Dark Field Microscopy

The effect produced by the dark field technique is that of a background against which object are brilliantly illuminated this is accomplished a equipping the light of microscope with a special kind of condenser that transmit a hollow cone of light from the source of illumination as shown in figure.

Most of the light directed through the condenser does not inter the objective. The field is essentially dark. However some of the light rays will be scattered (diffracted) if the transparent medium contains objects such a microbial cells. These diffracted light will intered the objective and reach the eye. Thus the objective of microbial cell, in this case will appear bright is an otherwise dark microscopic field. Dark field microscopy is particularly valuable for the examination of unstained microorganism suspended in the fluid with mount hanging drop preparations.

Reference: Microbiology by Pelczar, Chan and Krieg