In chapter 11 of Lord of the Flies, Using context clues, what is Piggy's "wall of myopia?"

It's easy to assume Piggy is farsighted until, at the beginning of chapter eleven, it says: "Piggy sat expressionless behind the luminous wall of his myopia." The concave lenses needed to fix myopia are incapable of focusing light to a point, thus incapable of starting a fire. Whether Golding was unaware of that distinction or had myopia and hyperopia confused, the net result is a clear-cut case of Did Not Do the Research.