This sub-series contains 23 files relating to Magneto-Ion Theory, c.1925. With the exception of Coll-37/C.221, the material was crammed into a bulky folder inscribed 'Some observations on the polarization of downcoming wireless waves'. The folder also has a later note ‘Magneto-Ionic Theory' by C. S. Gillmor, to whom is also owed the identification of W. Altar (see Coll-37/C.214-C.216). Appleton's own notes and drafts, which are almost never dated, appear at Coll-37/C.199-C.213. Correspondence and data exchanged with his colleagues appear at Coll-37/C.214-C.220.