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The compound microscope may be fitted with a dark field condenser that has a numerical aperture (resolving power) greater than the objective. The condenser also contains a dark-field stop. The compound microscope now becomes a dark-field microscope. Light passing through the specimen is diffracted and enters the objective lens, whereas undiffracted light does not, resulting in a bright image against a dark background. Objects are seen as light objects against a dark background.
These are three requisites for adopting an ordinary microscope for Dark ground illumination:
1 Dark ground condenser
2 Suitable light intensity lamp
3 Funnel stop (to reduce numerical aperture of objective)
1. To visualize Trepenoma pallidum in exudates.
2. To visualize Leptospira species in blood & urine.
3. To visualize Spirillum minus in blood.
4. To detect motility of other bacteria.
5. To identify sheathed microfilaria & other parasites.