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In a paper communicated to the Royal Society in February, 1910, I gave the results of measurements of absorption lines for the three alkali metals, potassium, rubidium, and cæsium. Up to that time I had not been able to obtain the corresponding spectrum for lithium vapour. This was owing to the fact that lithium vaporises at a considerably higher temperature than the other alkali metals, and at the high temperatures required for a sufficient density of vapour the lithium attacks the material of all tubes which I was able to use. Steel, brass, platinum, glass, silica, carbon are all attacked, and no satisfactory photographs could be obtained. Recently, however, by using a relatively large quantity of lithium in a steel tube sufficient vapour was obtained to give an absorption spectrum showing 27 lines of the principal series. The method was the same as that described in the paper referred to, with the exception that a second steel tube was placed in the outer steel absorption tube. This was simply for the protection of the outer tube, which would probably collapse after being acted on by lithium vapour at the bright red heat to which it was raised. The source of light used was the cadmium spark, as the limit of the series is so much further in the ultraviolet than in the case of the other alkali metals. The following table gives the wave-lengths and oscillation frequencies for the complete series of lithium lines. (See Table I opposite.)

The absorption spectra of lithium and cæsium