Intensity Measurements in the Arc Spectrum of Thallium

Measurements were made by the photographic method of the ratio of the intensities of the lines $2p_2-2s$, $2p_1-2s$ in the arc spectrum with a current of 1.6 amp. using alloys of lead and thallium in varying proportions. The ratio increased with decreasing concentration of thallium reaching a limit at about 0.01 per cent, the effects of absorption being negligible at this stage. The limiting value for $\frac{\overline{a_2^{}{}_{}{}^2}}{\overline{a_1^{}{}_{}{}^2}}$ is 0.388, where $I=a^2{\nu }^4$. Commercial lead was found to have a considerable thallium content making it necessary to use chemically pure lead. Measurements of the ratios of intensities of doublets in the principal series $2s-m{p}_1$, $2s-m{p}_2$ were made for $m=4, 5, 6 \mathrm{and} 7$ the values obtained for $\frac{\overline{a_1^{}{}_{}{}^2}}{\overline{a_2^{}{}_{}{}^2}}$ being respectively 4.5, 5.7, 6.4 and 7 of which the last two are very uncertain. The values were found to be independent of concentration of thallium and of the current over a restricted range of tests. The fraction of light transmitted through an antimony film reducer was found to vary with wave-length as follows: $\lambda$, 6550, 0.270; $\lambda$, 5530, 0.243; $\lambda$, 5350, 0.243; $\lambda$, 4890, 0.175; $\lambda$, 3780, 0.093. The diffraction effects recorded by Shenstone were observed and photometer diagrams are given in illustration.