The Wall and Four Hats

Four people are each given a hat. Two hats are black and two are white, and everybody is told this.

Three of them — call them One, Two and Three — stand in a line, all facing the same way, towards a solid wall. One is nearest the wall and can see nothing at all. Two stands behind One and can see One's hat. Three stands at the back and can see the hats of both Two and One.

The fourth person stands on the far side of the wall, facing it. He can see nobody, and nobody can see him.

Nobody can see their own hat. Nobody may talk, signal, or turn around. The moment anyone is certain of their own hat colour, they must call it out.

A long silence goes by. Then somebody calls out. Who is it, and how do they know?