I. Herzfeld and Wolf were able to conclude from the behavior of the refractive index of helium in the visible spectrum that the continuous absorption of the gas below the 507A series limit plays an essential part in producing the observed refractivity. II. The present investigation involves a quantum mechanical calculation of the actual strength of this continuum, and also of the resonance line at 584A. As a by-product, the atomic absorption coefficient for x-rays is computed. III. The accuracy of the calculations is discussed. The 30 percent deviation from the present value of Vinti's value for the strength of the resonance line is accounted for. IV. The $f$ sum rule is verified to 2 percent and the experimental refractive index is checked within 3 percent. V. It is discussed why the continuous spectrum is relatively so much stronger in helium than in hydrogen and the alkali metals.

• Received 3 January 1933

DOI:https://doi.org/10.1103/PhysRev.43.258