What illusion does Pip experience in the brewery and what does he do because of it in Chapter 49 of "Great Expectations"?

While he is walking throught the brewery, Pip imagines that he sees Miss Havisham hanging from a beam in the ceiling.  Although he knows it is just an illusion, he is startled, and returns to Satis House to ease his mind and see Miss Havisham one last time. 

When he enters the room, he sees the woman in her ancient wedding dress, sitting before the fire with her back to him.  Suddenly her dress catches fire, and she rushes toward him screaming, engulfed by flames.  Pip instinctively grabs Miss Havisham and smothers the flames first with his overcoat and then with a tablecloth.  He wrestles with her struggling form until the fire is extinguished; the servants rush in, and a doctor is called. The doctor lays Miss Havisham on the table.  Although she is badly burned, her injuries alone are not necessarily life-threatening, but the doctor is worried more about the trauma to her mind.  It is only after everything is over that Pip realizes that his own hands have been burned (Chapter 49).