Many leading features in the Stark effect are best illustrated by studies in the spectra of the rare gases. In each spectrum at least one phase of the Stark effect stands out prominently. Thus in helium the Stark types for singlet lines are most clearly revealed, while in neon an analogue of the Paschen-Back effect makes its appearance, together with some departures from the normal Stark patterns for parhelium. The present experiments with xenon constitute evidence in support of a quantum-mechanical explanation of the origin of Stark displacements and reveal new features concerning the nature of Stark patterns. It was first observed in Stark displacements, in helium that sharp and principal series lines were displaced very little in comparison with diffuse series lines. The relatively small displacements received an early explanation on the grounds of an atomic model in which the
s
-and
p
-terms corresponded to electron orbits of high eccentricity which revolve rapidly in their planes. This action prevented the external field from producing an appreciable shift of the electrical centre from its normal position in the nucleus. Since the excess of the term in question over the hydrogen term of the same principal quantum number (the so-called hydrogen difference) measured the speed of revolution of the orbit, it seemed clear that hydrogen differences should serve as valuable guides to probable Stark displacements. Up to the present well organised data have appeared to support this view.
The stark effect for xenon