264 ME. A. E. TUTTOX ON A COMPAEATIYE CRYSTALLOGEAPHICAL STL'DY 
Refractive Indices of Rubidium Magnesium Selenate. 
f 
Vibrations parallel ; 
2 nd median line I 
. ^ 
Vibrations parallel 
sjmimetry axis h 
y 
Vibrations parallel 
1 st median line 
LiAit. ’ 
O 
Prism 1. 
Prism 2. 
1 
Prism 3. 
Prism 4. 
! 
Prism 5. 
Prism 6. 
1 
Mean. 
Li 
1-497.5 
1-4979 
1-4981 
1-4977 
1-4978 
C 
1-4981 
— 
— 
1-4984 
1 - 4985 
1-4984 
1-4983 
Na 
1-5008 
■ - 
1-5011 
1-.501.3 
1-5011 
1-5011 
T1 
1-50.39 
— 
— 
1-5041 
1-5043 
1-5042 
1-5041 
F 
1-5076 
— 
-- 
1-5077 
1-.5079 
1-5076 
1-5077 
G 
1-5131 
— 
— 
1-5132 
1-5135 
1-5132 
1-5133 
Li 
1-4994 
1-4997 
1-4997 
_ 
1-5000 
_ 
1-4997 
C 
1-4999 
1-5002 
1-5002 
— 
1 - 5003 
— 
1-5002 
Na 
1-5029 
1 - 5032 
1-5031 
— 
1-5033 
— 
1-5031 
T1 
1-5059 
1 -5059 
1-5061 
— 
1-5062 
— 
1-5060 
F 
1-5097 
1-5096 
1-5102 
— 
1-5096 
— 
1-5098 
G 
1-5151 
1-5150 
1-5155 
1-5153 
— 
1-5152 
Li 
1 - 5098 
1-5101 
1-5100 
_ 
1-5100 
1-5100 
C 
— 
1-5103 
1-5107 
1-5105 
— 
1-5106 
1-5105 
Na 
— 
1-5133 
1-5135 
1-5135 
— 
1-5137 
1-5135 
T1 
— 
1-5165 
1-5168 
1-5167 
— 
1 -5167 
1-5167 
F 
— 
1-5203 
1-5207 
1-5204 
— 
1-5204 
1-5205 
Gr 
— 
1-5262 
1-5266 
1-5265 
— 
1-5263 
1-5264 ' 
1 
The mean refractive index (mean of all three indices) for sodium light is 1’5059. 
The intermediate refractive index ^ is represented accurately to near F of the 
spectrum by the following formula :— 
/3 = 1-4857 + 
739 403 4 215 900 000 000 
+ 
The a indices are nearly reproduced by the formula if the constant 1-4857 is 
diminished by 0-0019, and the y indices if the constant is increased by 0-0105. The 
indices thus yielded by the formuhe are corrected for a vacuum, being 0-0004 higher 
than those given in the table, which are not so corrected. 
Alteration of Refraction hy Rise of Temperature. —Determinations of refractive 
index at 75° indicated that the indices ])ecome reduced by 0-0019 by a rise of 60° in 
temperature. 
Axes of the OjAical JfUipsoid.—These are as follows :— 
Axes of optical indicatrix : 
a\ (B -.y ^ 0-9987 : 1 : 1-0069. 
Axes of optical velocity ellipsoid : 
a : li : r = 1-0013 : 1 : 0-9931. 
Molecular Optical Constants. —The calculated values of these constants are the 
folloAA-ino' :— 
