As long as Mr. Knightley remained with them, Emma’s fever continued; but when he was gone, she began to be a little tranquillised and subdued – and in the course of the sleepless night, which was the tax for such an evening, she found one or two such very serious points to consider, as made her feel, that even her happiness must have some alloy. Her father – and Harriet. She could not be alone without feeling the full weight of their separate claims; and how to guard the comfort of both to the utmost, was the question.
– Jane Austen
Emma, Chapter 50. A number of metaphors describe the kind of euphoria Emma experiences spending time with her fiancé Mr. Knightley. Her emotions are compared to a “fever,” which is “tranquillised” a little by his departure, then she must pay the “tax” of enduring a sleepless night. The word “alloy” is used as another metaphor to describe how she feels that even her own happiness must have some flaws. She is thinking of the impact her engagement will have on Harriet and her father Mr. Woodhouse.