When Jack accused Piggy of not helping much with the building of the fire in chapter2: Fire on the Mountain Simon defended Piggy by mentioning that they had used Piggy's spectacles to light the fire so he had helped in that way.
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Piggy is the voice of reason in chapter 2 because he insists that they make a fire so they can be rescued. Also in this chapter they use his glasses to light the fire.
At page42, Simon commented on Piggy. "We used his specs, he helped that way." shows that Simon see the best out of people.
Ralph wants to get Piggy's specs back and Piggy wanted to kill Jack there and then.
They took Piggy's glasses, and fire from Ralph's camp.
It is Jack's idea to use Piggy's glasses to start the fire. Jack steals the glasses right off of Piggy's face without even asking him first.