Limitations of the Theory of Complex Spectra

The theory of two-electron systems is extended to include $d^{9}p$ and $d^{9}d$, and comparison is made with experimental data for the $3d^{9}4p$ and $3d^{9}4d$ configurations of Ni I. Greater discrepancies appear here than in $d^{9}s$ or $p^{5}p$, but the multiplets are placed in the correct order. Though the neglected interactions are important, yet the values obtained for the parameters of the largest interactions seem not to be seriously disturbed.