High-power sliding-spark capillary discharges in vacuum may be operated so as to obtain soft x-ray radiation that is dominated by either line emission or continuum emission. In the latter case, black-body emission is obtainable with emission efficiencies exceeding 30% of input power. Adjustments in the spectral output of the discharge are achievable either with a conically tapered capillary or with electron-beam assisted heating of the discharge. Such devices are useful for high-resolution contact microscopy of wet, unstained, thin biological samples as well as for lithography. © 1981 Springer-Verlag.