Critical radiation potentials for iron, determined photo-electrically.-Electrons were accelerated from a filament heated by a constant current, to an iron plate, and the photo-electric effect of the radiation excited on a plate carefully shielded from stray electrons, was measured. All surfaces exposed to the radiation were of metal and were maintained at definite potentials, and the tube after thorough baking was maintained at a pressure of 10-6 mm or less during measurements. (1) M series, 40 to 175 volts. Nine critical potentials were observed, the corresponding νR values being 3.46, 6.03, 7.05, 8.21, 9.63, 10.40, 10.88, 11.33 and 11.83, where R is the Rydberg frequency. The first and third are interpreted as resonance potentials of the M3, M4 series, the others as belonging to the M5 series. The difference in νR for the two levels comes out 2.58. (2) L series, 600 to 700 volts. The three points observed correspond to Sommerfeld's L∊, Lη and Lα, the νR values being 45.5, 47.0, and 51.5. These agree well with x-ray data. Ionizing potentials for iron. The convergence limit of M5 series is νR=13.1 which should correspond to an ionizing potential for that level. For the L series, the value for L1 comes out νR=59.0. Extreme ultra-violet spectrum of iron, 1188 to 18 A.-The critical potentials are interpreted as corresponding to emission lines of wave-length 264, 151, 129.6, 111.0, 94.6, 87.8, 83.9, 80.5 and 77.0 A. Differences between various pairs of the νR values correspond closely with eleven wave-lengths, from 294 to 1188 A, measured by Millikan, indicating that these belong to combination series. The L potentials correspond to 20.0, 19.4 and 17.7 A.