I disagree that "the look" in this case meant that Emma was saying she found Mr. Knightley attractive. It was meant to invite him over to hear her joyful gratitude at his kind and noble act.

However, I don't deny that Emma may be feeling some attraction towards Mr. Knightley at thsi point- after all, she spends quite some time at the ball ogling him. She thinks that Mr. Knightley is as good looking as Frank Churchill, at least... ;-)

She was more disturbed by Mr. Knightley's not dancing, than by any thing else. There he was, among the standers-by, where he ought not to be; he ought to be dancing, -- not classing himself with the husbands, and fathers, and whist-players, who were pretending to feel an interest in the dance till their rubbers were made up, -- so young as he looked! He could not have appeared to greater advantage perhaps any where, than where he had placed himself. His tall, firm, upright figure, among the bulky forms and stooping shoulders of the elderly men, was such as Emma felt must draw every body's eyes; and, excepting her own partner, there was not one among the whole row of young men who could be compared with him. He moved a few steps nearer, and those few steps were enough to prove in how gentlemanlike a manner, with what natural grace, he must have danced, would he but take the trouble. Ch. 38