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A microscopical technique used to examine living cells or microorganisms. These are not suitable for examining under a normal light microscope (bright-field illumination) as they are usually transparent. Thus little detail can be seen against a light background. In dark-ground illumination direct light is prevented from forming an image by illuminating the specimen from the side so only diffracted light from the specimen passes through into the objective. The image of the specimen then appears luminous against a completely dark background. This technique exploits the differences in refractive index between organelles and the surrounded protoplasm that causes the organelle boundaries to reflect more light.
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