If the function $\psi(x, y, z)$ is harmonic in the half space $x>0$, and if on $x=0$, $\psi=1$ inside a closed curve $C$ and $\psi=0$ outside $C$, prove that $2 \pi \psi(x, y, z)$ is equal to the solid angle subtended by $C$ at the point with coordinates $(x, y, z)$.