  We consider the harmonic measure on a disconnected polynomial Julia set in
terms of Brownian motion. We show that the harmonic measure of any connected
component of such a Julia set is zero. Associated to the polynomial is a
combinatorial model, the tree with dynamics. We define a measure on the tree,
which is a combinatorial version on harmonic measure. We show that this measure
is isomorphic to the harmonic measure on the Julia set. The measure induces a
random walk on the tree, which is isomorphic to Brownian motion in the plane.
