On page 19 what is the rhetorical effect of the comparison between Nancy and the translucent like ale?

Sorry, you will need to quote the passage. Page numbers are seldom the same from publication to publication.

"But it was her eyes, wide apart, darkly translucent, like ale held to the light, that made her immediately likable, that at once announced her lack of suspicion, her considered and yet so easily triggered kindliness"

The author uses simile, "like ale", as well as vivid pronoun (describing her eyes) that reveal her personality.