A TWENTY-ONE FOOT VACUUM SPECTROGRAPH FOR THE EXTREME ULTRAVIOLET

Abstract

The construction of a vacuum spectrograph containing a six inch concave speculum grating of 21 ft. radius of curvature is described. Exposures of the order of magnitude of one hour suffice to give satisfactory spectrograms having a dispersion of 2.7A per mm in the first order, the entire range 2000–1000A being recorded at one setting on a 16 inch plate. By means of a vacuum‐tight screw up to thirty exposures can be taken on a single plate, and introduction of a stop‐cock in the slit tube allows source and slit to be changed at will. A gate is provided which allows air to be admitted to the 40 liter camera box without destroying the vacuum in the 830 liter body of the spectrograph, to facilitate the changing of plates. By means of a Cenco Hypervac pump and three 4‐stage Gaede diffusion pumps the instrument can be evacuated from atmospheric pressure to below 10⁻⁴ mm in an hour or less. High resolving power has been obtained in hot spark spectra of the elements of the first long period, which should facilitate wave‐length measurements on the spectra of ionized atoms; the main purpose of the instrument, however, is to enable intensity measurements to be satisfactorily made in the Schumann region.