Ralph has called an assembly and this particular one is later in the day than any previous assembly, so, for that reason, the sun is at a different angle and lighting the faces of the boys from above rather than from below. Ralph realizes that the faces look different with this new perspective and he wonders about reality - what is it? He's been trying to keep the boys civilized and orderly and one of the reasons for this assembly is to remind them they must abide by the rules in so that they can remain civilized. This, to Ralph, is what is real, i.e., civilization is reality and reality is civilization. His awareness of the differences in the faces leads him to wonder if they can maintain civility. He can't quite articulate yet what he is wondering, but he does worry about the problem of Jack's growing popularity and fondness toward savagery. Ralph will come to see that civilization and reality are not the same thing, that indeed, civilization is a facade put on by people to hide their savagery. The different light perspective on the faces is like getting a glimpse of the savage behind the facade.