What sort of hallucinations plague Hester when she looks into her child's eyes?

From the text:

Once this freakish, elvish cast came into the child's eyes while Hester was looking at her own image in them, as mothers are fond of doing; and suddenly for women in solitude, and with troubled hearts, are pestered with unaccountable delusions she fancied that she beheld, not her own miniature portrait, but another face in the small black mirror of Pearl's eye. It was a face, fiend-like, full of smiling malice, yet bearing the semblance of features that she had known full well, though seldom with a smile, and never with malice in them. It was as if an evil spirit possessed the child, and had just then peeped forth in mockery. Many a time afterwards had Hester been tortured, though less vividly, by the same illusion.