  The exact expression derived by Bougourzi, Couture, and Kacir for the
2-spinon contribution to the dynamic spin structure factor $S_{zz}(q,\omega)$
of he one-dimensional $S$=1/2 Heisenberg antiferromagnet at $T=0$ is evaluated
for direct comparison with finite-chain transition rates ($N\leq 28$) and an
approximate analytical result previously inferred from finite-$N$ data, sum
rules, and Bethe-ansatz calculations. The 2-spinon excitations account for
72.89% of the total intensity in $S_{zz}(q,\omega)$. The singularity structure
of the exact result is determined analytically and its spectral-weight
distribution evaluated numerically over the entire range of the 2-spinon
continuum. The leading singularities of the frequency-dependent spin
autocorrelation function, static spin structure factor, and $q$-dependent
susceptibility are determined via sum rules.
