Where was I? Oh. Right. Harriet turns up with a letter in hand - a lovely, well-written marriage proposal from Robert Martin, full of sense and feeling, and Emma is momentarily speechless. Harriet shows the letter to Emma, which is a bit unusual, really - it's a pretty intimate sort of letter to be flashing around - and Emma is quite keen to read it. She immediately quips that Mr Martin's sister must have helped him with it, but later contradicts herself by describing the style of writing:
"No doubt he is a sensible man, and I suppose may have a natural talent for — thinks strongly and clearly — and when he takes a pen in hand, his thoughts naturally find proper words. It is so with some men. Yes, I understand the sort of mind. Vigorous, decided, with sentiments to a certain point, not coarse. A better written letter, Harriet (returning it,) than I had expected."Now we get to the tricky part. See, Harriet wants to know what to do next, and Emma immediately tells her to be quite clear in her refusal - thereby poisoning that particular well, since it's only after Emma's statement that she (and we) learn that Harriet had meant to accept him. Only Harriet is now swayed by Emma's disapprobation, and things rapidly get out of hand as Emma manipulates Harriet into deciding to turn down Robert Martin and builds her hope for a relationship with Mr Elton.
You can see this scene play out (and some of Mr Elton's pandering about Emma's artistry) in this bit from the recent BBC production of Emma. I'm sorry that embedding is unavailable for this video. If you watch it and haven't read on yet, stop at the 5:57 mark.