  The works of William Rowan Hamilton in Geometrical Optics are presented, with
emphasis on the Malus-Dupin theorem. According to that theorem, a family of
light rays depending on two parameters can be focused to a single point by an
optical instrument made of reflecting or refracting surfaces if and only if,
before entering the optical instrument, the family of rays is rectangular
(\emph{i.e.}, admits orthogonal surfaces). Moreover, that theorem states that a
rectangular system of rays remains rectangular after an arbitrary number of
reflections through, or refractions across, smooth surfaces of arbitrary shape.
The original proof of that theorem due to Hamilton is presented, along with
another proof founded in symplectic geometry. It was the proof of that theorem
which led Hamilton to introduce his \emph{characteristic function} in Optics,
then in Dynamics under the name \emph{action integral}
