J. W. Nicholson

The Secondary Spectrum of Hydrogen

THE recent investigation by Dr. Merton of the effect of an admixture of helium on the intensity distribution in the hydrogen spectrum appears to have given a very strong clue towards the elucidation of that spectrum. On the photographs taken by Dr. Merton (reproduced in part in Proc. Roy. Soc, October, 1919) the spectra appear completely different in the cases of pure hydrogen and of hydrogen mixed with helium. Many lines, in the first case quite strong, are totally absent in the second; others remain practically unaltered in intensity; while a third set appears in the second case, though practically or completely invisible in the first. Such results seem, at first sight, to point to the existence of at least three classes of lines which are mutually independent, one class being unaffected by helium and the others affected in opposite senses.