is bertha the dark double of jane(eyre)
Since this is not my area of expertise, I searched Google under the key words "'Jane Eyre' Bertha" to get these possible sources:
(Broken Link Removed)
http://en.wikipedia.org/wiki/Jane_Eyre
http://www.sparknotes.com/lit/janeeyre/themes.html
http://www.jiskha.com/english/books_novels_and_plays/jane_eyre_more.html
http://www.novelguide.com/janeeyre/themeanalysis.html
Use "Find in Top Window" to search within each site for Bertha.
I hope this helps. Thanks for asking.