But how, do you say, can I see things right side up when they are upside down in my eye? This is a very good conundrum and it will keep a long time, till you are about seventy years old and have spare time to sit down and think about it.
Now you see how your eye is a camera; the pupil is the hole and the back of the eye, called the retina, is the ground glass.
But you will find that the camera you have just made does not show things distinctly and beautifully as the photographer's camera does; how can they be distinct in the eye then?
Because in the photographer's camera, in the hole is a lens, which is a piece of glass, shaped like a sun glass; and so in your eye just behind the pupil is a lens, not made of glass, but still almost as transparent as if it were. In order to see what effect this lens has, take your box camera, make the hole larger and put a lens in it; one of your magic lantern lenses will do; and if the lens has the right focus you will see the images sharp and distinct on your ground glass. The focus probably will not be just right, so make a paper tube, into which fasten your lens and slide the tube in and out of the hole until you find the right focus.
When you have got that right so that you see a boy on the sidewalk upside down and see his teeth when he laughs, put some small object, the little doll will do, about three feet in front of your lens, and you will find the image of it is blurred and indistinct, and that you must pull your tube out to get the focus on the doll; or if you had another lens of just the right shape to hold in front of your camera, you would with that get the focus on the doll.
FIG. 2.
Thus you can see how it is with your eye, and why you cannot see things distinctly held close to it. The lens in the eye can change its shape a little, so that it will focus objects a mile off, or ten inches off, but it cannot be pushed in and out like the tube in your camera. You can do this, however, if you take another lens and hold it outside your eye and let the light go through that first before it comes to the lens in your eye, and in this way you can get a focus in your retina, and the outside lens thus forms a part of that optical instrument called your eye. Does your grandma know that her spectacles are a part of the cameras that she calls her eyes?
How is it that a lens bends (refracts is the big word for it) the rays of light? You will learn by and by. You can see that it does so by a few experiments with your sun glass or any such lens. Hold it between the sun and a piece of white paper until the white spot in the centre is as small as you can make it. You will see that the rest of the lens casts a shadow although it is all glass; this is because the rays of sunlight that fall on the lens are all bent towards the centre, and so you have a small white spot on which is concentrated the light and the heat, and before you have found out how it is all done, your paper takes fire and the experiment ends in smoke.
Take another piece of paper, and when the white spot is at its smallest, measure the distance between the lens and the paper, and you will have the focal distance of the lens.