Harry Potter Question (Book 4)?

The foe glass mirror tells one when enemies are coming near. When they are very (dangerously) close, one can see the whites of their eyes. The function of this in the story is that Harry finds the fake Mad-eye as a foe with this glass, and mad-eye (fake one) could tell when one of the good people (enemies to him), such as the teachers, were coming. (I'm pretty sure-haven't read this one in a while)