Now when the waves in the ether sent out by the flame of a candle, or by the Sun, reach the eye, they pass through the cornea, then through the pupil, or hole in the iris, and finally through the lens which focuses the waves on the retina and forms the image there.
The different colors of light are caused by waves in the ether of different lengths; when very short waves strike the retina we say the color is violet; waves a little longer we call green and the longest waves which the eye can see form in our brains the sensation of red.
Waves in the ether which are longer than the wave-lengths the eye can see produce heat and when these waves fall on any part of the body the nerves detect them and we call the sensation heat. On the other hand waves in the ether which are too short to affect the nerves of the eye will impress a photographic plate.
The iris of the eye acts as a self-regulating shutter, which makes the hole, or pupil in front of the lens larger or smaller according to the amount of light which is needed to see an object well. If the light is strong the iris contracts, which means that the hole gets smaller and so cuts off some of the light. If the light is weak the hole gets larger and we say the pupil expands and this lets more light through the lens.
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