I learned something today.

[billyskank]

When you quickly swivel your eye to gaze in a different direction, your brain edits out the blurring that your eye actually sees while it's turning.

What's more, there is a network of capillaries over your retina, which would be visible, but your brain edits that out as well. But when you look at a blue sky, you can sometimes see white blood cells moving through the capillaries... this effect is called Scheerer's Phenomenon. It's because the blue light passes through the white cells but is absorbed by the more numerous red ones, and so the white cells appear as little white dots moving quickly along twisting paths.

And I'm not done yet... sometimes if you see little transparent blobs slowly floating across your vision - these are actually impurities in the vitreous humour. They're called, unimaginatively, "floaters".