40G 
DR. A. E. H. TUTTON ON THE 
always perpendicular to the second median line, which in this substance corresponds 
to the a axis of the indicatrix. 
The order of dispersion for the first median line is, therefore, that the latter is nearer 
to the inclined axis a by four minutes for red C light than it is for greenish blue F light. 
The third axis of the indicatrix, the intermediate axis /3, is identical with the crystal 
axis b, the symmetry axis of the monoclinic crystals, normal to the paper in fig. 3. 
These measurements with Plate 1, which have afforded all the data concerning the 
orientation of the axes of the optical indicatrix and of the optic axes, were supple¬ 
mented by similar ones derived by the use of two other tabular crystals parallel c { 001}, 
Plates 2 and 3. The results as regards the dispersion of the median lines were 
identical in both cases with that derived from Plate 1, namely, that the first median 
line is nearer by 4' to the inclined axis a for red C light than it is for greenish blue F 
light. 
The Extinction Angles with the Normal to c(00l), behind that normal towards the 
direction of the vertical axis c, derived from Plates 2 and 3, were 2° lP from Plate 2 
and 4° 39' from Plate 3. The two plates were not quite so perfectly developed as 
Plate 1, as regards the planeness and perfection of surface of the two c-faces, Plate 1 
having been a remarkably perfect crystal. But the mean value of the extinction 
angle for sodium light for the Plates 2 and 3 is 3° 25', only 4' removed from the 
excellent value derived from Plate 1, which was 3° 2F for sodium light. Flence the 
mean value of the results from all three plates, 3° 23', may be taken with confidence as 
very near the truth, quite as near as is possible in the cases of the ordinary extinction 
angle determinations in sodium light with the stauroscope. 
The final result for the orientation of the optical ellipsoid is therefore as follows. 
The crystallographic axial angle ac being 76° 10', the angle between the normal to the 
c-face (001) and the vertical axis c is 13° 50'. The second median line (axis a of 
ellipsoid) is inclined 10° 27' for sodium light from and in front of the vertical axis c, 
and 3° 23' behind the normal to c (001). The first median line (axis y) lies 3° 23' 
above the inclined axis a of the crystal and 10° 37' below the normal to ct(lOO). 
Both median lines thus lie in the obtuse crystal-axial angle ac. These positions will 
be clear from fig. 3. 
The values for the optic axial angle derived from Plates 2 and 3 were as under :— 
Light. 
Plate 2. 
Plate 3. 
Mean 2V 0 . 
Mean 2V a . 
Li. 
116 28 
o / 
117 8 
o / 
116 48 
o f 
63 12 
C . . .. 
116 27 
117 7 
116 47 
63 13 
Na. 
116 18 
117 1 
116 40 
63 20 
T1. 
116 12 
116 52 
116 32 
63 28 
Cd. 
116 8 
116 45 
116 27 
63 33 
F. 
116 2 
116 38 
116 20 
63 40 
