It is the back surface of the vitreous that is attached at the optic nerve. When the vitreous detaches, this looks like a ring.
Hmm, I think maybe I have one of these among my new floaters. The dominant new floater in my left eye's visual field is ring shaped and is seen at 9:00 O'clock and about 45 degrees off the optic axis. That corresponds to a position in the eye directly anterior to the "blind spot", where the optic nerve exits. The ring flops around a lot but the overall position has been pretty stable since it showed up a few days ago. Is this typical for a Weiss ring? If a Weiss ring shows up in the right eye, would it typically be seen at 3:00 O'Clock in that visual field?