It happens in chapter 11, when ol’ doc Frankenstein is meeting up with his monster-homie, and the monster is telling him about being out in the woods:
“Soon a gentle light stole over the heavens, and gave me a sensation of pleasure. I started up, and beheld a radiant form rise from among the trees.”1
Then, at the end of the chapter:
NOTE
1. The moon
There are only two other footnotes in the book, and those are for when the narrator quotes passages of The Ancient Mariner and Tintern Abbey and cites them. I want to find something “in the scholarship” about how that footnote came to exist. Maybe it was the the first instance of figurative language so Shelley felt she didn’t have a choice.