"Don't you see that if you had a candle—or even a lamp as big as this one—shut up in a glass case, the light wouldn't do much good? Most of it would be wasted on things very near the lighthouse, instead of getting out to the horizon."
"Oh, it's magnified, of course," said Joan; "I do know that."
"More than magnified," Jim corrected. "You see, you must concentrate the rays and throw them as far out to sea as possible, if they're to be of any use. Well, here's a cylindrical lens, with the lamp in the middle; that lens refracts,—bends all the rays that go through it so that they shoot straight away from the lantern."
"Oh, I see," Joan said.
"But," pursued Jim, "how about all the rays that get outside the cylinder at the top and bottom? They can't be wasted, lighting up the floor and ceiling!"
Joan gave up the solution.
"Well, behold! Rings of separate prisms"—he indicated them—"above and below the central refractor; bigger rings nearest the lens, smaller ones farthest away. The three surfaces of each prism, combined, twist the ray, reflect
