M. DES CLOIZEAUX OX AMBLYGONITE AND MONTEBEASITE. 
579 
, p s 
edge-: edge s 
f p : h (fig. 1) 
m : L (fig. 1) 
, P : S (%• 1) 
fm : or d : g (figs. 1 and 4) 
C S : L superior 
( S : L inferior 
L : H 
S , 8 
edge : edge (figs. 1 and 2) 
S p 
edge 7- : edge — 
0 L 0 m 
Calculated. Observed. 
O 1 it 
= 101 39 33 
= 37 6 
= 37 10 
— 179 56 salient. 
= 180 4 reentering. 
= 101 39 50 
= 78 20 10 
j 89 8 
{ 90 52 
90 48 
f 89 46 
{ 90 14 
J 89° 12' 
>9 
99 
(12° 29' with m 
The plane of the optic axes makes an angle of ^ ’ In the twins tig.'l 
the plane of the axes of 1, 3, 5, 7, 9 meets that of 2, 4, 6, 8 at an angle of 58° 22' 
(which is the mean of several observations). In the twins tigs. 2 and 4 the plane of 
the axes of one region intersects that of the other region at an angle of 123° 14', by 
observation 122° 42' to 123° 31'. The bisector of the acute angle of the axes is negative , 
and it is very little removed from coincidence with the twin plane H, and makes 
respectively angles of 11° 39' 58" with the edge 53° 26' 31" with the edge and 
51° S' 18" with the edge g- The mean index was measured on a prism of 59° 23', the 
refracting edge of which is almost exactly normal to the plane of the optic axes ; I have 
found /3= 1*594 (the yellow sodium ray). 
The apparent separation of the axes admits of the two corresponding systems of 
coloured rings being both visible in air. This separation, though sufficiently constant 
in the different regions in the same plate, being about 86°, is, however, as low as about 
