  We revisit the theory of the Kondo effect observed by a scanning-tunneling
microscope (STM) for transition-metal atoms (TMAs) on noble-metal surfaces,
including $d$ and $s$ orbitals of the TMA, surface and bulk conduction states
of the metal, and their hoppingto the tip of the STM. Fitting the
experimentally observed STM differential conductance for Co on Cu(111)
including both, the Kondo feature near the Fermi energy and the resonance below
the surface band, we conclude that the STM senses mainly the Co $s$ orbital and
that the Kondo antiresonance is due to interference between states with
electrons in the $s$ orbital and a localized $d$ orbital mediated by the
conduction states.
