The spectrum of single transits, forming part of the pressure broadening, has been quantitatively investigated for Na—A and for Hg—A transits. It was found to be very nearly identical with the occurrence distribution of the perturbed eigenfrequencies, the intensity distribution being in agreement with the theoretical prediction I(v) ~ Δv-3/2 within the limits of its validity. As this agreement is a direct experimental test on the sixth power potential law of the van der Waals forces, given by London’s wave mechanical theory, it seemed to be of interest to investigate a case in which the velocity of the atoms is considerably less, so that any possible influence of the motion is smaller still. The interaction between two mercury atoms, i. e., the broadening of the mercury resonance line by the mercury pressure itself, was therefore chosen for quantitative investigation. An excited atom interacts with a normal atom of the same kind at very large distances with a potential of force proportional to 1/r3 is the internuclear distance. The influence of this “resonance force” on pressure broadening has been treated theoretically by Weisskopf. Though it is considerable at very low pressures, resulting in large “optical impact diameters” for the Lorentz effect, it is very small at higher densities. This is connected with the dipole property of the forces: the interactions of several neighbouring atoms mainly cancel each other, leaving a very small residue only, the so-called coupling broadening. It was thus expected! that in the wing effect the resonance forces can be completely neglected.