If you look up into the blue sky long enough, and with enough attention, you should be able to see tiny blue-white dots flashing around the sky.The phenomenon was so well-known that it acquired the nickname "blue-sky sprites," but they're actually white blood cells moving through your eye.
White blood cells make up a very small percentage of blood — as little as only 1%. Red blood cells floating in plasma are the bulk of blood. In the tiny blood vessels in the eye, we don't see blood in bulk. Cells file through the vessel one at a time. The red blood cells block out light, particularly if the light is blue. White blood cells don't absorb light well, so they let the light shine through.
When these blood vessels are strung over the receptor cells in the retina, the red blood cells block out the sun. That's no problem. The eye adjusts so we don't see dark nets across our vision. Because the eye adjusts, the 1% of the time that a white blood cell moves along the blood vessel, it lets the light through. The retina sends a signal of increased brightness to the brain, an, to the viewer, it looks like there is a tiny spot of blue-white light moving through the sky.
The phenomenon itself is interesting, but what's really intriguing is how scientists figured it out. They couldn't use human eyes, as people whose eyeballs are being prodded tend not to notice blue spots in the sky.
First researchers noted the frequency and motion of the blue-sky sprites, then they went looking for other thin tissue to shine light through. They found the wing of a hibernating bat, and the cremaster (the thin muscle covering the testes) of a rat. By shining light through these tissues, they could note, with instruments, the movement of bright spots through the blood vessels. By examining the frequency, motion, and size of the bright spots, the researchers were able to confirm white blood cells as their cause.
If you want to see blue-sky sprites, go outside on a sunny day. Look fixedly up into the sky, but make sure it's a part of the sky that's well away from the sun, as looking at the sun damages your eye. Tiny white dots should begin appearing soon. You'll know them because of their size, and because they will travel the same paths (the paths of your blood vessels) over and over again.
Image: Andrés Nieto Porras.