The invention relates to a transmitted-light base (1) for illuminating an object (15) for imaging through a zoom microscope (21), the transmitted-light base (1) comprising an integrated light source (3) having an associated electrical power regulation system (31) for generating a suitable radiation flux, and means (5, 6), capable of being added downstream, for generating a specified spectral intensity distribution. To allow compensation for brightness changes of low dynamic range, such as those occurring in particular during zooming, a continuously controllable mechanical brightness regulator (4) is proposed which is activatable by means of an associated adjustment device (41) in such a way that the illumination intensity of the transmitted-light base (1) is controllable with no change in the spectral intensity distribution. The brightness regulator (4) is arranged close to the light exit side of a reflector lamp (3) used as the light source, or, when a lamp (17) and collector lens (18) are used as the light source, close to the collector lens (18).