Diver Eyes

I've always been fascinated by the diving birds ability to see while swimming under water. Here is a quick view into their eyes.
Humans and mammals have one upper and one lower eyelid, birds on the other hand have a third eyelid, which is called the Nicitating Membrane.
Instead of opening and closing up and down as do our eyelids, the nicitating membrane opens and closes sideways across the eye, it is used to keep their eye's surface moist and clean.
To help see underwater, many diving birds have a special window type area in the center of this membrane, it is like the goggles Michael Phelps would wear while racing for another gold medal, allowing them decent underwater vision.
Another interesting note on these bird eyes, is the size of their pupils, they are tiny, as you look through the following pictures you will see that their eyes are mostly a large iris with a very small pupil in the center.
I would imagine their pupil grows much larger as these birds dive deeper into darker depths of the water column, just as our pupils would do in dim light.
Click on the photos to enlarge, these birds have some awesome eyes.

White-winged Scoter have a beautiful pale blue iris.

Long-tail iris's can be from brown to pink

One of the most incredible eyes in the animal kingdom.

A dark brown iris, changing to a lighter brown with the tiny pupil.

Hooded Merganser.

Golden Eyes, but not Goldeneye.

The Horned Grebe has a two toned iris, red to yellow.

Land birds show a much larger pupil as seen in this

young Great-horned Owl.
Imagine this pupil in the darkest of nights.