74
Open the aperture diaphragm (= pos. PH).
Engage the built-in Bertrand lens* into the light
path by turning the knurled wheel (50.2) = pos. B,
and focus the annular structures (Fig. 52) with
the lever (50.3). See page 83 for how to operate
the Bertrand lens on the polarized light
microscope.
If your microscope does not have a Bertrand
lens: insert an auxiliary telescop e* (Fig. 51) into
the observation tube in place of an eyepiece.
Slightly loosen the clamp ring (51.2) and focus
the annular structures by adjusting the eyelens
(51.1). Retighten the clamp ring.
Push in the two centering screws at the back of
the condenser (48.10 or 14.3) and rotate until the
dark ring (phase ring in the objective) coincides
with the slightly narrower bright ring (light ring
in condenser).
Disengage the Bertrand lens and watch the
quality of the phase contrast image. If using the
auxiliary telescope, watch the image with one
eye through the eyepiece. Then repeat the
centration process for the other objective light
ring combinations.
Possible errors
Specimen: too thick, too thin, too brightly
stained; refractive index of mounting medium
and specimen identical so that there is no phase
jump.
Specimen slide too thick, so Koehler illumination
not possible.
Wedge-shaped coverglass position, so
centration of light and phase ring is no longer
effective.
Wrong light ring, or light ring has been put in the
turret upside down (see assembly on page 23).
Aperture diaphragm not open. Wrong
condenser top (only 0.90 S 1).
Fig. 52
Centration process for phase contrast, observed with
a Bertrand lens or auxiliary telescope
a
Condenser in brightfield position (H),
b
Condenser in PH
position, light ring LR not centered,
c
Light ring and phase ring
centered
PH
LR
