Constitution of the Ionosphere and the Lorentz Polarization Correction

SOME years ago considerable discussion took place concerning the relation between the constitution of the ionosphere and its refractive index for radio waves. The question at issue was whether the force per unit charge exerted by the electric field of a radio wave upon an elementary charged particle in the ionosphere should be taken simply as the Maxwellian electric intensity E (the Sellmeyer theory), or whether there should be added a contribution (4π/3)P (the Lorentz theory), P being the electric moment per unit volume produced by the radio wave in the neighbourhood of the charged particle under consideration. The discussion culminated in a paper by Darwin1 which is the most exhaustive treatment of the subject so far published. This seemed to point to the conclusion that on theoretical grounds one should expect the Sellmeyer theory to hold good in the ionosphere at radio frequencies.