A capsule (20) having a glass wall (21) and containing mercury is positioned in a radiation-transmitting discharge vessel, after which the discharge vessel is provided with a rare gas and closed, means for maintaining an electric discharge are arranged in or adjacent the discharge vessel. The capsule is opened by fusion after the discharge vessel has been closed by heating the capsule by irradiation (42) with a parallel beam of radiation through the wall of the discharge vessel. The wall of the capsule has for this radiation an absorption coefficient which amounts at least ten times that of the wall portion of the discharge vessel.