Topic: yes but what is it?

[vidrioguapo]

A long shot, but could it be a glass insulator for old fashioned electricity pylons/telegraph poles?

Although because of its small size, more likely for a domestic property. I used to have a very old house in Spain and we had these on the front of the house where the electricity cable entered the property.

[peejyweejy]

What about an old Christmas tree light cover  :huh:

[peejyweejy]

Electric...that is. Possibly 1880's?

[Ivo]

I'd go with Bernard's ceiling or side light.  It does not look as if you could hope to put a wire around or over it - so I don't think it is an insulator. And it would drag down a Christmas tree because it is really heavy....