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Oblique Illumination and COL

The oblique illumination is a technique used to emphasize the subject by making the preparations of the same vision with an almost three dimensional effect.
To use this technique, the simplest method is to decentralize the condenser, this will allow the light from below to be directed at an angle to the specimen and then light the subject laterally, but unfortunately, not all microscopes allow this type of operation.
It will be necessary, then, apply a stop sign of opaque material, shaped like a crescent, triangles or other shapes, in the holder below the condenser, then we will proceed to observation.
It will take several tests to verify the correct size and shape of the stop at last reaching the best result.

The term COL mean "Circular Oblique Lighting". This technique is a variant of true oblique lighting, except that the condenser is inserted in the center of a circular stop of opaque material that will illuminate the preparation from all sides. The result is an even greater three-dimensionality with a more uniform distribution of gray areas on the preparation.