The Body: A Guide for Occupants

by Bill Bryson

You may have had the experience of looking at a clear blue sky on a sunny day and seeing little white sparks popping in and out of existence, like the briefest of shooting stars. What you are seeing, amazingly enough, is your own white blood cells, moving through a capillary in front of the retina.17 Because white blood cells are big (compared to red blood cells) they sometimes get stuck briefly in the narrow capillaries, and that is what you are seeing. The technical name for these disturbances is Scheerer’s blue field entoptic phenomena (named for a German ophthalmologist of the early twentieth century, Richard Scheerer), though they are more commonly and poetically known as blue-sky sprites.