An engraving from the eighteenth century

 An engraving from the eighteenth century

An engraving from the eighteenth century

It depicts a sumptuous banquet with a large number of wealthy visitors, as well as the unusual illumination of the building, which does not appear to be lighted by candles or oil lamps.

So it begs the question of using atmospheric electricity or perhaps the lights of St. 

Elmo, which could have been used to illuminate structures in the past.

 Well, it appears that St. Elmo's lights just learned to receive artificially in the twentieth century. 

It begs the question, what exactly is it?

Source : Forbes