Emma's fanciful imagination had at the first mention of Mr Dixon's service to Jane at Weymouth -- saving her from falling overboard into the sea by catching hold of her habit -- suggested an improper attachment between JF and the newly wed husband of JF's best friend – and confirmed it to herself by the fact that Mrs Dixon nee Campbell “has no remarkable degree of personal beauty, is not, by any means to be compared to Miss Fairfax” – “Miss Campbell always was absolutely plain -- but extremely elegant and amiable”. (chap 19)

This fanciful and rather malicious idea is contradicted by the narrator in chap 20 --
"They [the Campbells and Jane Fairfax] continued together with unabated regard however, till the marriage of Miss Campbell, who by that chance, that luck which so often defies anticipation in matrimonial affairs, giving attraction to what is moderate [Miss Campbell] rather than to what is superior [Miss Fairfax], engaged the affections of Mr. Dixon, a young man, rich and agreeable, almost as soon as they were acquainted; and was eligibly and happily settled, while Jane Fairfax had yet her bread to earn."

Mr Dixon is rich and would have no need for the Campbells' "moderate" fortune -- the reason Emma gave for him marrying Miss Campbell instead of JF. The narrator's brief description of Miss Campbell is that of a very warm-hearted, amiable young lady -- she saw "Jane's decided superiority both in beauty and acquirements", but she had no feelings of envy and was warmly attached to her. Perhaps Miss Campbell's warm-heartedness and amiability was what led Mr Dixon to fall in love with her.

It would have been a girl of very poor character who would have "seduced Mr Dixon's affections from his wife", the friend who had been so kind to her! This was the "mischievous" idea that Emma's imagination at first suggested to her. When she first saw JF again, Emma was willing to acquit her of this, and believe only that JF had been "unconsciously sucking in the sad poison" of "successless love on her side alone". Emma felt some “softened, charitable feeling” towards JF, and wished she could find a young man to match-make her with.

But after "Jane had spent an evening at Hartfield with her grandmother and aunt, and every thing was relapsing much into its usual state. Former provocations re-appeared. The aunt was as tiresome as ever; more tiresome, because anxiety for her health was now added to admiration of her powers ... and Jane's offences rose again. They had music; Emma was obliged to play; and the thanks and praise which necessarily followed appeared to her an affectation of candour, an air of greatness, meaning only to shew off in higher style her own very superior performance. She was, besides, which was the worst of all, so cold, so cautious! ... suspiciously reserved ... Emma ... returned to her first surmises ... Mr. Dixon, perhaps, had been very near changing one friend for the other, or been fixed only to Miss Campbell, for the sake of the future twelve thousand pounds."

Emma's imagination reflects her poor and unjust opinions of both JF and the former Miss Campbell -- that JF would be capable of wishing to engage the affections of her best friend's fiance, and that the warm-hearted Miss Campbell, because she was comparatively plain, would be incapable of gaining a man's affection for herself.