Abstract: An opthalmic biomicroscope for observing a narrow zone of a patient's eye has an optical axis traversing one of several sections of a flat semitransparent mirror serving to illuminate that zone with the projected image of a slit diaphragm in the ray path of a light source. The light source and the associated diaphragm and projection system are disposed in a housing which is swingable about a pivotal axis skew to the optical axis of the microscope so as to train a high-intensity beam upon the eye by way of a selected mirror section. These sections are relatively inclined in a plane perpendicular to the pivotal axis, including the optical axis of the microscope, and are tangent in that plane to an ellipse whose foci lie on the pivotal axis and on the projected slit image.