOUBLE SELENATES OF THE NICKEL GROUP. 
203 
the second median line, the first median line being at 90° thereto, as shown in the 
accompanying fig. 2. 
As the normal to c {001} lies 14° 2 7' in front of the vertical axis c (the 
morphological axial angle ac — 75° 33'), the second median line lies 6° 59' in front of 
the vertical morphological axis c. The first median line lies to the same extent 
(6 59') below the normal to a {100} and 7° 28' above the inclined morphological axis a. 
Very similar positions for the median lines were obtained by Tops0E and 
Christiansen, # that for the first median line being given as 6° 57' above the axis a. 
Optic Axial Angle .—Three excellent pairs of section-plates, perpendicular to the 
first (numbered 1, 2, 3) and second (numbered la, 2a, 3a) median lines, were prepared, 
yielding perfect interference figures in-convergent polarised light. They afforded the 
measurements of 2E (the apparent optic axial angle in air) and of 2H a and 2H 0 (the 
apparent acute and obtuse angles in monobromonaphthalene) recorded in the two 
following tables ; the true optic axial angles calculated from 2H a and 2H„ are given 
in the penultimate column, and the mean value of the true angle in the last column. 
Apparent Optic Axial Angle in Air, 2E, of KNi Selenate. 
Light. 
Plate 1. 
Plate 2. 
Plate 3. 
Mean 2E. 
Li . 7 . 
o • 
129 33 
O 1 
128 40 
o f 
129 2 4 
o / 
129 12 
C. 
129 38 
128 45 
129 29 
129 17 
Na. 
130 16 
129 36 
130 17 
130 3 
T1. 
131 14 
130 26 
131 2 
130 54 
Cd. 
131 35 
130 50 
131 34 
131 20 
F. 
132 6 
131 16 
131 55 
131 46 
* ‘ Ann. Chim. Phys.,’ series 5, vol. 1, p. 82 (1874). 
