A _convex lens_, see Fig. 146, is a lens which is thicker in the middle than it is at the edges. A convex lens is used for magnifying an object; or for forming an image so that it can be magnified by another lens as in a telescope, for forming an image on the screen of a camera, and for bringing the heat waves of the Sun to a focus, as with a burning glass. The point where the rays of light are brought together is called the _focus_. You can easily find the focus of a lens by holding a sheet of paper, or the hand under the lens and letting the sunlight pass through it; where the spot of light is smallest and brightest there is the focus. The distance of this point from the lens is called the _focal length_. [Illustration: FIG. 145.—PRISM FORMING A SPECTRUM.] [Illustration: FIG. 146.—CONVEX LENS.] A _concave lens_ is a lens which is thinner at the middle than it is at the edge, as shown in Fig. 147. It is used in small telescopes and opera glasses to turn the inverted picture formed by the convex lens around so that the object can be seen in its right position, as shown in Fig. 151. =Shadows.=—_Shadows_ are useful as well as sunshine, but shadows are such common, everyday things it seems almost useless to talk about them; still you may or may not know that there are different kinds of shadows. [Illustration: FIG. 147.—CONCAVE LENS.] Of course we all know that when a candle or a gaslight or the Sun shines on an apple or any other _opaque_ object—that is, an object that will not let the ether waves go through it—the light is cut off back of it and this dark space is called a shadow; this is also true when the Sun shines on the Earth, or on any of the other planets or their moons. There are always two parts to the shadow of an object unless the light is a mere point or the object is very close to the screen, or surface on which the shadow falls. The dark part of the shadow is called the _umbra_. The edge of the object where the light and shade run together and form a partial shadow is called the _penumbra_ and during a total eclipse this partial shadow surrounds the dark shadow, or umbra of the Earth or Moon. CHAPTER IX THE SPYGLASS OR TELESCOPE =The Boy Who Discovered the Telescope.=—Spectacles have been made and used for nearly a thousand years and the art of making lenses is very much older. A little over three hundred years ago there lived in Amsterdam, Holland, a spectacle maker named Lipperhey, and it is said of him that he made good lenses. There was apprenticed to this lens grinder a Dutch boy, and I am sorry I cannot tell you his name, for he was a boy who did things; but his name is not recorded, which is a shame, for if it had not been for this boy Galileo might never have had a telescope.