"But what makes them look so strange and lovely in this grotto?" asked Molly.
"Perhaps because most of the light that comes into the grotto shines through the water," said her father. "In some way the red and orange and yellow rays of the sunlight have been lost, but the blue and green and violet rays seem to be all here. So everything looks cool and blue instead of warm and rosy, as it does just outside."
"How was such a great cave ever made under this island?" asked May. "Did the Italians make it?"
"No, indeed!" answered the boatman. "The sea made it ages and ages ago. This cave used to be a resort for the Roman emperors and their friends two thousand years ago. One emperor about that time built wonderful palaces on the island, and on hot summer days he and his friends would come down into this cool grotto to swim and to rest. Since then the island has sunk a little, so the water in the grotto is higher than it used to be. But even now the walls of the cave are forty feet high and a hundred and seventy feet long. It is almost as large as a good-sized church, you see."
"But the door is only three feet high," said Molly. "What would happen if a big storm came up while we were in here?"
"We couldn't get out," said the boatman. "Boats are not allowed to come into the grotto unless the sea is perfectly quiet. When it is rough it is very dangerous to be in here. We are allowed to stay only fifteen minutes, and our time is up now."
So they tossed a shining silver piece to the little boy who had made them think he was a really, truly water nymph, and their boats slipped quickly through the narrow passage back into the bright Italian sunshine.
