MELLO: MICROSCOPICAL STRUCTURE OF ROCKS. 
157 
be distinguished from the crystal ? The polariscope at once helps us. 
When we place a glass between the crossed NichoUs we find it has 
no power to remove the darkness, but substitute for the glass, say a 
plate of selenite, and at once light is restored, coloured according to 
the thickness of the film. However much you may rotate your 
section you produce no change if the object before you is a glass, for 
it has no double refraction, but the great majority of crystals are 
possessed more or less of this property, they are what is termed 
anisotropic, those however which belong to the cubic system are 
isotropic, and do not polarize, the others are variously coloured 
according to circumstances, but are never absolutely dark between 
the crossed prisms, unless the section is one cut at right angles to 
the principal axis of tlie crystals belonging to the tetragonal or 
or hexagonal system, or at right angles to an optic axis in a bi-axial 
crystal, or again when an axis of elasticity coincides with the shorter 
diagonal of the polariscope, and then it is not absolutely dark as is 
glass, for on rotating the section light will pass through it.''' 
* Note,— The following tables, taken from Mr. Rutley's text book on " The 
Study of Rocks ", will be found useful for reference : — 
When a section shows single refractive, i.e., remains dark between crossed 
nichoUs, it may be — 
/ 1 Amorphous (glassy) — Singly refractive in all directions. 
^ 1 2 Cubic 
3 U ' ' 1 ^ Tetragonal } Singly refractive in the direction of the princi- 
niaxia | Hexagonal ^ pal crystaUographic axis. 
i Rhombic ] Singly refractive in one or the other of the two 
Monoclinic I optic axes, or when an axis of elasticity coinci- 
Triclinic J des with the short diagonal of polariser. 
II. 
A section showing colour between crossed prisms, giving four consecutive 
changes from darkness to colour, then when the principal directions of 
vibration are parallel and at right angles to the crystaUographic axes the 
sections may be tetragonal, hexagonal, rhombic, or monoclinic. 
If all the sections of the same miaeral do not behave alike, the mineral is 
tetragonal, hexagonal, or monoclinic. , 
If besides sections which show colour under crossed prisms there are others 
of the same mineral appearing dark, and if the former show dichroism and 
pleochrism, it is tetragonal or hexagonal. 
If besides the sections in which the principal directions of vibration are 
