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An account has been given in a previous paper of the structure of three of the helium bands (λλ 6400, 5730, 4546), and it is now proposed to deal, in this and a subsequent paper, with the remaining bands constituting the spectrum between the limits λλ 7000 to 3000. A preliminary survey of these has been carried out by Fowler, who arranged certain of them (the "doublet bands") in two series, in each of which the band heads were distributed according to a line series law. The internal structure of these bands was not investigated, however, by Fowler, and the present paper is mainly concerned with this question. The wave-lengths to be given subsequently have all been derived from grating plates taken by the author in 1914. Particulars relating to the conditions under which these were taken will be found in the previous paper, so that it may suffice to remark here that the wave-lengths have in general been obtained from one plate only, and are in consequence not entitled to be regarded as necessarily accurate in the absolute sense. For the purpose of this investigation, however, it is the wave-lengths relative to one another of the lines in a single band which are of importance, and these are much more reliably determined. The order of accuracy of these relative wave-lengths is estimated to be as follows:— For band λ 4650 (3rd order plate) about 0·005A. For bands λλ 4000 and 3680 (2nd order plates) about 0·01A. For remaining bands (1st order plates) about 0·015 to 0·02A.

The structure of the band spectrum of helium.— II