For the extreme ultraviolet (λ < 1000 Å) a light source has been developed using the lateral emission from a condensed capillary discharge. In that way damages of the spectrometer entranceslit or grating are prevented. The periodic discharges of a condensor through a low-inductance circuit were switched by a rotary spark-gap, which makes possible a control circuit for photon flux stabilization. The light source is rugged, suitable for continuous load, and has a long life time. The effect on emission of the following parameters has been investigated: pressure, repetition frequency, condenser voltage, capacity, discharge energy, and inductance. The dependence of intensity on pressure, voltage, and inductance permits conclusions on ionization stages. The light source can be operated with all noble gases as well as nitrogen or oxygen, giving reproducible many-line spectra with high constancy, among which the spectra of higher ionization stages are predominant. Except the lines of oxygen, originating on the capillary material, no extraneous lines were observed. The spectra of the different gases have been examined in more detail within the region between 350 Å and 945 A. These contain many new lines of argon, krypton, and xenon. It has been shown by other investigations, that the light source emits an intense and copious line spectrum at shorter wavelengths, too, reaching down lower than 100 A.