"With a piece of ice which we shall cut out."
"Why, do you think—"
"Why not? We want to make the sun's rays converge to a common focus, and ice will do as much good as crystal."
"Is it possible?" asked Johnson.
"Yes, only I should prefer fresh to salt water; it is more transparent, and harder."
"But, if I am not mistaken," said Johnson, pointing to a hummock a hundred paces distant, "that dark green block shows—"
"You are right; come, my friends; bring your hatchet, Johnson."
The three men went towards the block which, as they supposed, was formed of fresh water.
The doctor had a piece, a foot in diameter, cut through, and he began to smooth it with the hatchet; then he equalized the surface still further with his knife; then he polished it with his hand, and he obtained soon a lens as transparent as if it had been made of the most magnificent crystal. Then he returned to the snow-house, where he took a piece of tinder and began his experiment. The sun was shining brightly; the doctor held the lens so that the rays should be focused on the tinder, which took fire in a few seconds.