Shift of the1S1State of Helium

In this paper the shift of the $1{}_{}{}^1{}_{}{}^{}S$ state of helium is reconsidered and it is shown that recent discussions of the problem are subject to considerable doubt. The doubt arises from the unreliability (to the required precision) of the current theoretical determinations of the energy of the ground state. The result of an improved calculation for the latter is presented; and if this is accepted as a sufficient approximation, an unex-plained shift of 21.5 ${\mathrm{cm}}^{-1\mathrm{}}$ (opposite in direction to that previously suggested) would result for He. Similar unexplained shifts (but in the same direction as those suggested earlier) are also predicted for the other He-like ions.