What "It"?

Given,

$C=\cfrac { \sqrt [ 3 ]{ n_{ 2 } } -\sqrt [ 3 ]{ n_{ 1 } }  }{ \sqrt [ 3 ]{ n_{ 1 }n_{ 2 } }  } -\cfrac { n_{ 2 }^{ 2 }-n_{ 1 }^{ 2 } }{ (n_{ 1 }n_{ 2 })^{ 2 } }$

when $n_2\lt n_1$, for absorption lines, $n_1\leftrightarrow n_2$,

$D=\cfrac { n_{ 2 }^{ 2 }-n_{ 1 }^{ 2 } }{ (n_{ 1 }n_{ 2 })^{ 2 } }-\cfrac { \sqrt [ 3 ]{ n_{ 2 } } -\sqrt [ 3 ]{ n_{ 1 } }  }{ \sqrt [ 3 ]{ n_{ 1 }n_{ 2 } }  } =-C$

And so we can expect the spectra line $n_{1\rightarrow 2}$ to cancel the reverse spectra line $n_{2\rightarrow 1}$.

For the sake of it....