"The focus is the gathering together, or concentration, of the rays of light; and as every ray of the sun has heat in it, the concentration of the heat of all the rays at that point makes the paper burn."
"Then, the larger the glass,"—
"The convex lens, you mean," said Johnny.
"Yes: the larger the lens, the more rays of the sun would be brought together at the focus, and the more heat there would be?"
"Of course."
"Then, if I had a real big convex lens in my room, in front of the window, I could sit in the focus of it, in the winter, and keep warm without any fire."
"You could keep too warm, perhaps; for you know a sun-glass will burn your hand: but even if the focus would be just right on a winter day, the light would be too bright for your eyes; and sometimes the sun wouldn't shine in at your window, and you would get very tired of sitting in one place. Besides, a convex lens of that size would cost a great deal more than a very nice stove and ever so much coal. So I guess convex lenses will never take the place of wood and coal, which are the best provisions the sun has made for warming people, that we know of yet, when it is not nearly enough over their heads to warm them itself, or when its rays are shut out by bad weather."
"I don't see what the sun has to do with wood and coal," replied Felix, sitting down by the table, and holding the magnifying-glass over Johnny's books and various other objects on the table.
"Why, the sun has stored up a lot of its heat in wood and coal, and all those things which we call combustibles."
"Come, now! none of your fooling," said Felix, staring at Johnny incredibly: "there isn't one bit of heat in wood or coal till you burn them."