Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Transparent, Affectionate, Subtle
It's easy to forget that the full title of Jane Eyre is Jane Eyre: An Autobiography. But that "autobiography" bit is super-important when it comes to understanding the tone of this tome.
Because there is so much autobiographical material in Jane Eyre, it’s often difficult to separate Charlotte Brontë’s authorial tone from the narrative style of her protagonist. Often the author’s voice and attitude seem to get caught up in the story, making it more like a memoir than a novel. We’re calling this a "transparent" tone, where we seem to be looking straight through the author’s personality at the first-person narrator.
When something of Brontë’s own personality starts to come through, it’s usually in dialogue, when another character is talking to Jane. Mr. Rochester and Diana Rivers, for example, seem to be two of the characters who occasionally express the author’s attitude toward Jane herself.
For example, check out this exchange between Mr. Rochester and Jane:
"You are a beauty in my eyes, and a beauty just after the desire of my heart,—delicate and aërial."
"Puny and insignificant, you mean. You are dreaming, sir,—or you are sneering. For God’s sake don’t be ironical!" (2.9.19-23)
These ain't just the words of a dude hoping to sweet talk a woman into marrying him; they’re the author’s affirmation that Jane is beautiful and should have a greater sense of self-worth.
Diana’s and Rochester’s affection for Jane and their awareness of the passionate, fun-loving woman behind the repressed Lowood graduate provide a subtle commentary—it's not bashing us over the head—on Jane’s own narrative that seems to channel Charlotte Brontë’s own feelings.