SINCE atmospheric refraction plays an important part in the bending of ultra–short radio waves round the surface of the earth, an adequate study of the propagation of these waves requires a knowledge of the refractive indexes of gases at very high frequencies. It was thought desirable to test the assumption, made in all previous theoretical work, that the values of these indexes were the same as their values at lower frequencies. When this work was begun, no figures were available for the refractive index of any gas at a frequency higher than about 4 Mc./sec. Since then, however, a result for water vapour at 42 Mc./sec. has been published by Tregigda1.