These two drawings show how Paul faced the two pictures one after the other.
“Now tell me,” commanded Uncle Henry, “which picture you see the plainest—is it the one you see when your back is to the lamp—or is it the one you see when you face the lamp, and look across it toward the picture on the wall beyond?”
“The lamp is so bright without a shade that it blinds me when I try to see the picture beyond it,” said Paul.
“Oh, I see! I see!” said Betty, beginning to hop up and down. “Can I tell, Uncle Henry?”
“Surely,” laughed Uncle Henry, “what do you see?”
“When Paul faces the picture with his back to the lamp,” said Betty, “it’s night on his face, and day on the back of his head! Is that right?”
“Yes, go on,” encouraged Uncle Henry.
“And so he can see that picture better, ’cause the lamplight isn’t in his eyes. But when he faces the lamp and looks across it, then it’s day in his face, and night on the back of his head, and he can’t see the picture beyond the lamp very well, ’cause the sun-lamp shines in his eyes.”
“So that’s why we can only see the stars at night!” said Peter.