Lie back on a cloudless day and let your eyes rest on a deep blue sky. As you relax and stare at the sky, you should begin to see faint dots of light moving quickly around. It may take ten or fifteen seconds before you begin to see the dots. Or they may look like tiny flashes of light.
However they appear to you, those tiny dots are really blood cells moving in the retina of your eye.
What's Going On?
At the back of the eye in the part called the retina, are the photoreceptor cells that detect light and send signals through the nerves to the brain.
In the eyes of invertebrates, like insects, snails, and spiders, the nerves go straight from the back of the eye to the brain.
In the eyes of vertebrates, such as mammals, birds, and reptiles, the path that the nerves take from the photoreceptors to the brain begins at the surface of the retina in front of the photoreceptors.
Those nerves then come together to form the optic nerve, passing through the retina on their way to the brain.
What this means is that there is a layer of nerves between the retina and the light that the retina is detecting. But to operate, the nerves need blood and so tiny blood vessels also flow across the surface of the retina.
A spider looking at the sky wouldn't see the tiny flashes because there are no blood vessels between its photoreceptor cells and the sky. But when you look at the sky, you can see your own blood cells moving across your retina.
It's best to relax and stare at the sky because when your eyes focus on an object at any particular distance, you won't be able to see the retina of your own eyes.