There must also be some means of adjustment to make a sharp image, or picture, on the retina, however near or far away the object may be from the eye. In a camera this is done by moving the lens and the screen closer together or farther apart and this is the purpose of the bellows of a camera. But the eye has a much finer and quicker adjustment than this for distance. The lens is so made that the front part of it can bend just as the distance changes. You have only to look at an object and the lens is adjusted without the slightest effort or knowledge on your part. You may wonder how light waves can pass through a substance as solid and as hard as glass or through the eye. You will remember I told you in the beginning of this chapter how ether got into every little space, even in metals and glass, as well as that it filled all the great space between the stars. We think of glass as being very solid, and it is solid enough to keep water or air in a bottle from getting out through its pores. But glass and the substance of which the eye is made are just about as full of holes as a sieve, but the holes are so very, very small you couldn’t begin to see them even with the aid of a high-power microscope, yet they are large enough for the ether to run through just as water runs through a sieve. When I tell you that waves in the ether which are sent out by the light of a candle or the Sun are only about 15 ten-millionths to 30 ten-millionths of an inch in length, and that the holes, or pores, in the glass and the cornea and lens of the eye, and which are full of ether, are much larger, you can readily understand that the ether waves which we call light can merrily pass through either glass or the eye and that there is nothing in the way to stop them. To sum up briefly how the stars shine, how light travels and how the eye sees we will start with the light of the Sun and say (1) That the Sun is made up largely of gases, and that (2) These gases are formed of various substances, and that (3) The gases produce terrific heat, which means that (4) The atoms or particles which form the gases are in violent motion or vibration. (5) These vibrations start out waves in the ether which travel out into space at a speed of about 186,500 miles per second. (6) On reaching the eye the waves pass through the lens and form a picture or image (7) On the retina, or screen of the eye, which is made up of the ends of nerves, and these vibrate just as the atoms of the gases in the Sun which sent out the waves vibrated, and finally (8) These nerve vibrations are sent over the optic nerve to the brain, where they take on the shape, size and colors of the Sun. =Reflection of Light.=—The flame of a match, or a candle, an oil or a gas lamp, or Sun, comet or meteor, produces its own light, and for this reason these bodies are called _self-luminous_—that is, they are themselves the source of light.