From time to time it has been suggested that gravity on a sufficiently large scale might not follow an exact inverse square law. For example, in the nineteenth century this was proposed to explain the anomalous perihelion advance of Mercury (N.T. Roseveare, Mercurys Perihelion from Le Verrier to Einstein, Oxford, 1982) and more recently to account for the dynamics of clusters of galaxies without invoking dark matter (Milgrom, M., Ann. Phys., 1994, 229,384).