The ray equations are derived from geometrical considerations and ambiguity of sign that has appeared in previous papers concerned with acoustic refraction by vortices is thereby avoided. The antisymmetry of the scattered field about a line through the acoustic source and the centre of the vortex is investigated to first order in the Mach number of the vortex. This antisymmetry is shown to exist, which is consistent with results obtained by methods that do not depend on ray theory.