These fires represent a pagan element and refer to a past that pre-dates Christianity in England. At the beginning of the novel, the tradition of having fires on the heath on the 5th November is described as dating back centuries before the Gunpowder Plot and is also associated with warding off the forthcoming winter. The light from the fire offers a resistance to the impending long dark nights.
Eustacia also uses a bonfire to signal to Wildeve and, therefore, uses the traditions of the surrounding area to her own benefit. The light from the fire is for her also a form of resistance against the environment she hates.
His blindness is literally brought about with the eye strain that developed through excessive studies. However, his inability to see what is evident is given a solid form when he is no longer able to see well enough to read. When he courts Eustacia and proposes to her, for example, it is apparent that she expects him to one day return to Paris, but his short-sightedness means that he overlooks her desires in favor of his proto-socialist principles.
As well as this landscape being a thematic concern, it also acts as a symbol of the past as it evokes a sense of timelessness. Before the land was farmed and before the advent of enclosures, such a wilderness would not have been a rarity and the evocation of this place acts as a reminder of how the land used to be.
It is also a means of understanding some of the main characters as Wildeve and Eustacia hate the heath as though it is a prison, whereas Clym and Thomasin love it as home.
The Return of the Native: Metaphor Analysis