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 formula_6 . D is split into formula_7. 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. Lithium has a diffuse series with diffuse lines averaged around 6103.53, 4603.0, 4132.3, 3915.0 and 3794.7 Å. The sodium diffuse series has wave numbers given by: formula_8  The sharp series has wave numbers given by: formula_9  when n tends to infinity the diffuse and sharp series end up with the same limit. A diffuse series of triplet lines is designated by series letter "d" and formula "1p-md". The diffuse series of singlet lines has series letter "S" and formula "1P-mS". Helium is in the same category as alkaline earths with respect to spectroscopy, as it has two electrons in the S subshell as do the other alkaline earths. Helium has a diffuse series of doublet lines with wavelengths 5876, 4472 and 4026 Å. Helium when ionised is termed HeII and has a spectrum very similar to hydrogen but shifted to shorter wavelengths. This has a diffuse series as well with wavelengths at 6678, 4922 and 4388 Å. Magnesium has a diffuse series of triplets and a sharp series of singlets. Calcium has a diffuse series of triplets and a sharp series of singlets. With strontium vapour, the most prominent lines are from the diffuse series. Barium has a diffuse series running from infrared to ultraviolet with wavelengths at 25515.7, 23255.3, 22313.4; 5818.91, 5800.30, 5777.70; 4493.66, 4489.00; 4087.31, 4084.87; 3898.58, 3894.34; 3789.72, 3788.18; 3721.17, and 3720.85 Å  At Cambridge University George Liveing and James Dewar set out to systematically measure spectra of elements from groups I, II and III in visible light and longer wave ultraviolet that would transmit through air.