What I was surprised to learn about the eye is that the area of foveal vision, the central part of the image that can see fine details, is about the size of a quarter held at arms length from your face. You can only ever clearly see a tiny dot, but your eyes dart around and paint a picture for your brain to show you.
Another interesting fact: every time your eyes move, you're blind for a couple milliseconds. But you never notice this because your brain fills in these gaps in time.
You can see the reason for this if you move your eye slowly (by following a moving object)— say, place your cursor under a line of text and move it across the screen, following it with your eyes. If you do this right, you can notice motion blur in the surrounding text.
Now look back at where you started, a movement that takes tens of milliseconds. Imagine how much blur that creates— that image must be completely useless. You're doing yourself a favor to ignore it.
Edit: Since I mention it, I've wondered before if there'd be any value to training yourself to move your eyes smoothly without following an object. This test suggests probably not; the image just isn't good for anything unless it's correcting for a moving object.
There's a lot of cool chemistry involved in the eye adjusting to darkness and light. My favorite fun fact is that as you adjust to darkness, the spectrum of light your eye perceives becomes slightly blueshifted: http://en.wikipedia.org/wiki/Purkinje_effect
