The class HΦ of Φ-bounded
harmonic functions on Riemann surfaces first investigated by Parreau for the special
case where Φ is increasing and convex, was later characterized by Nakai in its
complete generality by assuming only that Φ was a nonnegative real-valued function
on [0,∞). In this paper we show that Nakai’s theory can be presented in the
axiomatic setting of Brelot. The theory of Wiener compactifications which is
indispensable in the study of potential theory on Riemann surfaces is extended to
harmonic spaces and shown to be equally useful in the potential theory
there.