Leica DM R Serie Mode D'emploi page 79

If the image is not bright enough, both the
polarizer and the
disengaged. A neutral density filter (30.4) can be
used in the empty slot of the analyser 360 (30.1)
to protect the viewer from glare when the
analyser is disengaged. Coloured birefringent
specimens may exhibit differences in brightness
and colour when the stage or polarizer is rotated
(when the analyser is disengaged). This
phenomenon is called dichroism or pleochroism
and is an important indication for crystal exami-
nations. However, this effect can also be
simulated on non-polarized light microscopes,
as these have no built-in depolarizing quartz
plate, or if an incident light reflector has been
left in the light path when transmitted light is
switched. This also applies for the use of the
tubelens 1.6x on the polarized light microscope
(54.11).
Incident light reflectors or fluorescence filter
cubes should disengaged during examinations
in p olarized transmitted light and transmitted light
interference contrast ICT.
Examinations
Crossed polarizers
The DIN and ISO standard vibration directions
are shown in the chart on page 77, but when the
polarizers are crossed the same polarization-
optic effects are observed when the polarizers
are transposed by 90°.
If the specimen contains many non-birefringent
or opaque particles, the analyser is frequently
turned out of the crossed position by a few
degrees so that these particles show up at least
analyser should
faintly (they remain dark when the polarizers are
be exactly crossed). It is not customary to examine
specimens with the polarizers parallel, as this
method of identifying birefringence is not sensi-
tive enough.
Change in brightness when birefringent objects
are rotated
When the stage is rotated, the brightness of
birefringent
periodically. During a full rotation the object
disappears four times after each 90° interval.
The four dark p ositions are called extinction or
normal p ositions. Exactly between each of these
extinction positions the object can be observed
with maximum light intensity. These are the four
diagonal or 45° p ositions. In the extinction
positions the object vibration directions run par-
allel to the transmission directions of the polar-
izers, at maximum intensity the object vibration
directions represent the angle bisectors of the
polarizer directions. The crosslines in the (right-
hand) eyepiece of polarized light microscopes
can either be aligned at N – S/E – W, i.e. in the
polarizer directions, or at 45° angles, i.e.
corresponding to the object vibration directions
in the diagonal position.
(anisotropic) objects changes
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