Piggy's glasses also represent intelligence and intellectual pursuit. Piggy is probably the brightest of the stranded boys on the island in Lord of the Flies, and his glasses help to set him apart from the others. Of course, it also gives him the power of sight, and when the glasses are taken from him, it robs him of one of his most important senses. He is left virtually helpless without them, which restricts his usefullness as Ralph's advisor. The glasses still serve useful to Jack's hunters when they are taken, but without Piggy's brains behind them, they are good only for starting a fire.
To underscore the point of Piggy's glasses representing intelligence, his glasses, which make him seem more adult along with his paunchy body and thinning hair, come to represent the ability of which a more adult intelligence is capable--namely reason. Thus, the breaking of the glasses, as symbolic of reason and rationality, represents the breakdown of civilized behavior in the boys. When the glasses are stolen, all rationality is taken away and anarchy with Jack and the sadistic Roger gains control:
The chief [Jack] led then, trotting steadily, exulting in his achievement. He was chief now in truth; and he made stabbing motion with his spear. From his left hand dangled Piggy's broken glasses. (Chapter Ten)
After the theft and gaining of the power to make fire, reason is blinded: "They've blinded me," says Piggy, who tells Ralph that he is going to go to Jack with the conch, "the talisman" of civilization and order, and hold it before him, asking to the return of his glasses:
Look, I'm goin' to say, you're stronger than I am and ....you can see...with both eyes. But I don't ask for my glasses back, not as a favor. I don't ask you to be a sport, I'll say, not because you're strong, but beacause what's right's right. Give me my glasses, I'm going to say--you got to." (Chapter Eleven)
In my opinion, Piggy's glasses symbolize science and technology. They are the things that can be used to make fire without having to rub sticks together. On this island where that is the only technology, really, that is available, the glasses represent the only real source of technological power.
Late in the book, the theft of the glasses by the savage tribe really shows that they have taken complete control of the island. They have been able to take control of science and technology and will now use it for their own ends.