Relative positions of certain multiplets in two and three electron systems of the second long period with wave-lengths for Zr III, Cb IV and Cb III.---Applying the irregular doublet law to the data already known for the triad of multiplets $^{3}DF$, $^{3}D{D}^{\ensuremath{'}}$, and $^{3}DP$, ($ds\ensuremath{-}dp$) in the two electron systems of Sr I and Yt II, the corresponding multiplets for Zr III and Cb IV have been identified. Similarly the triad of multiplets $^{4}F^{\ensuremath{'}}{D}^{\ensuremath{'}}$, $^{4}F^{\ensuremath{'}}F$, and $^{4}F^{\ensuremath{'}}{G}^{\ensuremath{'}}({d}^{2}s\ensuremath{-}{d}^{2}p)$ in the three electron systems of Yt I and Zr II has been extended to Cb III and the stronger lines of the $^{4}F^{\ensuremath{'}}{G}^{\ensuremath{'}}$ multiplet of Mo IV. In passing successively from element to element in both of these isoelectronic systems the shift in the frequency of the radiated lines for each of these multiplets is approximately constant.