Picture this: you're standing in a field, on a bright sunny day when suddenly you look up into the cloudless, clear blue of the sky. Racing back and forth in your vision in sometimes repeating patterns - but also sometimes seemingly at random with just a few repeating patterns, are these little white squiggly lines. Each heartbeat, they go soaring through a small spot of your vision before dissipating and there are hundreds of them - all moving at once. No matter where you move or seem to look, now that your attention has been called to them, you are starting to notice them. Well. Don't panic just yet.
When you look back to the ground, and your vision is not placed flatly against a solid color, they are significantly less noticeable. You may not see any, anymore - or you may find there are only one or two at the corners and edges of your vision. Thinking they are gone, you might continue to go on about your day... But eventually, you will look up again at the blue, or the grey, of the sky - and that is when they will once more become significant and noticeable. They can appear anywhere from being faint, dark spots which seem to move along tracks slowly and which seem to have a whitish color at their central points of travel - too thick and bright and made up of similar whites and greys, and moving quickly along those same pathways.
The blue field entoptic phenomenon or Scheerer's phenomenon (named for the German ophthalmologist Richard Scheerer) is a physiological phenomenon that takes place in the retina. Richard Scheerer first drew clinical attention to this phenomenon in 1924. It's the appearance of tiny bright dots (nicknamed by Richard 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 usually travel short distances along seemingly random, often curved paths. Some of them follow the same path as predecessors. Often described as "seeing stars" The dots may be elongated along the path like tiny worms, and The speed of the dots varies in sync with the pulse of the one experiencing them. They briefly accelerate at each heartbeat.
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, as the images are laid over one another in the subconscious observational process.
Most people can see this phenomenon in the sky, although it is rather weak, and many people do not notice it until asked to pay attention to it. The dots are highly conspicuous against a monochromatic blue background typically making up one's view of the sky - as the sprites themselves are small and white, and their trailing tails are usually darker, near grey.
The dots themselves are the result of white blood cells moving along capillaries (which is the reason for their color), in front of the retina. Blue light, (at a wavelength of about 430 nm) is absorbed by the red blood cells which fill the capillaries. Your brain automatically filters out the shadowing lines of these capillaries (similar to the way you cannot see things at a certain angle/point in your visual field while focusing at its center). They are then further filtered out partially by dark adaptation of the photoreceptors lying beneath the capillaries. In this case, The white blood cells, which are much rarer than the red ones and do not absorb blue light, create gaps in the blood column, and these gaps appear as bright dots (and this is also why they appear to move.)
The gaps are elongated because a spherical white blood cell is too wide for the capillary, and red blood cells get backed up behind them (creating the appearance of a white dot with a tail) the white blood cells travel slowly through, and the swell of regular blood cells behind them show up as the dark tail. This behavior of the blood cells in the capillaries of the retina has been directly observed in human subjects by adaptive optics, including scanning laser ophthalmoscopy, a real-time imaging technique for examining retinal blood flow. The dots will not appear at the very center of the visual field, because there are no blood vessels in the foveal avascular zone.
Scheerer's phenomenon consists of corpuscles of identical diameter and visual sharpness, of a simple dot or worm-like shape, brighter than the background. This is different from "floaters" or other visual disruptions within the eye because If the eye stops moving, the dots don't keep darting around, whereas with "floaters" in your eye, If the eye stops moving, they settle down - and since particles are being trapped in the vitreous humor, they are in liquid and being moving around sluggishly relative to your eye movements. This movement is subject to inertia. With blue field entoptic phenomena, If the eye moves, the dots follow instantaneously, because they are contained in the retina, and not the vitreous humor, and so, do not have the appearance of inertia.
Scheerer's phenomenon can be distinguished from visual snow, as well. This is because it appears only when looking into bright light, whereas visual snow is constantly present in all light conditions including the dark - and is a sign that there may be something wrong neurologically, whereas the blue field entopic phenomena is centralized in your eyes themselves and are therefore more like visual artifacts than symptoms of a more significant underlying problem.