Wow! This is so cool…from Wikipedia:
Blue field entoptic phenomenon
The blue field entoptic phenomenon or Scheerer’s phenomenon is the appearance of tiny bright dots moving quickly along squiggly lines in the visual field, especially when looking into blue light (such as the sky). This is a normal effect that can be perceived by almost everybody. The dots are due to the white blood cells that move in the capillaries in front of the retina of the eye, near the macula.
Blue light (optimal wavelength: 430 nm) is well absorbed by the red blood cells that fill the capillaries. The brain “edits out” the dark lines that would result from this absorption. The white blood cells, which are much rarer than the red ones and do not absorb the blue light well, create gaps in the blood column, and these gaps appear as bright dots.
In a technique known as blue field entoptoscopy, the effect is used to measure the blood flow in the retinal capillaries. The patient is alternatingly shown blue light and a computer generated picture of moving dots; by adjusting the speed and density of these dots, the patient tries to match the computer generated picture as best as possible to the perceived entoptic dots. This then allows calculation of the blood flow in the capillaries. This test is important in diseases such as diabetes which can cause retinopathy.
Scheerer’s phenomenon should not be confused with “floaters” or muscae volitantes. Scheerer’s phenomenon is distinguished by the appearance of multiple, identical-looking bright dots that follow each other rapidly along the same path. Floaters are variable in appearance; although they sometimes are dots, they often have the appearance of threads or shreds of crumpled cellophane. Floaters remain almost stationary or drift slowly and do not follow well-defined paths. They are due to debris floating in the vitreous humor of the eye.