On Mrs. Rochester

"Yet, whatever Charlotte's original conception of the character may have been, some men clearly still feel threatened by Bertha. In 1999, the distinguished politician Roy Jenkins described Margaret Thatcher as the 'great incubus of John Major's premiership, comparable with Mr Rochester's mad wife in Jane Eyre.' The first Mrs. Rochester, it seems, has resonance today as an emblem of power"

--Lucasta Miller, The Bronte Myth

Lucasta Miller's assertion about Bertha is intriguing.
I too confess to having varied reactions toward Bertha. When I was really young, I would be intimidated by the vision of her as an unsympathetic secret that threatens to destroy any peace the heroine (Jane) might harbor. Somehow, this impression of Bertha has changed little over the years, although I've definitely grown to sympathize with her character. She is not really evil. How can she be if she is mentally unstable? How can she be accountable for her actions if is not aware of them? Rochester claims that Bertha was unchaste from the first and this frustrates him greatly. But how can one continue to blame her if she progresses into madness? However unacceptable her habits might have been in the past, she deserves pity after her state of mental collapse. This is not to say that Rochester did not treat her well. He did not throw her out on the streets, and neither did he banish her to an asylum. He chose to look after her by means of employing a servent,Grace Poole.

However, the most important question is, How mad is Bertha?

Though she might be "mad", she still intimidates everyone around her, specifically Jane and Rochester. Is it Bertha's madness or her presence that threatens others? There is enough evidence to support both. Bertha's madness is striking because it also means that she is a danger to those around her, in case she commits a hasty action she has no knowledge of.

However, Bertha's presence is also intimidating because besides being Rochester's wife, she is also a foreigner, one with the potential of
charming [the] English gold from [the] British breeches' pocket". Thus she threaten's Jane desire of uniting with Rochester. She also challenges Victorian ideals of conjugal bliss, in that she raises questions about domestic harmony, equality of the spouses, divorce and property laws, and the "woman question".

In this sense, the powerless become powerful, because the others around them are faced with how to deal with them. The powerless challenge established norms and, through their inability, help open up much needed paths of Reform in Society.