Nucleation has been studied on atomic scale for Ag deposition on a Pt(111) surface at low temperature (50-120 K) by means of variable-temperature scanning tunneling microscopy. From island density versus coverage data, the transition from the initial steps of nucleation to growth and coalescence has been studied as a function of temperature. The data show that dimers are stable nuclei for T less-than-or-equal-to 110 K, which facilitates quantitative comparison with rate equations from nucleation theory. The migration barrier E(m) = 157 +/- 10 meV is determined for Ag adatoms on Pt(111) and compared with theoretical values.