The mains plug is not correctly plugged in There is no power at the socket Defective bulb Defective fuse The specified bulb or fuse has not been used The aperture diaphragm and/or field diaphragm are not opened wide enough The selector switch for the beam path is set to “Camera” The condenser is not correctly centred The brightness control has been set incorrectly The condenser has not been correctly centred The condenser is too low The objective is not positioned correctly on the beam path The selector switch for the beam path is between two settings The nosepiece is not correctly fitted The condenser is not correctly fitted An objective is being used which doesn’t match the lighting area of the condenser The condenser has not been correctly centred The field diaphragm is closed too tightly The bulb is not correctly fitted The interpupillary distance is not correctly adjusted Dioptre setting has not been carried out correctly Different eyepieces are used for the righthand and lefthand side The eyes are not used to using a microscope