The kind of lens we have been talking about is the _convex_ lens. "Convex" means bulging out in the middle. There are other kinds of lenses, some flat on one side and bulging out on the other, some hollowed out toward the middle instead of bulging, and so on. But the only lens that most people make much use of (except opticians) is the convex lens that bulges out toward the center. The convex lens makes a clear image and it is the only kind of lens that will do this. [Illustration: FIG. 78. Lenses of different kinds.] WHY YOU CAN SET FIRE TO PAPER WITH A MAGNIFYING GLASS. A convex lens brings light to a focus, and it also brings radiant heat to a focus. And that is why you can set fire to things by holding a convex lens in the sunlight so that the light and heat are focused on something that will burn. All the sun's radiant heat that strikes the lens is brought practically to one point, and all the light which is absorbed at this point is changed to heat. When so much heat is concentrated at one point, that point becomes hot enough to catch fire. _APPLICATION 36._ Explain why there is a lens in a moving-picture machine; why a convex lens will burn your hand if you hold it between your hand and the sun; why the front of a good camera is made so that it can be moved closer to the plate or farther away from it, according to the distance of the object you are photographing; why there is a lens in your eye. INFERENCE EXERCISE Explain the following: 211. Cut glass ware sparkles. 212. An unpainted floor becomes much dirtier and is harder to clean than a painted one. 213. If you sprinkle wet tea leaves on a rug before sweeping it, not so much dust will be raised. 214. Food leaves a spoon when the spoon is struck sharply upon the edge of a stewpan. 215. An image is formed on the photographic plate of a camera. 216. Ripples in a pool distort the image seen in it. 217. Cream rises to the top of a bottle of milk. 218. Your eyes have to adjust themselves differently to see things near by and to see things at a distance. 219. A vacuum cleaner does not wear out a carpet nearly as quickly as a broom or a carpet sweeper does. 220. You can see a sunbeam in a dusty room. SECTION 25. _Magnification._ Why is it that things look bigger under a magnifying glass than under other kinds of glass? How does a telescope show you the moon, stars, and planets? How does a microscope make things look larger? Everybody knows, of course, that a convex lens in the right position makes things look larger. People use convex lenses to make print look larger when they read, and for that reason such lenses are often called _reading glasses_. For practical purposes it is not necessary to understand how a convex lens magnifies; the important thing is the fact that it does magnify. But you may be curious to know just how a magnifying glass works.