A Simple Geometric Representative for $\mu$ of a Point

For $SU(2)$ (or $SO(3)$) Donaldson theory on a 4-manifold $X$, we construct a simple geometric representative for $\mu$ of a point. Let $p$ be a generic point in $X$. Then the set $\{[A]\mathrm{\mid }{F}_A^{-}(p)$ is reducible $\}$, with coefficient -1/4 and appropriate orientation, is our desired geometric representative.