The apparent tripling of certain lines in arc spectra

King discovered that the lithium line 6708, in the arc or in the furnace, changed from a doublet into a triplet by increasing the amount of Li vapour present, the separation of the triplet increasing with the amount of vapour. He suggested that the effect was due to the electrical resolution of the line by the action of interatomic fields. If this suggestion can be verified, we shall have therein a means of measuring the intensity of the interatomic fields in the arc, and from that a means of determining the interatomic fields in the sun, for in ‘Kodaikanal Observatory Bulletin, No. 73,’ the view was put forward that the abnormal displacements of unsymmetrical lines when the sun and arc were compared were due to interatomic electrical fields caused by ionisation, the field in the sun being roughly estimated at about 2,000 volts/cm. less than in the centre of the arc employed. 2. Consequently it seemed important to investigate more fully this observation of King, in order to find whether his suggestion of its being due to electrical fields could be maintained. It was obvious that if his interpretation were (direct, not merely a few exceptional lines, but the majority of spectrum lines should show a complexity on increasing the amount of material in the arc, if he resolving power of the spectroscope were sufficient to show it. As a liminary it therefore seemed desirable to search through the whole visible of the spectra of as many substances as were readily available in order make a list of those lines which could be made triple or multiple by merely increasing the amount of material in the arc. From previous knowledge it med a foregone conclusion that the number of such lines must be small, whether small or large, the most suitable case could then be chosen for a complete investigation of the cause, and the hypothesis revealed could be on the remaining examples.