What do piggys glasses serve as?

They are a symbol of order and intelligence, and a link to making fire for rescue. As they get slowly destroyed throughout the novel (taken, one lens smashed, then stolen) it shows the breaking down of order. However, he is short sighted, so his lenses would not have magnifiedd the sun and therefore not started a fire. In this case Golding has chosen to put symbolism above the reality of the story