Sodium and Magnesium Spark Lines in the Far Ultra-Violet, and the Quantitative Application of the Irregular Doublet Law to Isoelectronic Sequences

New levels in Na III, Mg IV, and Mg V are identified through the transition 2s←2p. The value of the negative term 2s2p⁵³P of O I is predicted.The relativistic screening number σ₂(L₂₁L₂₂), is 3.19 for $Z=10,11$ compared with the value 3.49 for heavier atoms, usually considered independent of Z. The difference is probably due to outer screening.Wentzel's result σ₁(n,l+1)-σ₁(n,l)=0.58·2ˡn for the first order screening number σ₁ in x-rays is applied instead of the usual qualitative form of the irregular doublet law to determine the rate of dependence of the transition energy 2s²2p⁵←2s2p⁶ upon Z in the fluorine-like isoelectronic sequence. The extremely high result, 49 ·{} 10³{cm}^{{-}1}$, is verified by the new Na III and Mg IV data. Certain regularities in the curves showing the rates of dependence of the irregular doublet transition frequencies upon the atomic number in isoelectronic sequences, as functions of the number of electrons present, may be used for interpolation of these rates.