Chemistry Reference
In-Depth Information
a
stage
bearing the thin section. The stage can rotate freely around the
axis of the microscope and can carry a mobile holder (mechanical stage)
for moving the thin section and precisely locate a point. The edge of the
stage is graduated so that the angles can be measured; most models are
also equipped with a vernier for precise measures;
the
objective
, actually there are a series of objectives with different mag-
nifications placed on rotating nosepiece. The observations are made
from the lowest magnification to the highest one. You must take care
to handle the nosepiece at the base and not by objectives, which may
decenter the objectives;
the
upper polarizer,
or
analyser
, is removable and whose polarization
plane of is at right angle to that of the lower polarizer. You verify that
the two polarizers are at right angles by observing that no light emerges
from the analyzer when the two polarizers are crossed;
the
ocular.
Various removable accessories are installed on the microscope:
1
a diaphragm placed over the light source used to modulate the size of
the light beam; it is used for increasing the contrast between two fields
of the image (for instance for a Becke line);
2
removable auxiliary condenser lenses which converts the incident paral-
lel light beam (orthoscopic illumination) into a conical one (conoscopic
illumination); it is used to make interference figures in convergent
light;
3
accessory (compensator) plates, that are introduced into a slot located
between the objective and the analyzer. The accessory plates have a
known birefringence. The most useful is a quartz plate whose birefrin-
gence is equal to
0,018 (for a thickness of 30 microme-
ters) (which corresponds to purple/indigo color that marks the boundary
between the red at end of the first order and the blue of the beginning of
the second order in Michael Lévy's interference color chart). The acces-
sory plate is used to determine the sign of elongation of a section or the
optical sign of a mineral;
δ
=
e(n
γ
−
n
α
)
=
4
the Bertrand lens, removable, is used to observe interference figures in
convergent light.
An
objective
lens must be
centered
so that the axis of the microscope
coincides with the rotation axis of the stage. It is centered either through
rotating rings located on the objective or small screws at the lens collar or
in the rotating nosepiece. The method involves: identify a distinctive point
in the thin section, place it at the intersection of the cross hairs of the reticle
and rotate the stage of 180°. If the objective is not centered, the identified
point is no more located at the intersection of the cross hairs. Then move the
