A normal pressure arc having local thermal equilibrium, is a useful source for measurements of atomic parameters in the vacuum ultraviolet region (Boldt, 1970). However, it imposes severe difficulties if the spectral lines ending on low-lying levels should be observed. The reason is the absorption effective in cold and inhomogeneous gas layers intervening in the optical path between the homogeneous hot plasma column and the vacuum monochromator. Here a version of a cascade arc with a differential pumping system is described which makes possible the observation of argon resonance lines at 1067 and 1048 Å, considerably free of absorption. For this purpose it was found necessary to use a bored anode as a pumping port.