  Donaldson has shown that the moduli space of monopoles $M_k$ is diffeomorphic
to the space $\Rat_k$ of based rational maps from the two-sphere to itself. We
use this diffeomorphism to give an explicit description of the bundle on
$\Rat_k$ obtained by pushing out the index bundle from $M_k$. This gives an
alternative and more explicit proof of some earlier results of Cohen and Jones.
