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, α to ε of Stenstr\"om and a new line ε₂ of wave-length 2.85 A. By use of screens made of tissue paper soaked in thorium nitrate, five absorption limits, M₁ to M₅, were found, checking the results of Stenstr\"om 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 M₁ to M₅, both δ and ε₁ corresponding to M₄, and ε₂ to M₅.Vacuum x-ray spectrograph is described in which a screen with a slit is rotated in front of the plate at twice the angular speed of the crystal so as to prevent scattered rays from striking the plate.