1. The placing of a body under water or other liquid. 2. In microscopy, filling the space between the objective lens and the top of the cover glass with a fluid, such as water or oil, to reduce spherical aberration and increase effective numerical aperture by elimination of refractive effects that result from an air-glass interface; the best resolution is achieved when the space between the condenser lens and the specimen slide is also filled with the fluid. [L. im-mergo, pp. -mersus, to dip in (in + mergo)]
homogeneous i. in i. microscopy, use of a fluid, such as oil, that has a refractive index virtually identical to that of glass, providing the highest possible numerical aperture.
oil i. , water i. See immersion (2).
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