
os 
Parr IL. § vii] DETERMINATION OF ROCKS. 189 
the Nicol prisms. If the object be singly refracting, such as a piece 
of glass, or an amorphous body, or a crystal belonging to some 
substance which crystallizes in the isometric or cubic system, the light 
will reach our eye apparently unaffected by the intervention of the 
object. The field will remain dark when the axes of the two prisms 
are at right angles (crossed Nicols), in the same way as if no inter- 
vening object were there. Such bodies are zsotropic. If, however, 
the substance under examination be doubly refracting—a mineral 
belonging to one of the other erystallographic systems—it will 
modify the polarized beam of light. On rotating one of the 
prisms we now perceive bands or flashes of colour, and numerous 
lines appear which before were invisible. The field no longer 
remains dark when the two Nico] prisms are crossed. Such a 
substance is anisotropic. 
It is evident, therefore, that we may readily tell by this means 
- whether or not a rock contains any glassy constituent. If it does, 
then that portion of its mass will become dark when the prisms are 
crossed, while the crystalline parts which in the vast majority of cases 
do not belong to the cubic system, will remain conspicuous by their 
brightness. A thin plate of quartz makes this separation of the 
glassy and crystalline parts of a rock even more satisfactory. It is 
placed between the Nicol prisms, which may be so adjusted with 
reference to it that the field of the microscope appears uniformly 
violet. The glassy portion of any rock, being singly refracting or 
isotropic, placed on the stage will allow the violet light to pass 
through unchanged, but the crystalline portions, being doubly re- 
fracting or anisotropic, will alter the violet light into other prismatic 
colours. The object should be rotated in the field and the eye should 
be kept steadily fixed pon one portion of the slide at a time, so 
that any change may be observed. ‘This is an extremely delicate 
test for the presence of glassy and crystalline constituents. 
In searching for the crystallographic system to which; a mineral 
in a microscopic slice should be referred, attention is given to the 
directions in which the mineral appears dark, in other words, to the 
directions of its extinction, between crossed Nicols. It is extinguished 
when two of its axes of elasticity for vibrations of light coincide with 
the principal sections of the two prisms. During a complete rotation 
of the slide in the field of the microscope the mineral becomes dark 
in four positions, each of which marks that coincidence. When on the 
-- other hand the prisms are placed parallel to each other, the coincidence 
¢ = rs: 
of their principal sections with the axes of elasticity in the mineral 
allows the maximum of light to pass through, which likewise occurs 
four times in a complete rotation of the mineral. The different 
crystallographic systems are distinguishable by the relation between 
their crystallographic axes and their axes of elasticity. By noting 
this relation in the case of any given mineral (and there are usually 
sections enough of each mineral in the same rock-slice to furnish the 
required data) its crystalline system may be fixed. But in many 
