This sub-series consists of 12 files containing correspondence between Robert Naismith and Edward Appleton. Naismith worked at the Department of Scientific and Industrial Research's Radio Research Station at Slough, where he was largely responsible for the ionospheric work. He became a close collaborator and friend of Appleton, a member of the Tromsö expedition, and a co-author of many publications with Appleton and L.J. Ingram. The correspondence, which is often accompanied by data, graphs, drafts, etc., is presented as a chronological sequence as far as this is ascertainable. It deals with a variety of ionospheric research topics and should be consulted in conjunction with similar material in Coll-37/C.