"objective correlative"

The objective correlative is a concept in English literature invented by T S Eliot.  It refers to something in the external world that acts as an equivalent for things in the mental world.  So, for instance, any events or objects that serve to symbolise or evoke an emotional response which is not directly expressed.

In this case, the (imaginary) debauched and depraved activities happening in Ware Common are the external manifestation of all the debauched and depraved thoughts in Mrs Poulteney's subconscious.