To understand this, take out one of your eyes and look at it with the other one. You see that it is a little round camera; most boys have seen a camera and some boys can make one. The simplest way to do that is to take a box, say a cigar box (empty, of course); pull off the cover and fasten in the place of it a piece of ground glass if you have one: if not a piece of white letter paper, oiled, will do; bore a hole in the middle of the bottom with a small gimlet and your camera is done. Point the bottom with the hole in it out of the window, and throw a piece of cloth over your head and over the box, as the photographers do, to shut out the side light, but mind and not cover up the hole; look at the ground glass (or oiled paper) and you will see things upside down. (Fig. 1.) But what has it to do with my eye? you say. Why, your eye is just like it, only round, as in fig. 2. And if you hold a doll or anything else about ten inches in front of the eye you have taken out and look at the inside of it (the eye, not the doll) just as you look at the ground glass of your box camera, you will see the doll upside down on the back of the eye. 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. [Illustration: FIG. 2.]