Microscopical Society of Victoria. 
13 
as a means of discriminating between them and other 
minerals which resemble them, but are not dichroic* 
Fig. 2. Biotite Mica , Hexagonal System. —On being placed 
between crossed nicols, so that the lines indicating the basal 
cleavage (which is perpendicular to the chief axis) are in 
accordance with the optic section of the polariser (placed as 
before described), the mineral will become dark. Thus A, 
chief axis, and a, axis of elasticity, agree, as do also B 
and b. This mineral is therefore, according to its optical 
conditions, either tetragonal, hexagonal, or orthorombic. 
Morphological characters and dichroism, to be described 
by-and-bye, will suffice to determine which of the three. 
Fig. 3. Hornblende , Monoclinic. —On rotating such a sec¬ 
tion of hornblende between crossed nicols, one axis of elasticity 
will be found in the position of and the second, of course, 
perpendicular to it, as b. They form small angles with the 
two crystallographic axes A and B , the former of which 
may be inferred as to position from the prismatic cleavage, 
which is almost always evident. 
Fig. 4. Augite y Monoclinic. —In sections of this mineral, 
such as the one figured, one axis of elasticity will be found, 
as at a, and another at b } both forming more considerable 
angles with and in different positions towards the crystallo¬ 
graphic axes, or the crystalline planes which are parallel to 
them. 
In the last cases we have mineral species which, in some 
cases, are morphologically difficult to distinguish, and it is 
often the case that illdefined laminae are met with, of which 
at sight it is difficult to say to what species they may belong 
If, however, any characters are present from which tlie 
crystallographic conditions of the laminae may be inferred, 
the above principles may be used with advantage. 
Finally, the observation whether any thin plate, or 
lamina, is or is not dichroic will often determine its character, 
as, for instance, in discriminating biotite from Muscovite 
mica, or hornblende from au^ite. 
In observing light which has traversed doubly-refracting 
coloured minerals, it may be observed, with greater or less 
distinctness, that in different directions they show different 
colours, or intensity of colour. This is called polychroism, 
and depends upon a partial absorption of the traversing 
* Polychroism is a more proper term, but in any one slice only two colours 
can bo observed; hence the term dichroic is appropriate. 
