There are an infinite number of these for each azimuthally symmetry of the germane points: the zeroth one, which is actually a point, occurs where the change in the angle of the source trajectory is 0; the first one, which is the most prominent and observable, occurs where that change is pi; and so on.  
This phenomenon was first seen experimentally in observations of MG1131+0456. There are an infinite number of these for each azimuthally symmetry of the germane points: the zeroth one, which is actually a point, occurs where the change in the angle of the source trajectory is 0; the first one, which is the most prominent and observable, occurs where that change is pi; and so on.  First observed using the VLA in 1987 by Jacqueline Hewitt, these are named for the physicist who first derived their existence from general relativity. FTP, name the type of gravitational lensing that occurs as a result of the collinearity an observer, a lens, and a point-like source.
This phenomenon was first seen experimentally in observations of MG1131+0456.
Einstein rings