I'm always looking for new ways of decorating my yard for Halloween (from Pirate Ships to Ghostly Projections), but I am always disappointed that the fun only lasts a day. I wanted to build something that I can leave all of October and apply the "hollow face optical illusion". One great example and inspiration for this project is the Slime Monster with Following Eye Instructable, another is the Dragon example originally by Jerry Andrus.

The basic concept, is that your brain expects an eye to be convex (bulbous out) because you are (presumably) a human and we have evolved to recognize faces, an important skill in a society. Facial recognition is harder than you may think (only recently have computers been getting good at it) and your brain takes shortcuts to get the job done. So when you seen an eye shape, your brain says: "I know this! the eye is convex, and it is looking at you in a threatening or friendly way". In this case your brain has failed you, because it is actually concave, and the eye parts shift as you walk past it, it's glare appears to be following you.

***I have found that this illusion is more convincing on video (along with the other examples linked above): I think this must be due to the "flattening" of the image into 2D: forcing your brain to make some extra assumptions while watching it. I am just about the worst judge, because I definitely KNOW the actual shape. Please leave a comment to let me know how you perceive it. There is also a video in the last step of this instructable.

Step 1: Supplies and Testing

I actually had everything on hand for this project, including an eager helper. You probably won't be doing the exact same setup as myself, so I recommend testing paints, and materials before going full blown.

4'x8' of white coroplast (or 2 x 2'x4' and one 4'x'4'): I had harvested some election poster after the last election for project such as this, and was happy to use it.
Poster paint (black and iris color)
Sharp utility knife
Set square
Tape: I used hockey sock tape because it is flexible an easy to use and painted over nicely. I would try other types of tape, but test the paint out first.
Glue gun
Leaves

Step 2: Cut Triangles

I used two sheets of 2' by 4' to make all the triangles that are assembled to make the cone. I cut these sheets of coroplast into 14 identical isosceles (2 equal sides for those of us not in high school) triangles (8" and 2 x 25"). If you measure out the 8 inch lengths along one of the long edges and space the peaks 8 inches apart (4 inch offset of course), then all your leftover pieces are actually the same as the ones you are cutting out!

To cut out the pieces, make sure you wear gloves, firmly hold the set square against the coroplast. Then, slowly and firmly run the utility knife along the set square. This is not a kid job for obvious reasons, please be careful and pay attention to the path of the knife (your knee, your other hand, your child...). You may need to run it twice: take your time, if you diverge, or displace the set square, you may ruin two triangles.

If you change the dimensions of the triangles or use a sheet that is malleable enough to fold into a cone, you should aim for about 1/4 of the circle to be missing.

Step 3: Tape and Paint

My helper was eager to start painting right away so I set her up with half the triangles in the first picture . The tape I used is hockey sock tape, which is rather rubbery and became practically invisible once painted. I elected to paint the finished side of the eye because it ensured a flexible and seamless finish.

I used a ruler to make a mark on each triangle at 7 inches from the center and 15 inches from the center. I then connected each mark to make a circle. The inner circle was painted black and the outer circle can be painted any color (my helper was fond of purple). Note that I eventually expanded the outer circle to about 18 inches from the center and removed the cartoon reflection spot. For the iris, we noticed that smearing a thin coat of paint gave a nice radial texture, just like the real thing.

Step 4: Eye Socket

I used the 4 x 4 foot piece of coroplast to make the eye socket. Once the "eye cone" is closed, it is 3" across, and you want the "socket" to partially cover the eye hole, so start by making a few marks showing where the edges of the eye would be located. I decided to leave about 3 inches around the edge of the eye hole, and make a socket shape somewhat lemon shaped and symmetric.I tried to leave at least 1.5 inches at the right and left edges to ensure I didn't have any exposed edges and keep some overhang.

After marking up the desired shape, carefully cut out the eye shape with a utility knife.

Step 5: Close Eye and Tape It Into the Socket

You'll need a friend for this part. Carefully close the cone (painted side inward) and place a piece of tape on the outside as best you can. With a couple extra hands, carefully flip the cone up-side down onto the socket. The cone is relatively floppy until you've fastened it to the socket, so try not to fold it completely and put undue stress on the joints and paint. Move the cone to the center of the socket: drag it one edge till you see an opening, then the other extreme then drag it half way back again. Now use, any kind of wide tape to fasten the cone to the socket.

Step 6: Leaf Up Your Monster

I used some ferns from the forest out back, cut off one side of the leaves, and hot glued them on both edges to make eyelashes. To camouflage the rest of the socket, I hot glued some leaves that we collected from the forest floor.

You will need a whole bunch of leaves to cover this giant eye. My neighbors were more than happy to donate. I used some branches to make an arch as a brow and piled some leaves onto it.

It is hard for me to tell if the illusion is any good because I made it... Please leave your comments below.