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First, here is a picture with all the parts. They fit together like a puzzle, and are not glued or taped any further. This way I can store it without taking up a lot of space. On the bottom right is the base, made out of corrugated cardboard (my favorite material). Basically a rectangle with strips folded and glued to the side edges. The ring in the center holds the eyeball in place. Near the center of the photo is a square piece of cardboard which holds the lens. This lens is borrowed out of the magnifying glass visible in this photo.Cut out a hole the size of the lens, and glue a small cardboard ring on one side, and a half-ring on the other side. The inside diameter of the rings is a tiny bit smaller than the lens. This way you can get the lens in and out easily. The third piece of cardboard has a hole, covered by the thinnest white paper you have in the house, like tracing paper or tissue paper, but cheap typing paper works fine too. This piece represents the retina. I also tied a bundle of phone wire to the back of this piece, for the optical nerve. Also shown are 3 sheets of paper, which can be hung in front of the lens. They have holes punched in the corners, corresponding to two toothpicks glued in the cardboard of the lens holder. The sheets are the iris (colored, with a small hole in the middle), the cornea (big hole, covered with saran wrap, or a blank overhead transparency sheet), and the eyelids (colored and cut out). Finally you can see the half-spheres which make up the eyeball.

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Here are the first pieces put together. Not clearly visible is that on the inside surface of the side strips of the base are glued small pieces of cardboard with 1/8" gaps, into which the other two pieces of cardboard slide and fit snugly. First fit the piece with the lens. Now you have to decide how far back you should place the retina. Point the lens at a distant, bright object, like a tree across the street. As you move the retina back and forth you'll find a spot where the distant object is in sharp focus on the white paper of the retina. That is where you want to mount the back cardboard. In the picture, I have pointed the assembly at a window, and you can see the image of the window on the retina. Now that this distance is established, you can make the spere of the eyeball. I used the balloon-and-newspaper method. Here are a few tips: the colored balloons from my local party store don't blow up to form nice spheres. However, their clear balloons, which are more expensive, form almost perfect spheres. Rip the newspaper into about 3/4" x 1' strips. Make some glue by boiling flour and water. Now I wet a strip of newspaper, then lay it on the balloon, then brush it with the glue paste. If you do it in this order, the paper will not crinkle, and the sphere will turn out smooth. Lay the strips randomly till you run out of paper. This will take a while to dry, maybe more than a day. Then I carefully mark the center line and cut the ball in half. Mark and cut holes for the lens and retina. A few tabs (see photo) allow you to fit the two halves back together. I painted the inside orange and the outside white. Here is the whole thing assembled:

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In this photo you can see the toothpicks that hold the iris etc.

One more thing you can do is to make a water balloon out of another clear balloon, representing the vitrious humor. This should fit inside the sphere. In the end, I left the vitreous humor balloon home, since it would turn everything into a horrible mess if it broke.

You can observe that the image on the retina is fainter with the iris in place. Less light is transmitted to the retina.