Copy letter from Robert William Frederick Harrison, Assistant Secretary of the Royal Society to F Stanley, 71 Hornsey Rise, N 'In reply to your letter of the 4th instant, the paper you refer to is probably contained in substance in: Poynting, JH; "A graphical method of explaining the diffraction bands at the edge of a shadow" 91890) Bimingham, Phil Soc Proc. 7(1889-91). 210-219. "A method of making a half-shadow field in a polarimeter by two inclined glass plates", Brit Ass Rep, 1899, 662-663.' Our references do not go beyond 1900. You would find other useful references in section 4000 of the Royal Society's Subject Index to Physics, PArt 1; and in th Subject Index (PHysics) of the International Catalogue of Scientific Literature.' 6 August 1915