1. 1) I. Newton: “Opticks” (Dover, New York, 1952).
2. 2) W.B. Hardy and J.K. Hardy: The London, Edinburgh,and Dublin Philosophical Magazine and Journal of Science 38, 32 (1919).
3. 3) M.T. Kirk: Nature 194, 965 (1962).
4. 4) R. Gohar and A. Cameron: Nature 200, 458 (1963).
5. 5) A. Cameron and R. Gohar: Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 291, 520 (1966).