Divination by means of mirrors.
“Mirror, mirror, on the wall. Who is the fairest of them all?” At some time or other, almost anything you can think of has been used to foretell the future, from straws on a red-hot iron to disembowelled chickens. But the mirror, that most strange thing that allows not only reflection but self-reflection, has always been special. As a result there are many superstitions about mirrors — such that they must be covered or removed after a death to prevent the soul of the dead person from being stolen. In part it also explains why it is considered bad luck to break one (until modern times, they were also rare, so breaking one really was bad luck).
There are records from many ancient civilisations of mirrors being used for magic, and some not so ancient: John Dee, the sixteenth-century English magician of the royal court, had a mirror made of a highly polished piece of coal. Fortune-tellers and magicians would use such stones, or perhaps polished metal mirrors or reflections in bowls of water to answer questions or predict the future. The word comes from the Greek word katoptron for a mirror, plus manteia, divination. The same root appears in catoptrics, the part of optics that deals with reflection. (worldwidewords.org)