Emma uses her imagination to elevate Harriet (fancied to be the daughter of a gentleman) and lower Jane Fairfax (Chapter 19: At this moment, an ingenious and animating suspicion entering Emma's brain with regard to Jane Fairfax, this charming Mr. Dixon, and the not going to Ireland...). Jane's history is revealed at the start of Chapter 20 but by the end, Austen gives us another glimpse into Emma's brain:

Why she did not like Jane Fairfax might be a difficult question to answer; Mr. Knightley had once told her it was because she saw in her the really accomplished young woman, which she wanted to be thought herself; and though the accusation had been eagerly refuted at the time, there were moments of self-examination in which her conscience could not quite acquit her. .... It was a dislike so little just -- every imputed fault was so magnified by fancy...

In short, she sat, during the first visit, looking at Jane Fairfax with twofold complacency; the sense of pleasure and the sense of rendering justice, and was determining that she would dislike her no longer. When she took in her history, indeed, her situation, as well as her beauty; when she considered what all this elegance was destined to, what she was going to sink from, how she was going to live, it seemed impossible to feel any thing but compassion and respect; especially, if to every well-known particular entitling her to interest, were added the highly probable circumstance of an attachment to Mr. Dixon, which she had so naturally started to herself. In that case, nothing could be more pitiable or more honourable than the sacrifices (governess) she had resolved on. Emma was very willing now to acquit her of having seduced Mr. Dixon's affections from his wife, or of any thing mischievous which her imagination had suggested at first.