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Rather more than a year ago it was announced that the bands which go down to the two 2p 3 IIab levels had been found, but owing to the inclusion of a considerable number of wrong lines little progress in understanding them has been made until quite recently. The discovery of these bands is important for several reasons, of which we shall mention one at this stage. It proves that a system of triplet states analogous to the states of the orthohelium line spectrum really exists in the spectrum of H2 and to that extent confirms the view we have taken of the structure of this spectrum. This follows since the singlet 2p 1 IIab levels have now been firmly identified with the C level of Dieke and Hopfield; the final levels of the present band systems are undoubtedly 2p IIab levels, and there is no room for any other 2p II level in the singlet system. The notation here used is that proposed by Mulliken. It has been described by one of us in the 'Transactions of the Faraday Society,’ vol. 25, p. 628. It is assumed that in all the electronic states of H2 with which these bands are concerned only one electron gets excited, the other being in ans state (l = 0). Thus the resultant azimuthal quantum number L of the two electrons is equal to that of the azimuthal quantum numberl of the excited electron. The magnitude of both these quantities is thus expressed by the letters s (forl = 0),p (forl == 1), d (forl ==2), etc., in such symbols as 2p 3 II. In addition we have to specify A the resolved part of L about the molecular axis. This is indicated by the symbols Σ for Ʌ = 0, II for Ʌ = 1, Ʌ for Ʌ = 2. etc. The first number, such as 2 in 2p 3 IIab , indicates the principal quantum numbern and the second such as3 shows that the level is believed to be a triplet level. The suffixesab distinguish the double character of II, Δ, etc., levels which arise according to whether the value of Ʌ is positive or negative.

The spectrum of H 2 .-The bands ending on 2p 3 II levels

The spectrum of H ₂ .-The bands ending on 2p ³ II levels