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3.1 DARKFIELD MICROSCOPY AT HIGH MAGNIFICATIONS
Careful attention should be paid to the details of oiling a high numerical aperture condenser to the
bottom of the specimen slide. It is very difficult to avoid introduction of tiny air bubbles into the area
between the condenser top lens and the bottom of the microscope slide, and this technique should
be practiced to perfection. Air bubbles will cause image flare and distortion, leading to a loss of con
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trast and overall image degradation.
Problems are also encountered when using microscope slides that are either too thick or too thin.
Many darkfield condensers contain the range of usable slide thickness inscribed directly on the con
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denser mount. If the slide is too thick, it is often difficult to focus the condenser without resorting to a
higher viscosity immersion oil. On the other hand, slides that are too thin have a tendency to break
the oil bond between the condenser and the slide. It is a good idea to purchase precision microscope
slides of the correct thickness to avoid any of the problems mentioned above.
High numerical aperture condensers, whether intended for use dry or with oil, must be accurately
centered in the optical path of the microscope to realize optimum performance.
To achieve this, many darkfield condensers are built with a small circle engraved onto the upper sur
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face to aid in centering the condenser. Centering is performed with a low power (10x-20x) objective
by imaging the engraved circle and using the condenser centering screws to ensure the circle (and
condenser) are correctly centered in the optical path.
