Letters to the Editors

I AMgrateful to Prof. M. Cotte for directing my attention to the fact that he has already published the explicit forms of the deviation due to the earth's magnetic field and discussed the order of the separation of the reflexion points and its effect on the relative fading of the ordinary and extraordinary waves. The forms of the deviation quoted by me were derived some years ago (though not yet published) from Booker's analysis. They are implicit in Booker's paper and were doubtless known to him when he originally discussed the phenomenon.