Make a prediction about the spreading of the blindness.

What's this, he thought to himself,
and suddenly he felt afraid, as if he himself were about to
turn blind any minute now and he already knew it. He held
his breath and waited. Nothing happened. It happened a
minute later as he was gathering up the books to return
them to the bookshelf. First he perceived that he could no
longer see his hands, then he knew he was blind.