Cauchy never stopped being an engineer. In 1822 Claude-Louis Navier had presented to the Academy a theory of elastic solids based on forces between molecules, and Augustin Fresnel was explaining light as a transverse vibration of an elastic medium, the ether. Cauchy, a colleague of both at the Academy, followed this work closely. On 30 September 1822 he announced to the Academy a new theory, one that needed no molecules.