MINERALOGY OF BLACK LAKE AREA. 
43 
as before, and was found to be 59° 48'. From this value the true internal 
optic axial angle (2V), the apparent angle*in air (2E), and the position of 
the acute bisectrix (giving the extinction angle on (010)), were calculated 
to be as follows : 
2V = 59° 15', 2E = 111° 54', Bx a A c - axis = 38° 21'. 
The axial angle for red light is greater than for yellow (p > v ) , but it 
was not measured. Judging from the appearance, in ordinary white 
light, of the rings surrounding the optic axes, there seems to be a dispersion 
of the bisectrices in the plane of symmetry (inclined dispersion), but the 
effect may be due, in part at least, to a distortion of the interference 
figure under the conditions of the experiment. 
The value of 2V, calculated directly from the refractive indices 
a, (2, y, is 59° 29', giving also 2E = 112° 30'; these are not very different 
from the corresponding angles as derived from the actual measurement. 
From this angle 2V, together with a knowledge of the extinction angle 
on (010) ( = 38° 21') and the angle 010 A 110 (=43° 35'), the extinction 
angle on (110) is found, by the usual formula, to be 32° 19^'. A large 
number of measurements of this angle, made on cleavage fragments, 
fell within the limits Z2\ a — 33°. It was not possible to measure the ex- 
tinction angle on (010) directly, owing to the small size of this face on 
the crystals. 
The acute bisectrix is inclined at 51° 39' to the normal of (100), and 
at 22° 31' to the normal of (001). 
The value calculated for the refractive index for light vibrating 
in the crystal along the c-axis (using, as data, a, y, and the angle 
Bx Ac-axis) is 1-686; this is in fair agreement with the observed 
a 
values noted above. 
As will appear from the above description, the main interest of the 
colourless crystals, from an optical standpoint, lies in the refractive 
indices, which are as low as, or lower than, any which have hitherto been 
recorded for this mineral. The optical characters of diopside naturally 
vary with the chemical composition, and this is especially true with regard 
to the relative amounts of FeO, Fe 2 0 8 , and A1 2 0 3 which may be present. 
In general it has been found that the nearer the mineral approaches to 
the theoretical composition, CaMg(Si0 8 )2, the lower are the indices. 
In this respect, the Montreal chrome pit crystals may be compared with 
the limpid crystals from Nordmark, Sweden (FI ink’s type V), and also 
with certain colourless crystals from Alatale, Piedmont. Analyses of 
these are reproduced above, and their principal optical characters are 
tabulated on page 44. No. 1 refers to the Montreal chrome pit crystals, 
