Hold a finger between a candle and the wall, and it will cast a
shadow of a certain darkness; then place another candle in the same
line with the other from the wall, and the shadow will appear doubly
dark, although there will be more light in the room than before. Then
separate the candles, and place them so as to produce two shadows of
the finger, one partly overlapping the other, and that part will be of
double darkness, as compared with the remainders.