The diffuse series limit is the same as the sharp series limit. In the late 1800s these two were termed supplementary series.
Spectral lines of the diffuse series are split into three lines in what is called fine structure. These lines cause the overall line to look diffuse.  The reason this happens is that both the P and D levels are split into two closely spaced energies. P is split into  . D is split into . Only three of the possible four transitions can take place because the angular momentum change cannot have a magnitude greater than one.
In 1896 Arthur Schuster stated his law: "If we subtract the frequency of the fundamental vibration from the convergence frequency of the principal series , we obtain the convergence frequency of the supplementary series". But in the next issue of the journal he realised that Rydberg had published the idea a few months earlier.
Rydberg Schuster Law: Using wave numbers, the difference between the diffuse and sharp series limits and principal series limit is the same as the first transition in the principal series.
This difference is the lowest P level.
Runge's Law: Using wave numbers the difference between the diffuse series limit and fundamental series limit is the same as the first transition in the diffuse series.
This difference is the lowest D level energy.