At that ball, while I had the honour of dancing with you, I was first made acquainted, by Sir William Lucas's accidental information, that Bingley's attentions to your sister had given rise to a general expectation of their marriage.
He spoke of it as a certain event, of which the time alone could be undecided.

From that moment I observed my friend's behaviour attentively; and I could then perceive that his partiality for Miss Bennet was beyond what I had ever witnessed in him.

Your sister I also watched.

Her look and manners were open, cheerful, and engaging as ever, but without any symptom of peculiar regard, and I remained convinced from the evening's scrutiny, that though she received his attentions with pleasure, she did not invite them by any participation of sentiment.

If _you_ have not been mistaken here, _I_ must have been in error.