WHERE HAVE ALL THE LENSES GONE

We compute the probability that the extended emission of extragalactic radio sources is gravitationally lensed. This leads to the prediction of a new class of lenses - small ring images (mean diameters of ∼0″.7) embedded in much larger regions of radio emission. Three out of every four lenses in the MIT-Green Bank (MG) survey should be of this type. The current lens search based on the MG survey does not find this class because of the effects of resolution and selection criteria. These lenses in principle allow the lens potential to be studied on a wide range of scales, unlike the current radio rings which sample the lens potential only near the critical line. We suggest a mixed radio-optical survey for locating these lenses, and examine the ability of existing and planned radio observatories to study them in detail.