MR. A. E. TUTTOX OX A COMrAKATIVE CRYSTALLOGRARHICAL STUDY 
length to whicli thi« reading corresponds from the curve prepared in the calibration 
of the instrimient. {Vide ‘ Phil. Trans.,’ A, 1894, p. 925.) When the prism-circle of 
tlie illuminator is further rotated towards the violet, the brushes oj^en out again, and 
separate along the vertical diameter of the held of the polariscope, the ])lane of the 
o])tic axes having crossed from the symmetry plane to a plane at right angles to the 
symmetry 2 )lane. Tliese Ijeautiful changes are illustrated by the lirst six photo¬ 
graphic reproductions given in the accompanying Plate ; they rej^i-esent the inter¬ 
ference figures observed at the ordinary temperature in Li-, Na-, Tl- and F-light, 
light of wave-length 4G6, and G-light respectively. 
The actual measm-ements are recorded in the foliowiim- two tables. The 
phenomena when the section-plate is immersed in the cedar-oil cell are preciselv 
similar to those in air, the angles alhjrded being merely smaller, namely, the true 
angles of inclination of the optic axes within the crystal. 
In both cases the ancle for G-lio-ht was determined with the section rotated 90° 
O O 
in its own plane, so as to bring the new direction of separation horizontal, and there¬ 
fore convenient for the measurement of the angle. 
The determinations of the circle readinn corresponding: to uniaxialitv were made 
repeatedly for each of the two positions of the section, and the mean of the whole is 
the value recorded in the table. 
Determination of Apparent Angle in Air of Ccesium Magnesium 8elenate. 
Plane of optic 
axes. Light. 
Section 1. 
Section 2. 
Section 3. 
Mean 2E. 
r Li 
30° 7' 
31° 43' 
30° 39' 
30 50' 
T V 
29 32 
31 _ 19 
30 20 
30 24 
In symmetry J 
, y jN <l 
26 31 
28 30 
27 20 
27 27 
21 4 
23 33 
22 11 
22 16 
L F 
12 7 
14 53 
13 23 
13 28 
Wave4ength 
in blue 
0 0 
0 0 
0 0 
0 0 
Perpendicular to '[ 
21 13 
18 8 
18 59 
19 27 
symmetry plane. J 
Determination of True 
Optic Axial Angle of Ltesium 
Immersion in Gedar Oil. 
Magnesium 
iSeleiiate by 
Plane of optie 
axes. Light. 
Section 1. 
Section 2. 
Section 3. 
Mean 2Ya. 
r Li 
21° L 
20° 53' 
20“ 31' 
20 49' 
T C 
20 49 
20 39 
20 14 
20 34 
In symmetry J 
xa 
plane. 1 
18 47 
18 36 
18 22 
18 35 
15 40 
15 25 
15 10 
15 25 
L F 
10 38 
10 23 
10 29 
10 30 
tYavedength 
in lilue 
0 0 
0 0 
0 0 
0 0 
Perpendicular to 1 q 
12 51 
11 57 
13 38 
12 49 
symmetry plane. J 
