Note on vacuum grating spectroscopy

Abstract

In the spectroscopy of the extreme ultra-violet region, it is necessary to work either with arcs in vacuo or with sparks in one or other of the gases hydrogen or helium. As all other gases are opaque to ultra-violet radiations of short wave-length, their use is precluded. With many of the elements, arcs are difficult to maintain in a vacuum, and consequently one is driven to the use of sparks in either one or other of the two gases mentioned. Lyman, in his brilliant researches, has shown us that, with hydrogen, it is possible to obtain spectra extending to about λ 900 Å. U. For radiations below this limit, it would appear, however, that hydrogen is more or less absorbing.. With helium, on the other hand, the evidence available goes to show that this gas is transparent to radiations having wave-lengths as short as λ 400 Å. U. or λ 500 Å. U., and possibly shorter still. It would appear, then, that in the spectroscopy of the extreme ultra-violet, the procedure to be followed, which would permit of the most rapid progress being made, would be, in so far as the emission spectra are concerned, to work with a vacuum grating spectrograph, and to use an atmosphere of helium.