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IMMERSION OF A LENS
With almost all high-power lenses, it is intended that the spaces between the condenser and the slide, and the specimen and the frontlens of the objective be filled with an immersion liquid. Owing to the limitations imposed by the critical-angle phenomenon, numerical apertures are impossible exceeding 1.0 without immersion. In addition, immersion makes possible the use of the naturally aplanatic points of the frontlenselement of the objective.