[WayneS]

I'd always heard that an "Einstein ring" is when you put black ink on the end of the eye-piece of a telescope so that the next guy who uses ends up with a black circle around his dominant eye.

That guy was such a practical joker

[Charles Martel]

I’m pretty sure that you’re thinking of the “Galileo Ring”, which failed to lighten the mood of the Roman Catholic Inquisitors.