Abstract: An optical ancillary device is configured to be attached to a microscope. The optical ancillary device includes an imaging optical device configured to be placed in an imaging beam path of the microscope and an illumination optical device adapted to be placed in an illumination beam path of the microscope. The imaging beam path and the illumination beam path are separated. The illumination optical device is configured to match the imaging optical device, thereby acting on the illumination beam path to cast light into a field of view of the microscope. The illumination optical device of the optical ancillary device matches the imaging optical device, so that the illumination is adapted to the changed observed area, that is, the adjusted field of view of the microscope, so as to ensure the illumination quality of the changed observed area.