  The large N limit of D3-branes is expected to correspond to a superconformal
field theory living on the boundary of the anti-de Sitter space appearing in
the near-horizon geometry. Dualizing the D3-brane to a D-instanton, we show
that this limit is equivalent to a type IIB S-duality. In both cases one
effectively reaches the near-horizon geometry. This provides an alternative
approach to an earlier derivation of the same result that makes use of the
properties of a gravitational wave instead of the D-instanton.
