Abstract
In the presence of foreign gases, a narrow and structureless absorption maximum was observed on the short wave-length side of certain members of the principal series of alkali metals. A résumé of the results obtained so far is given. It is suggested that the band is due to the increase in the energy of the optical electron through the process of collision with a foreign gas atom when the electron is at that energy level whose orbital radius is close to the distance of optical collision.