By "the spell," we are accustomed to hearing the spell which binds him in secrecy to the silver, and indeed he has just admitted his theft. But I also think that "the spell" here means his love for Giselle, and the ideal of utopian socialism. By declaring himself a thief, he renounces that ideal and binds himself to the laws of society. It is this meaning, I believe, that causes Giselle to suddenly moan, and Nostromo, casting his eyes "down" (as if from the height of the ideal), to lose sight of her. Note below, where Nostromo says that the spell of the silver still holds him.