crystallized surfaces upon light. 1*51 
The following observations were made with rhombs taken 
from a different mass of calcareous spar : 
Position of the crystal. 
Azimuth. 
Polarising No. of 
angle, observations. 
Short diagonal in plane of reflection o* 57 0 36' 5 
One of the edges in plane of reflection 50 57!' 58 50 ' 7 
Long diagonal in plane of reflection 90 59 44 7 
In these experiments the results were the same, whether 
the obtuse angle of the rhomb was nearest or farthest from 
the eye, or whether it was to the right or left hand of the 
observer. 
In order to determine the angle of polarisation for surfaces 
differently inclined to the axis, I selected some fine Faroe 
crystals of calcareous spar from the cabinets of Sir George 
Mackenzie and Mr. Allan. One of these crystals was an 
acute dodecahedron, having a highly polished surface inclined 
about 5 0 to the axis, and with it I obtained the following 
results. 
Position of the crystal. Polarising angle. No. of observations. 
Axis in plane of reflection - 54 0 18' 13 
Axis perpendicular to plane of reflec- 
tion - - - 58 14 10 
I was now desirous to obtain a surface perpendicular to the 
axis; but I have searched in vain for such a specimen, and 
have invariably found that the summit of the prism is rough 
and unpolished. With a surface polished by art, and cut per- 
pendicular to the axis, I found the polarising angle about 
58° 15'; but I do not regard this result as deserving any par- 
ticular attention. 
In order to supply the defect of natural faces, I ground 
and polished a great variety of surfaces, inclined at all angles 
