Published by phillauren.org
THE FOUR-EYED FISH (Anableps spp.)
"The fish does not actually have four eyes, but the eye is divided to allow the fish to see both above and below the waterline. A narrow band of epithelium divides the upper and lower halves of the eye.
Each half of the eye has a separate pupil, iris and cornea, but the retina is divided. Both halves of the eye use the same lens, with the upper light path traveling through the short axis of the lens, while the lower light path travels through the long axis.
This dual use of the lens corrects for the different behavior of light in air and in water, with the underwater lens face more strongly curved. The underwater half of the eye projects an image to the upper half of the retina, while the part of the eye above water projects to the lower retina.
The upper eye must be occasionally wetted to prevent dehydration, but when the fish is completely submerged, the image from the upper half of the eye is out of focus."
Here is another very informative article about the eye, diagrams and all:
We mentioned the Trilobite eye, and it's correction for spherical aberration by the use of calcite lenses in the ommatidia.
Here now is another remarkable eye. IT CORRECTS FOR seeing in the water, and seeing out of the water, and can apparently see both AT THE SAME TIME. And obviously, makes sense of what it does see!
I wonder what Dawkins would say about this one.
1. Both types of eye are built in to a single eye structure
2. The fish obviously has stereoscopic vision
3. So it has the necessary nervous structure, and instinctive behaviour to match
4. Most remarkably of all, the eye corrects for above water, and underwater viewing. The refractive indices of the two mediums are widely different.