There are several alternative type of microscopes other than bright field microscope that increase the contrast between the background and the specimen without staining, thus allowing for visualization of living specimens. One such microscope is dark field microscope.
Principle:
In a dark room dust particles in the air are practically invisible. But if a beam of light crosses the room the dust immediately becomes visible as bright particles in light.
This is due to the fact that the dust particles refract the rays of light and direct some of them towards the observer. By this method of indirect illumination against a dark background, particles are visible.
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In a dark field microscope also, the specimen under observation is brightly illuminated against a dark background.
In other words, the images of specimens appear as luminary bodies against a dark background. This is in contrast with bright field microscope where the specimens will be darker against a light background.
The dark field condenser:
The dark field microscope is essentially a light microscope, but with a different condenser. The normally used Abbe condenser is replaced by dark field condenser.
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In a dark field condenser, the centre of the top lens is opaque, so that none of the central rays of light can pass through, and the specimen on the stage is illuminated only with the peripheral oblique rays.
The light rays are focused in such a way that they do not reach the specimen directly (as in the ordinary microscope) but instead the light rays traverse across the specimen horizontally almost at right angle to the objective lens.
As a result of this the field appears dark but the specimen stands out as a bright refractive structure in the same way as a dust particle in a beam stands out due to its shining.
In normal operations of dark field microscopy a modified version of Abbe condenser with low power objectives would suffice.
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But for higher magnifications, special dark field condensers with cardio and parabolic lenses are employed. In some instances ordinary light may not be enough. Sources of intense light such as carbon arc lamps may become necessary.
Utmost care should be taken to use perfectly clean and scratch free glass slides and cover slips. Many a time unwanted particles or dust are capable of reflecting light and may illuminate the background which should always remain dark.
Uses of dark field microscope:
The dark field microscope is used for examination of live, unstained preparations of microbes or other specimens suspended in fluid.
It is highly useful for the study of very small bacteria which are generally invisible under ordinary microscope. Motility of microbes also can be observed since they (microbes) are in live condition and are not killed.