Spectral Theory and the Origin of the Nebulium Lines

Abstract

THE fact that the so-called nebulium lines are found to be associated with hydrogen and helium lines only, has frequently been taken as an indication that these lines are somehow emitted by hydrogen or helium, although it has been a great difficulty for this hypothesis that terrestrial experiments always have failed to reveal the lines. The physical conditions prevailing in nebulae are very probably characterised by exceedingly low density combined with the absence of nearly all possible sorts of disturbing influences, apart from a field of radiation which, however, in only a few cases can be comparable in intensity to that of faint moonlight. Similar conditions of low density may also be essential characteristics of the outer atmospheres of long period variables, of the solar chromosphere, and the upper atmosphere of the earth, in all places of which there appear spectral lines of unknown origin or under anomalous conditions of excitation.