Critical Potentials of the Thorium M Series Lines

M-series of x-ray lines of thorium.—Spectrograms taken with various potentials decreasing in steps from 10 kv to 3.2 kv, are reproduced which show six lines, $\alpha$ to $\epsilon$ of Stenström and a new line ${\epsilon }_2$ of wave-length 2.85 A. By use of screens made of tissue paper soaked in thorium nitrate, five absorption limits, ${\mathrm{M}}_1$ to ${\mathrm{M}}_5$, were found, checking the results of Stenström and Coster. The critical potentials of the six lines were determined and found to agree with the values predicted by the quantum law from the corresponding absorption limits ${\mathrm{M}}_1$ to ${\mathrm{M}}_5$, both $\delta$ and ${\epsilon }_1$ corresponding to ${\mathrm{M}}_4$, and ${\epsilon }_2$ to ${\mathrm{M}}_5$.