but it was when they finally turned their eyes to the sky that another colour , even if no more than a hint of it , seemed the surest promise of the fabled sun : for though a layer of pale ashen clouds covered most of it , there were small narrow rifts in the grey that opened up here and there as fast as they closed , through which they could glimpse a pale blue dome , its light faint yet certain as a stubborn spark in a hearth full of cinders .