10 
Quarterly Journal. 
(ie ) which do not become totally obscure in any position, but 
show light and colour. The former are doubly refracting 
minerals ; the latter are aggregates. 
Examples : (cl) Fibrolite, augite, hornblende. 
(e) Serpentine rock , Szc. 
When a pencil of polarised light passes through a 
plate of some doubly-refracting mineral, it is resolved into 
two rays which are polarised, and vibrate in accordance 
with a well-known physical law, in two planes, which are 
perpendicular to each other. One of these rays vibrates 
in the direction of the greatest elasticity pervading the 
mineral, while the other ray vibrates in the direction of the 
least elasticity. Where such a thin plate is cut parallel to 
the optic axial plane of a mineral having two optic axes, 
one of these polarised rays vibrates in a direction bisecting 
the acute angle formed by the optic axes, and this direction, 
which is that of the greatest or least elasticity, is called 
the bisectrix or intermediate section. The other polarised 
ray being perpendicular to the first, bisects the obtuse 
angle formed by the two optic axes, and is termed the 
normal or supplementary section. 
Tins I may illustrate by a diagram (Plate I. Fig. 1) copied 
after Rosenbusch; it represents a flake of gypsum in the 
clino-diagonal cleavage, and in which lies the optic axial 
plane. 
It is principally to the application of these principles to 
micro-petrographical researches that I propose to refer. 
We find that in the isomorphic system of crystallisation 
the optical conditions pervading the crystal are the same in 
every direction. It is a simply-refracting substance ; the 
crystallographic axes of the system are all perpendicular to 
each other, and of equal value, and the optical properties of 
such minerals conform to the symmetry of the system. In 
whichever direction a thin plate may be cut from an 
isometric mineral, the same observation will be made, that 
between crossed nicols it remains dark. 
In the next system, the tetragonal minerals, commence 
the series of the doubly-refracting substances. They have 
one optic axis, which is also the axis of the greatest or 
of the least 'elasticity. We find that one of the three ciystal- 
looraphic axes is differentiated from the two others it is 
either longer or shorter, but all three are perpendicuLn to 
each other. This differentiated axis also conforms to the 
