  A Kondo peak in the differential conductance of a single-electron transistor
is measured as a function of both magnetic field and the Kondo temperature. We
observe that the Kondo splitting decreases logarithmically with Kondo
temperature and that there exists a critical magnetic field Bc below which the
Kondo peak does not split, in qualitative agreement with theory. However, we
find that the magnitude of the prefactor of the logarithm is larger than
predicted and is independent of B, in contradiction with theory. Our
measurements also suggest that the value of Bc is smaller than predicted.
