In Lord of the Flies by William Golding, Ralph is gradually losing control and there's only Ralph, Piggy and Samneric left supporting Ralph, having been designated leader since the beginning because he was the one "with the trumpet thing," (ch 1)-the conch. Jack's "tribe" has stolen Piggy's glasses in order to make a fire, effectively, as Ralph puts it - "They've got our fire....They stole it!"(ch 11) The "curtain" that "flickered in his brain" is referring to Ralph as he berates Jack for not even asking to use Piggy's glasses to start their fire. Ralph doubts his ability to resolve the issues anymore and his lack of confidence shows in his manner. His air of authority and belief in what they were doing - starting a rescue fire, building shelter, etc. is revealed in his uncertainty - the flickering curtain is representative of this. He is losing focus and needs Piggy to prompt him more than ever.
Piggy still believes in the power of the conch and demands to get his glasses back. He wants to tell Jack what he thinks of him and, wonders, despite Ralph's warning that he will get hurt, "What can he do more than he has?" Piggy thinks he is ready to take on Jack and show him "the one thing he hasn't got" because the conch makes him feel - if only momentarily - that he has some power.
Piggy is suggesting then that, whatever Jack has taken from them, he cannot take the ultimate power away because he does not have the conch. Piggy thinks that the conch allows him to demand his glasses from Jack because "...you got to!"