According to the well-known Kohler principle, the image of a light source is projected through a collective lens by way of a first diaphragm onto an image plane of a microscope objective for illumination of its object plane. It is also known in such a system to insert a pair of axially spaced diaphragms between the light source and the objective, i.e. a first diaphragm proximal to the source serving as a field stop and a second diaphragm remote from the source acting as an aperture stop; the latter diaphragm lies in the image plane of the microscope objective. Since the filaments of the ordinary incandescent lamps produce a disjointed image, a diffuser such as a plate with a matt finish is generally disposed behind the lamp to scatter its light rays. In many instances, moreover, it is necessary to provide a protective heat filter or special cooling means.