## Abstract

A new method of measuring lifetimes of excited states of atoms has been devised and applied to helium. A short pulse 10$^{-8}$ s duration and 10 kc/s repetition rate) of low-voltage electrons causes atoms of a gas to be raised to excited states. The subsequent decay to lower states is observed by intercepting the emitted photons with a photomultiplier, those belonging to the desired transition being selected by means of a filter. By recording delayed coincidences between the photomultiplier pulses and the electron pulses the rate of decay of the excited state can be measured. If certain conditions are satisfied, the decay is exponential with a decay constant equal to the reciprocal of the natural lifetime of the excited state. The mean lifetimes of the 4$^{3}$S, 3$^{3}$P, 4$^{3}$P and 3$^{3}$D terms of helium are found to be 6$\cdot $75 $\pm $ 0$\cdot $10, 11$\cdot $5 $\pm $ 0$\cdot $5, 15$\cdot $3 $\pm $ 0$\cdot $2 and 1$\cdot $0 $\pm $ 0$\cdot $5 respectively, in units of 10$^{-8}$ s. These values agree well with the theoretical calculations of Bates & Damgaard. The apparent lifetime of the 3$^{1}$P level is modified by the imprisonment of resonance radiation in a manner in qualitative agreement with Holstein's theory. The 2$^{1}$S-3$^{1}$P transition probability is found to be (1$\cdot $35 $\pm $ 0$\cdot $02) $\times $ 10$^{7}$ s$^{-1}$.