Microscopical Society of Victoria. 
11 
position of the optic axis, and to one of the axes of elasticit} 7 ’. 
The optical emanations show also that the crystal does not 
permit the passage of the light undulations in every direction, 
as was the case in the isometric system, with equal facility. 
A plate cut perpendicularly to the chief axis still behaves as 
an isotropic or isometric substance. In any other direction 
the light becomes doubly refracted, the plate showing light 
and colour between crossed nicols. 
In the minerals of the hexagonal system , the optical 
characters are absolutely the same as in those of the 
tetragonal. The distinction will here depend mainly upon 
morphological characters. 
In minerals of the orthorhombic system we find a further 
divergence in character. We have three crystallographic 
axes, which are certainly perpendicular to each other, but 
which are of different values. We no longer find a single 
optic axis conforming in position to the chief crystallo¬ 
graphic, but we have two optic axes, which are symmetrically 
placed as regards two of the crystallographic axes, one of 
which bisects their acute and the other their obtuse angles. 
We have also three axes of elasticity, the greatest, the least, 
and the mean, which accord in position with the three 
crystallographic axes ; and the optic axial plane, that is, the 
plane in which lie the optic axes, lies in one of the chief 
crystallographic sections. 
In the raonoclinic system we have a still further departure 
from the primary symmetry of the isometric system. 
We find here three crystallographic axes of unequal value, 
two of which are inclined to each other, whilst the third is 
perpendicular to the two others. It is this crystallographic 
axis—the orthodiagonal—which alone falls into position with 
one of the axes of elasticity. The optic axial plane may be 
found in the plane of symmetry, but it may be perpen¬ 
dicular to it. Thus the position of the optic axes and of the 
two remaining axes of elasticity which bisect their angles 
has no constant position throughout this system. 
In the triclinic we have a complete departure from the 
symmetry of the isomorphic system. We have here three 
crystallographic axes of unequal value, unequally inclined ta 
each other. We find the optic axial plane not conforming 
in any case to the crystallographic section, and none of the 
three axes of elasticity conform to any one of the crystallo¬ 
graphic axes. 
