Dome!
And this one is fun for all you capable of normal convergence....look toward your nose till you see THREE images. Then study the middle image and see if you can get it into focus (hint: if you converge too far, you'll see four, so try to get the inner-most two images to merge into one). Yes, it requires some oculomotor skills (convergence, fusional vergence, etc etc), but it's worth the effort! See if you can do it....you should see a 3D image if you can!
