Abstract: A transmitted-light illuminating device (FIG. 1) for microscopes consists of a condenser (1), an iris-aperture diaphragm (2) and a turret (3). The turret permits interchangeable mounting of auxiliary lenses (4) or diaphragms (5) by means of which large-object fields can be illuminated, if desired; alternatively, the microscope can be used in phase contrast. Condenser, diaphragm and turret are combined into a unit in which the turret is functionally coupled to the iris diaphragm. For this purpose, the index shaft of the turret (3) is eccentrically mounted to the actuating ring (9) of the diaphragm (2), the ring is configurated for detent engagement to retain the newly indexed turret station in axial register with the optical axis of the condenser. Thus, rotation of the turret first drives the actuating ring (9) against its diaphragm stop to thereby open the diaphragm before there is any indexing movement of the turret, from one to its next indexed position.