42 
MUSEUM BULLETIN NO. 27. 
comparison, and also owing to the close correspondence in optical char- 
acters between the material from these localities, and the crystals from 
the Montreal chrome pit, as detailed below. 
Optical Characters . 
Optically, the colourless crystals of diopside are interesting in possess- 
ing unusually low indices of refraction, as might have been expected in 
view of the almost complete absence of iron from the mineral. 
The orientation of the indicatrix relative to the crystal axes is as 
usual in diopside. The mean line, ft, coincides with the crystallographic 
axis of symmetry, and the axial plane lies in the plane of symmetry, 
with the acute bisectrix emerging in the obtuse angle between the crystal 
axes a and c. 
The indices a and y were determined by the minimum deviation 
method, using the natural prism 130 A 130 (= 35° 12^')* By the same 
method, using the prism 310 A 310 ( = 38° 36'), the index ft was obtained, 
and also the value, between « and y, for light vibrating vertically, 
along the c-axis, in the crystal. The two latter values were also deter- 
mined by the total reflection method ; the crystal in this case was mounted 
with the zone axis (100 : 001) vertical, immersed in Thoulet solution 
of index 1*7344, and the angle of total reflection from the face (100) 
measured. The indices thus obtained are as follows; 
Method 
a 
r 
fi 
Light vibrat- 
ing along c~ 
axis 
M.D., using prism 130 A 130 . . 
M.D., using prism 310A310 . . 
T.R. from lace (100). 
1*6691 
1*6983 
1*6765 
1*6761 
1*6873 
1*6811 
In each case, yellow (sodium) light was used, and the same applies 
to the determinations of the other optical characters referred to below. 
Owing to the small size of the prism faces, the illumination was 
somewhat poor in the minimum deviation determinations, but from the 
dose agreement in the values found for ft by the two methods, it is 
believed that the figures may be relied on to the third place of dedmals, 
and the refractive indices may be taken as 
a = 1*669, fi = 1*676, y = 1*698. 
The birefringence is thus strong (y— a = 0*029) and positive. 
One of the optic axes is nearly normal to (001), while the other is 
inclined at an angle of about 20 degrees to the normal of (100). The 
angle between the optic axes, as they emerge from these faces, was 
measured with the crystal mounted and immersed in Thoulet solution 
