Piggy's specs serve two purposes early in the novel: Piggy seeing clearly, but also as a tool to light the signal fire.
As the novel develops, Piggy's specs become less powerful: once they are broken, Piggy cannot see clearly, and once they are stolen, they become a tool for feasting rather than to support the rescue effort.
'His specs - use them as burning glasses!' (Jack, ch2)
"Give 'em back! I can hardly see! You'll break the conch!" (piggy, ch2)
'Ralph moved the lenses back and forth till a glossy white image of the declining sun lay on a piece of rotten wood' ch2
'Piggy's glasses flew off and tinkled on the rocks' ch4
'Have some sense. What can Piggy do with only one eye?' (Ralph, ch6)
'Nowadays he sometimes found that he saw more clearly if he removed his glasses and shifted one lens to the other eye' ch10
'He was a chief now in truth... from his [Jack's] left hand, dangled Piggy's broken glasses' ch10
'You came sneaking up like a thief and stole Piggy's glasses' (Ralph, ch11)