The potential flow of an ideal fluid in the presence of an open spherical cavity, with a circular opening and enclosing a concentric sphere, is analyzed in the framework of the theory of dual series equations. It is shown that the flow equations can be reduced to a pair of dual series equations whose exact solution is presented in the form of a perturbation series. The solution inherently contains the correct behavior near the edge of the aperture, i.e., that required by Meixner’s conditions. Various properties of the flow, in particular its separation at the solid boundaries, are discussed and illustrated by numerical results.