1. Bell J . S. 1953 Basic algebra of the strong focusing system. A .E .R .E . T /R 1114. L ondon: H.M . S tationery Office.
2. Proc. Roy;Crewe A. V.;Soc. A,1955
3. Proc. P hys;Le Couteur K .;Soc. B,1951
4. Proc. P hys;Le Couteur K .;Soc. B,1953
5. Nature;Pickavance T. G.;Bond.,1952