"I'll show you in a moment."
Just then the paper began to burn, and Johnny removed the glass.
"Why didn't you let it go on burning?" said Felix.
"Because, you see, it was blackening the paint on the sill, and might have burned into the wood if I had kept the glass there. Now, what made the paper burn?"
"Why, you held the glass so that it made a focus, and that made the paper burn."
"But why was there what you call a focus? and why did that make the paper burn?"
"I never thought any thing about that," replied Felix, looking a little confused. "Do you know the reason?"
"Oh, yes! I knew that the first time I ever saw a sun-glass: my father told me. Just look at the shape of the glass on both sides; it's convex, you see; that is, it rounds out toward the centre. The rounding of the glass causes the rays of light to strike it obliquely everywhere except in the very centre; and when a ray enters a transparent surface obliquely, or comes out of it obliquely, it is bent out of its course in a particular manner,—it would take too long to tell you exactly in what manner, although I can lend you a book that will show you exactly,—and in passing through this convex lens, the rays are all bent towards a point at a little distance from the glass, and exactly opposite the centre of it; so that, if you hold any thing that will burn easily exactly at that point where the rays all join together, their united heat is sufficient to burn the article. I know exactly why each side is made convex, and why the glass is so much thicker in the middle; but it would take a long time to explain it to you, although you could read about it all in fifteen minutes in one of those volumes of 'Science for the Young,' there on the shelf. Any time you want to look it up, I will show you the place."
"No: I don't want to be bothered with reading it. I guess what you've told me will do."
Johnny then held the glass over the paper again.