The refracting superposition eye has a gap between the lens and the Rhabdom.

http://en.wikipedia.org/wiki/Eye

The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are.