This is a 19th century witch ball. I have a thing for mirrored glass where the silver has started to corrode...having a room filled with different types of old mirrors where the glass aspect is of varying quality is an interior design dream :-)
There are modern attempts at witch balls available, mainly on ebay but they are nowhere near as cool as the real thing...also people try and pass those glass blue and green balls that fishermen used in their nets and call them witch balls just to make some fast, dishonest cash.
A proper witch ball was hung in windows years ago and inside them there is lots of thin spindly glass threads like a netting which is supposed to catch the witch's bad spells or spirit vibes...sounds a bit like Aladin's lamp. I think most people who have clocked it when visiting (probably) think it is a large Christmas bauble or one of those things you can get at garden centres......