## Abstract

According to modern quantum electrodynamics, the 1S level (ground state) of hydrogen and deuterium is predicted to lie 0$\cdot $272$_{6}$ cm$^{-1}$ above the energy given by the Dirac theory. In order to obtain an experimental value for this Lamb shift, the absolute wavelength of the L$_{\alpha}$ line of deuterium has been determined by means of a 3 m vacuum grating spectrograph in fifth order. Lines of the series 6$^{1}$S$_{0}$-n$^{1}$P$_{1}$ and 6$^{1}$S$_{0}$-n$^{3}$P$_{1}$ of $^{198}$Hg observed in the same order were used as standards. The wavelengths of these standards were obtained to $\pm $ 0$\cdot $0002 angstrom by the combination principle from lines in the visible and near ultra-violet regions, some of which were newly measured. Both the far ultra-violet Hg lines and L$_{\alpha}$ of deuterium were simultaneously recorded on the plate in absorption. In this way, from six independent plates the wavelength of the L$_{\alpha}$ line of deuterium was found to be 1215$\cdot $3378 $\pm $ 0$\cdot $0003 angstrom. This value refers to an unresolved doublet. If the relative intensity of the two doublet components (2:1) and the temperature of the absorbing column (80 degrees K) is taken into account and the result compared with the Dirac value of L$_{\alpha}$, a shift of the 1$^{2}$S level of 0$\cdot $26$_{2}\ \pm $ 0$\cdot $03$_{8}$ cm$^{-1}$ is obtained. The agreement with the predicted value is very satisfactory.