What is Jack's "dirty trick"?

Jack steals Piggy's glasses..... according to Ralph, the theft of the glasses was a dirty trick.

"You pinched Piggy's specs," said Ralph, breathlessly. "You've got to give them back."

"Got to? Who says?"

Ralph's temper blazed out. "I say! You voted for me for chief. Didn't you hear the conch? You played a dirty trick-we'd have given you fire if you'd asked for it--"

The blood was flowing in his cheeks and the bunged-up eye throbbed.

"You could have had fire whenever you wanted. But you didn't. You came sneaking up like a thief and stole Piggy's glasses!"