258 MR. A. E. TUTTOX OX A COMPARATIVE CRYSTALLOGRAPHICAL STUDY 
^[oJecldar Volume.—lUd = 495 6 ^ 2-863 = 209-73. 
Distaucp Ratios .—The molecular volume combined with the axial ratios and axial 
angle }deld the following distance ratios :— 
X : ^ : CO = 6-2233 : 8-3144 : 4-1829. 
Optics. 
Orientfition o f Acres of Optical Ellipsoid. —The symmetry plane is the plane of the 
o|)tic axes (binormals). The sign of the doulde refraction is. positive. 
The follov/ing extinction angles were afforded by two section plates ground parallel 
to the symmetry plane. They are referred to the normal to the basal plane :— 
Section 1, 3° 40'; section 2, 2° 20': Mean, 3° O'. 
Topsoe and fhiRiSTiANSEN give 2^ 0' as the mean of two determinations. 
The direction is behind the normal (that is, nearer to the vertical axis c). This 
direction is the second median line, the first median line being consequently also in the 
ol)tuse angle ac of the morphologicad axes and inclined 3° 0' to the axis a. The 
second median line is inclined 11° 18' to the vertical axis c. 
R<fractive Indices. —The six pi-isms employed were ground on six different crystals. 
The results are given in the accompanying table. 
liefractive Indices of Potassium Mao-nesium Selenate. 
O 
Vibrations } 
parallel ' 
2n(l median 
line 
\ ibrations 
parallel 
■symmetry 
axis h. 
. . t . i 
Vibrations j 
parallel -j 
1st median | 
line. 
nght. 
Prism 1. 
Prism 2. 
Prism 3. 
Prism 4. 
Prism 5. 
Prism 6. 
Mean. 
Values of 
Topsoe and 
Christiax- 
SEX. 
Li 
1 - 4933 
1-4937 
1-4935 
1-4937 
1-4936 
C 
— 
1 - 4938 
— 
1-4944 
1-4941 
1-4942 
1-4941 
— 
Xa 
— 
1 - 4965 
— 
1-4973 
1-4969 
1-4969 
1-4969 
1-4950 
T1 
— 
1-4994 
— 
1-5001 
1-5000 
1-4999 
1-4999 
— 
E 
— 
1-5030 
— 
1-5037 
1-5037 
1-5036 
1-5035 
— 
G 
— 
1-5087 
— 
1-5096 
1-5091 
1-5091 
1-5091 
— 
Li 
1-4967 
1-4950 
1-4957 
1-4957 
1-4958 
_ 
C 
1-4974 
1-4955 
1-4962 
1-4961 
— 
— 
1-4963 
1-4942 
Xa 
1-5001 
1-4984 
1-4989 
1-4989 
— 
— 
1-4991 
1 - 4970 
T1 
1-5031 
1-5013 
1-5021 
1-5021 
— 
— 
1-5022 
— 
F 
1-5070 
1-5050 
1-5057 
1-5056 
— 
— 
1 - 5058 
1-5039 
G 
1-5127 
1-5107 
1-5110 
1-5113 
— 
— 
1-5114 
— 
Li 
1-5108 
_ 
1-5103 
1-5101 
1-5099 
1-5103 
_ 
C 
1-5114 
1-5107 
— 
1-5106 
1-5106 
1-5108 
— 
Xa 
1-5143 
1-5137 
— 
1-5138 
1-5139 
1-5139 
1-5120 
T1 
1-5178 
— 
1 -5169 
— 
1-5170 
1-5170 
1-5172 
— 
F 
1-5216 
— 
1-5208 
— 
1-5208 
1-5208 
1-5210 
— 
G 
1-5272 
-• — 
1-5263 
— 
1-5266 
1-5263 
1-5266 
— 
The mean refractive index (mean of all three indices) for sodium light is 1 • 5033. 
