When a transition is of a mixed type, that is allowed for magnetic-dipole and electric-quadrupole radiation, the corresponding intensities of Zeeman components are not simply additive but are modified by interference terms which are different for different components and vary with direction of observation. The only instance in which this interference effect has been observed is the case of the transverse Zeeman effect in the line 7330 Å of Pb i (Jenkins and Mrozowski, 1941). In the present work both the transverse and the longitudinal Zeeman effects were investigated in two lines of lead using a Fabry–Perot interferometer and a three-prism spectrograph. The relative intensities of the π components of the line 7330 Å. (1D2−3P1) of Pb i were found to be about 20:20:10 and 38:11:2 for the transverse and longitudinal views, respectively, and for the line 7099.8 Å ( ) of Pb ii about 29:21 for the longitudinal view. The reproducibility of these relative intensity values was about ±10%. Using theoretical formulas developed by Gerjuoy for the Pb i line 7330 Å, good agreement between theory and experiment is obtained for 4%±1% electric-quadrupole radiation relative to total radiation intensity. For the Pb ii line 7099.8 Å the contribution of the electric-quadrupole radiation is found to be 3%±1%, the interference effect being inverted in comparison to the effect in the line 7330 Å.
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