Part of the time Paul was facing a picture on one wall and the next minute his back was toward that picture and he was looking at another picture on the opposite wall, across the lamp. These two drawings show how Paul faced the two pictures one after the other. [Illustration: Night on Paul's Face] [Illustration: Day on Paul's Face] "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. "Yes, that's why the moon and the stars come out only when it gets dark," said Uncle Henry. "You see the earth turns round and carries us to its dark side, the side that is away from the sun. We say 'The sun has set.' Then when the sun glare is gone from our eyes we can see the sky-pictures, just as Paul sees one picture better with his back to the lamp than he does the other when he has to look through the lamp-light toward it." "And the stars are in the sky all day long, whether we see them or not?" asked Paul. "Certainly," said Uncle Henry. "If you could look up at the sky from the bottom of a very deep well, or a tall chimney, so that the sun-light was kept out of your eyes, you could see the stars shining in the daytime. There is a long deep tunnel in the great pyramid of Egypt that goes up and out from the centre of its base toward its north side at just the right angle so that the ancient Egyptians could always see the pole star through it--no matter whether it was night or daytime. You see the pole star never rises or sets, because it is always right over the end of the axis that the earth spins on." This picture shows how the tunnel in the great pyramid always pointed to the north star because the tunnel is always parallel to the axis the earth spins on. [Illustration]