A known number of atomic layers of one metal was deposited on the surface of a single crystal of another metal by evaporation in a high vacuum. Direct results on depth of penetration were obtained from measurements on electron diffraction as a function of the thickness of the surface layer. A silver film having a thickness up to 15 atoms does not form in a single crystal when deposited on the (100) face of a copper crystal. A monatomic layer reduces the maxima of the beams from the copper lattice by at least 70 percent for energies up to 300 ev. A number of foreign silver atoms equal to a few hundredths of that contained in a monatomic layer can be detected by this method. A silver film forms in a single crystal when deposited on the (100) face of a gold crystal. At least 50 percent of the maximum of each diffraction beam from a thick silver crystal is contributed by the surface atomic layer of silver, for energies as high as 300 ev, while at least 90 percent is contributed by a surface layer two atoms thick. A possible selective effect was observed for low voltage reflection from a film 2 atoms thick.

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