  Starting from the type IIB Dirichlet 3-brane action, we obtain a Nambu-Goto
action. It is interpreted as the world volume action of a fundamental 3-brane,
and its target space theory as F-theory. The target space is twelve
dimensional, with signature $(11, 1)$. It is an elliptic fibration over a ten
dimensional base space. The $SL(2, Z)$ symmetry of type IIB string has now an
explicit geometric interpretation. Also, one gets a glimpse of the conjectured
self-duality of M-theory.
