If the spectrum excited by low velocity electrons in mercury vapor at low pressure be observed from a direction normal to the electron beam it has been found that many of the lines are polarized (Ellett, Foote and Mohler, and Skinner). Except when the excited atom is in a state having a minimum value for $j$, the usual theory predicts such polarization, and further indicates that the polarization (electric vector) should be parallel to the electron stream for lines which have $\Delta j=\pm 1$ and normal to the electron stream when $\Delta j=0\mathrm{}$. The observation shows that for the first subordinate series the polarization is in accord with this theory. For the second subordinate series the polarization is always much weaker, and the indication is that the sense of this polarization is not in agreement with that predicted by theory.

• Received 24 September 1926

DOI:https://doi.org/10.1103/PhysRev.28.1151