According to Sommerfeld's theory of the fine structure of spectral lines, the magnitude of the separation of the doublets for all the members of the Balmer series of hydrogen should be constant, and have a value of 0·365 cm.−1. Now, owing to the extreme lightness of the hydrogen atom, a direct proof of Sommerfeld's theory by observation of the hydrogen lines is a somewhat difficult undertaking. At ordinary temperatures the Doppler effect is considerable, so that the lines are broadened out and the components of the doublets tend to overlap. The atom of helium, however, is four times as heavy as that of hydrogen, while the nuclear charge is double, so in consequence the separation of the doublets should be, according to the theory, sixteen times as great.