The blue field entoptic phenomenon or Scheerer's phenomenon (after the German ophthalmologist Richard Scheerer, who first drew clinical attention to it in 1924) is the appearance of tiny bright dots (nicknamed blue-sky sprites) moving quickly along squiggly lines in the visual field, especially when looking into bright blue light such as the sky. The dots are short-lived, visible for a second or less, and traveling short distances along seemingly random, curvy paths. Some of them follow the same path as predecessors. The dots may be elongated along the path like tiny worms. The speed of the dots varies in sync with the pulse; they briefly accelerate at each heart beat. The dots appear in the central field of view, within 10 to 15 degrees from the fixation point. The left and right eye see different dots; someone looking with both eyes sees a mixture.
Most people are able to see this phenomenon in the sky, although it is rather weak, and many people do not notice it until asked to pay attention. The dots are highly conspicuous against a monochromatic blue background (~430 nm) instead of the sky.
Source: Wikipedia https://en.wikipedia.org/wiki/Evolution_of_the_eye