Helium I Like Spectra

New measurements of the $1s^2 {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}-1s·\mathrm{np} {}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ series of the Helium I isoelectronic sequence for Be III, B IV and C V together with previously published data on He I, Li II, N VI and O VII are critically examined and values for the $1s^2 {}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ terms for these spectra obtained. A slightly modified Hylleraas type extrapolation formula is used to calculate these same values. When relativity, spin, mass and mass polarization corrections are applied the agreement between observed and calculated values is very good for the heavier elements. A small discrepancy appears in the case of Li II.