The gravitational lens as an astronomical diagnostic

How a point gravitational lens images a diffuse background source is examined, and it is shown that under a wide range of circumstances the image contains a 'gravity ring.' The chief criterion for producing this phenomenon is that the angular scale of brightness variation in the source be smaller than the angular radius of the cone of inversion of the image. The observation of gravity rings, which should soon become resolvable with very long baseline radio interferometry, would provide a useful astronomical diagnostic. It would give an independent (or unique) estimate of fundamental quantities such as the mass or distance of the deflector star and the distance of the source. A variety of astronomical examples, such as nebulae and pulsars, where the phenomenon may be especially useful is described