OF THE HOUPLE SELEXATES OF THE SERIES R .H(Se0,),,6H,0. 
2(\5 
Axis of optical indicatrix 
a. 
yS. 
7- 
Specific refraction, , , 
1 
= „ . .i 
re 
0-1094 
0-1098 
0-1117 
^ {n~ + ; 
i)d 
I 
-G 
0-1122 
0-1125 
0-1146 
_ 
Molecular refraction, , 
■ 1 
M I 
— = ni . i 
iC 
64-33 
64-54 
■65-66 
'A -f 
. 2 
d 1 
iG 
65-96 
66-17 
67-38 
Specific dispersion, Ug — 
lie 
. 
0-0028 
0-0027 
0-0029 
Molecular dispersion, hIg 
— 1 
lUc • • • 
1-63 
1'63 
1-72 
Molecular' refraction, 
a 
-0 
1 . . . 
c 
109-31 
109-72 
111-99 
Optic Axial Angle .—Three pairs of excellent section plates, gronnd parallel to the 
first and second median lines, afforded the following values ;— 
Apparent Angle in Air of Itubidium Magnesium Selenate. 
Light. 
Section 1. 
Section 2. 
Section 3. 
Mean 2E 
Li . 
... 74° 
3' 
74° 
15' 
74° 
r 
74° 
6' 
C . 
. . . 73 
58 
74 
9 
73 
52 
74 
0 
Xa . 
... 73 
34 
73 
40 
73 
29 
73 
34 
T1 . 
... 73 
9 
73 
10 
73 
0 
73 
6 
F . 
... 72 
22 
72 
34 
■ 72 
27 
72 
28 
Determination of true Optic Axial Angle of Rubidium Magnesium Selenate. 
Light. 
Li . 
Xa . 
T1 . 
F . . 
Xo. of 
section 
perp. 1st 
median line. 
r 1 
Observed 
values 
of 2H./. 
42 ^ 
42 
42 
42 
42 
41 
41 
41 
41 
41 
00 
50 
47 
42 49 
42 47 
42 46 
42 18 
19 
20 
46 
49 
41 44 
0 
7 
5 
Xo. of 
section 
pei'p. 2nd 
median line. 
in 
•2a ■ 
3a 
la 
■2a 
3a. 
la. 
2a 
3a 
la 
2 a 
3a 
la 
2a 
3a 
Observed 
values 
of 2Ha. 
112 ° 
112 
112 
112 
112 
112 
112 
112 
112 
111 
111 
111 
111 
111 
111 
29' 
29 
33 
25 
26 
32 
6 
6 
4 
40 
36 
41 
5 
1 
7 
Calculated 
values 
of 2Va. 
46 37 
46 40 
46 35 
46 
46 
46 
Mean value 
of 2Va. 
47“ 26' 
47 24 
47 3 
46 37 
46 6 
All examination of the optic figures in white light when the section perpendicular 
to the first median line is immersed in benzene, whose refraction is almost identical 
VOL. CXCVII.—A. 2 M 
