Microscopical Society of Victoria. 
9 
It would be out of place to enter into any general 
consideration of the subject of polarisation of light, and I 
shall therefore start with the assumption that we lmve before 
us a compound microscope arranged for work, and fitted with 
the usual polarising apparatus, the polariser being placed in 
such a position that its shortest diameter is parallel to the 
perpendicular plane of the instrument in which the light is 
incident. The analyser being accurately crossed so°as to 
produce obscurity in the field, we are in a position to make 
some of the most interesting observations in connection with 
the optical relations of the minerals to be examined. To 
enter into a consideration of all the beautiful and important 
phenomena of chromatic polarisation from which the petro- 
logist draws some of his conclusions Avould extend these few 
notes to the length of a treatise, and I must, therefore, confine 
myself to those observations which are most readily made use 
of for the purpose of distinguishing to which of the six classes 
of crystallisation the mineral under observation may belong. 
And this will often be sufficient when taken together with 
the morphological appearances and the colour by ordinary 
light, and the presence or absence of dichroism even to 
determine to which particular species the mineral mav 
belong. J 
If we, therefore, suppose ourselves to be now observing 
some thin rock slice by polarised light, the instrument being 
adjusted as I have said, we may observe that the first 
distinction to be noted is this: 
The mineral constituents ma}' be placed in two classes: 
ijiass (1), those which remain permanently dark, even 
when rotated between crossed nicols. These sub¬ 
stances will be (a) amorphous; (6) crystallising 
according to the isometric system ; or (c) seen in the 
direction of an optic axis, or axis of no double refrac¬ 
tion. Ihese last are so rarely met with that we may 
pass them over from this discussion. 
Examples of (a) are glass, opal , tackylite . 
(b) gaiyiet, hauyne , &c. 
Glass (2), those which generally show light and colour 
when placed under crossed nicols, and on the nicols 
being placed parallel show the complimentary colours 
of those first seen. 
. These may be subdivided into :—(d) becoming obscured 
in four positions, when rotated between crossed nicols, and 
