Answer: White Blood Cells

It’s formally known as the “blue field entoptic phenomenon” (a.k.a. Scheerer’s phenomenon) and it’s a curiosity to children and adults alike. When you stare up at a clear blue sky (or any other bright uniform blue surface), you see little glowing dots moving quickly across your field of vision.

What exactly are those little bright dots? The explanation is just as curious as the phenomenon. When you see the little bright dots, you’re seeing white blood cells moving through the capillaries in front of your retina. Your eyes and brain do a marvelous job of editing out, if you will, the shadow lines of the capillaries so that your vision is clear and crisp. However, under very specific circumstances (conditions), the editing process fails and little artifacts appear in your vision.

The blue field entoptic phenomenon is one of those situations (the larger size of the white blood cells compared to your red blood cells as well as their rarity, combined with the blue light which is absorbed better by the red blood cells than the white) that allows you to actually see your white blood cells drifting on by.