Leica DM LSP Bedienungsanleitung Seite 13

Conoscopy of crystals
Birefringent crystals cause interference patterns
(Fig. 47) in the exit pupil of the objective (i. e.
inside the objective). These are also called
conoscopic images. The shape of these interfer-
ence patterns and the way they change when
compensators are used supply information on
the number of crystal axes (uniaxial or biaxial
crystals), the orientation of these axes and the
plus or minus sign of the birefringence (positive
or negative birefringent crystal).
As these interference patterns occur in the
pupil, they are not normally visible during normal
microscopic observation (orthoscopy). Their ob-
servation can be improvised by removing one of
the eyepieces and looking into the tube with one
eye from a distance of a few centimetres.
Observation is better with the auxiliary telescope
for phase contrast (Fig. 25.1). However, other
crystals in the field of view disturb the interfer-
ence patterns of a crystal in the centre, so that it
needs to be isolated. This can only be done with
the Bertrand lens (43.3) in the Pol module, as
here isolation is performed by a fitted dia-
phragm. The diameter of the isolated object field
is about 55 µm for a 40x objective, about
36 µm for a 63x objective and 23 µm for a 100x
objective.
Setting the microscope for conoscopy
The most suitable object areas for conoscopy
are those that show the lowest possible phase
differences (chart in Fig. 45). Exact centration
of the strain-free Pol objectives and exactly
crossed polarizers are essential for perfect
conoscopic observation.
Turn an objective with as high an aperture as
possible (e. g. 40x, 50x or 63x) into the light path.
Open the aperture diaphragm (43.7). Move the
crystal you want to examine as near to the cen-
tre of the field of view as possible.
Narrow the field diaphragm (43.12) as well, if
necessary.
Push in the Bertrand lens (43.3). To improve the
quality of the image, especially for small crystals,
lower the stage by about 0.2 mm.
The insert the centering keys (1.3) into the two
openings (43.3a) one after the other and adjust
until the circular area (objective pupil) is aligned
to the centre of the crosslines.
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