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Darkfield

The Darkfield microscopy is a technique recommended for preparations which have a different refractive index than their surrounding medium, but which are difficult to see because they have no color. To get a dark field, you must place a dark filter (a "stop patch") in the filter condenser. This filter prevents light from entering directly into the objective, the sample is illuminated of side and the light that should be achieved is lost. The sample appears bright on a dark background.
There are two ways to obtain a dark field image.
Using specialized condensers for this type of contrast, this is the best way, but also the most expensive solution. or by using a black filter (a "stop patch") inserted in the support of the filter condenser. The patch stop can be achieved in a variety of materials, cardboard, plastics, glass ...

Benefits of Darkfield microscopy:

This is a simple procedure that can be used on live specimens, avoiding the staining of samples, a technique that could lead to the death of the subjects.
The images look spectacular and visually appealing.
The Darkfield microscopy also allows the view of objects that are below (!) of the resolution of the microscope. These objects appear as bright spots on a dark background. You can not, however, see, in detail, the shape of these objects.

Some possible disadvantages of Darkfield microscopy:

The Darkfield microscopy is very sensitive to any type of material such as dust or anything that is in the path of light, then the media preparations must be kept free from any form of impurities.
It is not suitable for all samples. If the index of refraction of a transparent sample is similar to the surrounding medium, then the light will pass through the sample and the same will not be dispersed before reaching the objective.
The lighting system must be adjusted to a strong light intensity, because the portion of this will reach the sample is small, in practice, only one side.
It's necessary to open the aperture diaphragm of the condenser, and this limits the effective use of the diaphragm.
One patch-stop is usually sufficient to work with low power objective, but at a higher magnification you will need to experimenting with different formats of patch-stops for different objectives, for good picture quality and with objectives beyond 40x will be necessary to use a condenser suitable for this technique (Darkfield Condenser).