A critical review is presented of recent progress in classical diffraction theory.Both scalar and electromagnetic problems are discussed. The report may serve as an introduction to general diffraction theory although the main emphasis is on diffraction by plane obstacles. Various modifications of the Kirchhoff and Kottler theories are presented. Diffraction by obstacles small compared with the wavelength is discussed in some detail. Other topics included are : variational formulation of diffraction problems, the Wiener-Hopf technique of solving integral equations of diffraction theory, the rigorous formulation of Babinet's principle, the nature of field singularities at sharp edges, the application of Mathieu functions and spheroidal wave functions to diffraction theory. Reference is made to more than 500 papers published since 1940. Q 1. INTRODUCTION, SCOPE AND LIMITATION OF THE REPORT G L A N C E through the scientific literature of the last decade reveals an increasing interest in diffraction theory. Undoubtedly the impetus to

conclusive, since the differences in spacing between successive elements are about the same as the experimental uncertainties of electron energies. Tentatively, as indicated above, the 52-kev Y-ray is assigned to the /S~-transition and the 38-kev 7-ray to the electron-capture transition. This leaves unaccounted for the origin of the x-rays of americium, so if the above assignments are to be taken seriously, the conversion electrons accompanying the isomeric transition must lie among the Auger electrons. There is some evidence that this is the case.Lead and copper absorption curves showed no hard 7-rays or K x-rays and only the 50-kev soft 7-ray. When compared with the abundance of the conversion electrons, this 7-ray appears to be about 50 percent converted.From arguments (not all consistent) based on relative abundances of x-rays, conversion electrons, and the /3~-particles, Am 242m appears to decay about 60 percent by ^"-emission, 20 percent by L-electron capture, and 20 percent by isomeric transition. All three of the modes of decay give rise to L-series x-rays which, when properly assigned and abundances measured, should aid materially in arriving at a decay scheme and in shedding light on the nuclear processes which result in the particular x-rays of this interesting nucleus.The /3~-particle of the ground state of Am 242 has also been measured, but the accuracy of the end point has not yet been determined with desirable accuracy. The value obtained is 580±30 kev which is consistent with the supposed 52 kev associated with the isomeric transition of Am 242m .

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