As far as I knew from school biology, eyes are either compound or simple, and it is taken as read that they focus refractivly.
Now it turns out they can be a little bit compound, and incorporate reflective focussing.
“In nearly 500 million years of vertebrate evolution, and many thousands of vertebrate species living and dead, this is the only one known to have solved the fundamental optical problem faced by all eyes – how to make an image – using a mirror,” said Professor Julian Partridge of the University of Bristol.
The creature concerned is the deep water ‘four-eyed spookfish’.
While it looks like it has four eyes, it actually only has two, each of which is in two parts.
One half points upwards, the other ‘diverticular’ half looks like a bump on the side of the fish’s head and points downwards.
Apparently, the spookfish was first discovered 120 years ago, although no one noticed the reflective focus until Professor Hans-Joachim Wagner of Tuebingen University caught a live one near the Pacific island of Tonga.
Photographs looking down on the live fish produced eye-shine in the main tubular eyes that point upwards, but not in the diverticular eyes that point downward. Instead, these reflect light when seen from below.
Bristol’s Partridge saw sections of the eye and realised that the diverticular mirrors were something exciting.
They have tiny plates, probably of guanine crystals says Bristol, arranged into a multi-layer stack – the same sort of multi-later stack that makes the outside of silvery fish look silvery.
Computer simulation by Partridge show the internal spookfish plates are in just the right place to focus light on to its retina.
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