The works of William Rowan Hamilton in geometrical optics and the Malus–Dupin theorem
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 (i.e., admits orthogonal surfaces). Moreover, the 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 that led Hamilton to introduce his characteristic function in optics, then in dynamics under the name of action integral.