Leica DM R Serie Mode D'emploi page 76

Focus the specimen with the 10x objective, open
the field diaphragm (48.22).
Adjust the condenser in x, y and z direction
(48.12 and 48.16) until the field is homogeneously
illuminated, narrowing the field diaphragm
(48.22). You can now switch to a higher-power
objective. Make sure only the observed field of
view is isolated by the field diaphragm.
Immersion darkfield
Assemble the immersion condenser (see above).
Before putting the specimen on the stage, apply
a drop of oil to the front of the condenser,
making sure there are no air bubbles. Set as for
"darkfield with UC/UCR condenser", p. 75.
Fig. 53 Special darkfield condensers
1 D 0.80 – 0.95 (dry), 2 D 1.20 – 1.44 OIL, 3 Condenser bottom
76
1 2
3 3
Possible errors
Darkfield illumination is very sensitive to the
slightest inhomogeneities in the specimen. As
dust particles and fingermarks on the upper or
lower surface of the specimen and the front lens
of the condenser also cause scattering and
diffraction of light, it is essential to keep
specimen surfaces and neighbouring lenses
absolutely clean.
If the objective aperture is larger than the
threshold values listed above of 0.75 or 1.10, you
will get an image similar to brightfield. This will
also happen if the condenser is greatly
decentered.