278 ME. A. E. TUTTOX OX A COMPARATIVE CRYSTALLOGRAPHICAL STUDY 
Comj^arative Table of Ptefractive Indices. 
KMg 
RbMg 
CsMg 
Index. 
Light. 
selenate. 
selenate. 
selenate. 
a 
' Li 
1-49.30 
1-4978 
1-5143 
Vibrations 
C 
1-4941 
1-4983 
1-5148 
parallel 
Xa 
1-4969 
1-5011 
1-5178 
second median 
T1 
1-4999 
1-5041 
1-5210 
line. 
F 
1 - 5035 
1-5077 
1-5248 
G 
1-5091 
1-5133 
1-5304 
li 
! Li 
1-4958 
1-4997 
1-5145 
Vibrations 
C 
1-4963 
1-5002 
1-5150 
parallel 
Xa 
1-4991 
1-5031 
1-5179 
symmetry 
T1 
1-5022 
1-5060 
1-5211 
axis h. 
F 
1-5058 
1-5098 
1-5248 
G 
\ 
1-5114 
1-5152 
1-5305'' 
7 
Li 
1-5103 
1-5100 
1-5201 
Vibrations 
C 
1-5108 
1-5105 
1-5206 
parallel 
Na 
1-5139 
1-5135 
1-5236 
first median 
T1 
1-5172 
1-5167 
1-5269 
line. 
F 
1-5210 
1-5205 
1-5308 
G 
1-5200 
1-5264 
1-5364 
V 
Mean Pefractive Indices of the Three Salts, 
(a + + y) tor Na. 
KMg selenate 
PhMg „ 
CsMg 
1-5033 
1-5059 
1-5198 
Did: 26 
Ditf. 139 
The mean refractive indices of KMg, RhMg, and CsMg sulphates are respectively 
1-4664, 1-4713, and 1-4877. The diflerences are 49 and 164, greater than in the 
selenate grou}). 
The mean refractive index of nihldlum magnesium selenate is thus seen to he 
intermediate between the mean indices of potassium and ctesium magnesium 
selenates; it lies, however, much nearer to tliat of the potassium salt than to that of 
cEesium magnesium selenate, as has been found general in the double sulphate series. 
The proportion of the differences for the two replacements is 1 : 5 for this group of 
double selenates, as against 1 ; 4 for the double sulphates containing magnesium. 
* The vibrations for G4ight in the case of CsMg selenate occur in the symmetry plane. 
