“The common eel dwells in both fresh and salt water, among stones, in mud, at times even in drying air. Its eyes roll, well protected, beneath tough, transparent picture windows, clear places in its head skin.

Anableps is a Central American fish that “rides at water level, its bulbous eyeballs separated into halves adapted to vision in air and water. These diverse media demand two focal lengths from a single lens.” This is no problem to the anableps because he has “an egg-shaped bifocal lens. Air-vision rays pass through the short dimension; water-vision rays through the long.”