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This communication deals with the application of a transfer-matrix strategy for the quantitative evaluation of the tunnel current in a scanning tunneling microscope (STM). The image given by a simple atomic-size object deposited on a metal surface is specifically examined in both modes of STM operation namely the constant-height and the constant-current modes. The two-dimensional corrugation induced at low temperature by Xe atoms physisorbed on an otherwise clean, unreconstructed Ni (110) surface is studied in detail. It is shown that the simple consideration of the elastic scattering of electrons by the three-dimensional potential barrier between the tip and the metal substrates provides a quantitative description of the images produced by the instrument: (1) the Xe atom appears as a conic protrusion, approximately 7 A wide, with a corrugation 1.3 A high; (2) in Xe clusters, each adjoining atom is resolved, with a shape in full agreement with experiment. In order to obtain correct quantitative results, image-charge corrections to the potential cannot be neglected.