Augustin Fresnel and François Arago are typically credited with jointly establishing the wave theory of light in early nineteenth-century France. Yet the two men, working in different traditions, brought to their collaboration vastly different conceptions of what light was and how it should be studied. This paper traces the work that went into co-ordinating these disparate approaches into a united front, as well as the dissolution of the alliance after 1821. Although the fruits of their alliance proved remarkably stable, in fact agreement between them was never more than partial.