12 
Quarterly Journal. 
The application of the foregoing to practice is simple. 
A consideration of the crystalline outlines of an isotropic 
mineral will usually show whether it is one belonging to the 
isomorphic system, or whether the section has been cut 
perpendicular to fin optic axis. This is not common in 
tretragonal minerals, and becomes more rare in the succeeding 
systems. 
Were it possible to examine the mineral sections under 
the compound microscope by means of convergent polarised 
light, as is done in Norremberg’s polariscope, it might be, in 
many cases, readily determined whether the mineral be¬ 
longed to the uniaxial or biaxial systems. 
In default of this, inferences may be drawn from the 
positions of the directions of greatest and least elasticity 
in regard to some crystallographic constants, such as^ the 
outlines indicative of the prismatic planes or known direc¬ 
tions of cleavage. For instance, in sections parallel to the 
chief axis in the tetragonal system, the axes of elasticity will 
accord with the prismatic sides, or be perpendicular to them; 
that is, parallel to the basis. 
In the hexagonal system it will be the same, and this and 
the former may be then distinguished by their different 
morphological character. 
In orthorhombic minerals, both in sections parallel and 
perpendicular to the chief axis, the directions of the greatest 
and least elasticity will conform to the prismatic sides or to 
the basis. 
In the monoclinic system of minerals this will only be the 
case when the plate is cut precisely in the direction of the 
orthopinnacoid. 
In the triclinic system we shall find that in no case do 
the axes of elasticity fall into accord 'with the prismatic sides 
or the basis ; we might expect this from the unsymmetrical 
character of the system. 
The mineral species crystallising in the triclinic system 
which are of most frequent occurrence in rock sections are those 
of the triclinic felspars commonly and conveniently termed 
by petrologists, “ plagioclase.” They are sufficiently distin¬ 
guished from all other species by their remarkable structure. 
The diagrams on Plate I. will illustrate the above 
remarks, and will also enable me to refer shortly to the 
dichroic effects observable in some minerals, which also serve 
