SARAH:  Hamlet is here using an image of heaven personified as a face, looking down upon the "solidity and compound mass" of the earth and seeing Gertrude's immoral act.

RALPH:  This "face of heaven" is showing two reactions:  it glows — it's blushing with shame, because it is embarrassed by what it sees; and it has a tristful visage, or a sad expression, as if doomsday was approaching, because it is thought-sick — as if it had gotten sick just by thinking about what Gertrude has done.