  Applying the techniques of an earlier paper with Frenkel, we develop a
geometric realization of spin representations and Clifford algebras. In doing
so, we give an explicit parametrization of the irreducible components of
Nakajima varieties of type D in terms of Young diagrams. We explicity compute
the geometric action of the Lie algebra and are able to extend the geometric
action to the entire Clifford algebra used in the classical construction of the
spin representations.
