"What are you studying?"
"I am studying about heat and light."
"What can you learn about those things, I'd like to know? When it's light, it's light; and when it's hot, it's hot; and when it isn't either of them, it's dark and cold."
"If folks didn't know any more about heat and light than that, you would have to go without a good many things you have now; for instance, there wouldn't be any machine-shops and railroads, and you couldn't have your picture taken."
"Why not?" said Felix.
"I can't tell you very well to begin with, any more than you could learn the back part of the arithmetic before you had studied fractions. But here is a magnifying-glass: we'll use it for a sun-glass."
As Johnny spoke, he placed a piece of white paper on the window-sill where there was a patch of sunshine, and, taking a magnifying-glass from the stand, held it above the paper in such a manner as to bring the rays to a focus.
"That is a regular sun-glass," said Felix: "I have had one many a time. It will burn the paper in a minute."
"It is something like a sun-glass," replied Johnny, "because it is a double convex lens."
"What do you mean by a double convex lens?"