An illuminating system (21) is provided for an optical viewing apparatus (1) which can be operated in a fluorescence mode. The illuminating system includes at least one broadband light source (31) for illuminating a viewed object (3) and at least one narrowband light source (37) for exciting fluorescence in the viewed object (3) and/or for background illumination in the fluorescence mode. The illuminating system further includes a light conductor (23) having a light source end inlet end (49) and an outlet end (25) facing toward the viewed object. Furthermore, the illuminating system (21, 210) includes a superposer (43) for superposing the light of the narrowband light source (37) with the light of the broadband light source (31). The superposer (43) is mounted at the inlet end (49) or at the light source side ahead of the inlet end (49) of the light conductor (23).