Just as the air is finer than water so the ether is a million times finer than the air. It is so fine that it fills up all the little spaces between the particles or atoms of water and of the air, and it penetrates in between the atoms of the densest metals and the hardest glass, and further, and still more wonderful, it fills all of the great space in which the planets and the stars are placed. Now when the particles, or atoms of gas are set in motion, or vibration by a flame of any kind, be it a candle or the Sun, little ripples or waves are started in the ether and if these waves are very short they affect the eye and cause the _optic nerve_ to vibrate exactly like the vibrations which are sending out the ether waves and these waves are carried to our brains and we _see_ the light. The way in which a flame sends out waves in the ether and is received by the eye, is shown in Fig. 138. [Illustration: FIG. 137.—SOUND WAVES IN THE AIR SET UP BY BELL.] It takes a ripple, or wave on the water started by the impact of a stone about one-half second to travel one foot. A sound wave, set up by the vibrations of a bell, or other sound producing device, travels through the air at the rate of 1,090 feet per second, while light and heat waves set up by the vibrations of a flame, the Sun or other hot body, travel through the ether at the rate of 186,500 miles per second. It must be plain now that if there was no ether connecting the flame, or the Sun with our eyes the flame, or Sun might continue to send out light and yet we could not see it. =How the Eye Sees.=—If it was not for our ears we could not hear a bell ring nor any sound, for though the waves in the air might still be sent out we would have no means of receiving them; again if it was not for our eyes we could not see a flame, the Sun or any other source of light and, what would be worse, we could not see an object by its reflected light, for though the waves in the ether would still be sent out we would have no means of receiving them. [Illustration: FIG. 138.—WAVES IN THE ETHER.] The eye is simply a camera on a very small scale, but what it lacks in size it makes up by the excellence of its operation. If you will set up a sheet of white cardboard on one end of your starboard, place a lighted candle at the other end and then hold your burning glass between the flame of the candle and the cardboard, as shown in Fig. 139, and do all this in an otherwise dark room, you will see a picture turned upside down, called an _inverted image_, of the candle flame on the cardboard. To get a sharp picture, or image of the flame on the cardboard screen you will have to move the lens toward and away from the cardboard, and this process is called focusing. If you will fix the lens in the front of a light-tight box and place a sheet of ground glass in the back of the box you will have a simple, though crude, camera.