PRINCIPLE:
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.
PARTS OF DARK FIELD MICROSCOPE:
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)
USES:
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.
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