The apparatus described is designed to permit the observation of the smallest cellular components with the phase contrast microscope (which requires an objective of a high numerical aperture) while a part of the cell is irradiated by a very small spot of ultraviolet radiation. Theoretical considerations show a good distribution of the energy for a diameter of the irradiation spot as small as 0.2 microns. Indications are given about the limitations of the brightness caused by diffraction at the spot diaphragm and the energy absorption by volume-unit of the irradiated object. The practical applications of the apparatus are discussed.

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