260 ME. A. E. TUTTON OX A COMPAEATIVE CRYSTALLOGEAPHICAL STUDY 
Determination of Apparent Angle in Air of Potassium Magnesium Selenate. 
Light. 
Section 1. 
Section 2. 
Section 3. 
Mean 2E. 
Li 
61° 
12' 
61° 
3' 
61° 
18' 
61° 
11' 
C 
61 
11 
61 
2 
61 
17 
61 
10 
Xa 
61 
7 
60 
59 
61 
14 
61 
7 
T1 
61 
2 
60 
55 
61 
11 
61 
3 
F 
60 
55 
60 
49 
61 
7 
60 
57 
Determination of True Optic Axial Angle of Potassium Magnesium Selenate. 
Light. 
Xo. of 
section 
perp. 1st 
median line. 
Observed 
values 
of 2Hfl. 
Xo. of 
section 
perp. 2nd 
median line. 
Observed 
values 
of 2 Ho. 
Calculated 
values 
of 2Yrt. 
i\Iean value 
of 2Ya. 
1 
"1 
35° 
54' 
la 
118° 
17' 
39° 
30' V 
Li. . 
2 
35 
52 
•2a 
116 
33 
39 
48 
> 39° 4.3' 
1 
-3 
35 
58 
3a 
116 
45 
39 
52 J 
1 
-1 
35 
52 
la 
118 
12 
39 
29 ^ 
C . . 
2 
35 
49 
2 a 
116 
29 
39 
46 
> 39 42 
1 
-3 
35 
56 
3a 
116 
42 
39 
50 J 
fl 
35 
43 
la 
117 
41 
39 
26 1 
1 
Xa . 
• 
35 
40 
2 a 
116 
0 
39 
42 
)■ 39 38 
^3 
35 
44 
3a 
116 
10 
39 
45 
1 
fl 
35 
32 
la 
117 
5 
39 
22 ' 
1 
Tl. . 
. 
1 ^ 
35 
28 
2 a 
115 
24 
39 
38 
39 34 
35 
34 
3a 
115 
30 
39 
42 . 
1 
fl 
35 
15 
Irt 
116 
20 
39 
14 ' 
1 
F . . 
. 
35 
10 
2«. 
114 
40 
39 
29 
> 39 25 
u 
35 
14 
3a 
114 
45 
39 
32 - 
1 
ToI’SOE 
and 
CriRISTIANSEN 
obtained for 
the angle in 
air G 2 ° 
12', 
and for the tr 
angle 40° 22', both referring to sodium light. 
The Dispersion of the Median Lines was investigated by immersion in benzene, 
whose mean refractive index is approximately the same as that of the crystals. It 
proved to be exceedingly small, not exceeding 5' between F and C, and while the 
largely preponderating number of determinations gave the indication that the first 
median line lies nearer to the morphological axis a for blue than for red, the amount 
is really so small as to lie within the limits of experimental error. 
Effect of Rise of Temperature on the Optic Axial Angle. —IMeasurements at 80° 
showed that the angle in air increases 3° 10' for 60° rise of temperature. 
