Abstract
The present communication deals with a further study of the absorption spectra of the vapours of lithium and cœsium. These spectra were investigated, and the results communicated at the beginning of last year to the Royal Society. The present account deals with an extension of the principal series lines for the two metals, and a more detailed examination of the expression of the wave-lengths by a formula, as well as an investigation of the broadening effect in a pair of lines with increase of density of the vapour of cæsium. We shall deal first of all with the lithium spectrum. As has been remarked before, lithium vapour presents some difficulties, because of the high temperature required and the action of the vapour in this condition on the material of the containing vessel, but this difficulty can be surmounted by the use of a double steel tube and a large quantity of lithium. With the tube at a bright red heat, a dense enough vapour is produced to show in transmitted light a large number of the principal series lines ; the number that can be observed depends, I am convinced, only on the resolving power of the spectrograph used to investigate them. With the instrument at my disposal, I have been able to measure 41 of these lines, and I think in this case the resolution has been pushed as far as practicable, the last few lines being separated by only about 0·2 Ångström unit. Fig. 1, Plate 9, shows the absorption lines up to No. 25. The finer lines after this do not appear in the reproduction.
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