It is called Schereer's Phenomenon. Please red below.

www.amasci.com/freenrg/tors/floaters.htm

Excerpt below.

"While I was still a student, I worked as a Research Assistant at the Center for Visual Science in Rochester, NY. At one point I had an opportunity to visit a vision research laboratory in Boston MA, where I had an opportunity to observe a device which was used to make these moving leukocytes VERY visible. It was a de-focused laser which was aimed into
an eyepiece. It was violet in color, with a frequency chosen which is absorbed by hemocytes. When I looked into this eyepiece, I saw a uniformly illuminated field of light, of violet-white color. In this field I saw several hundred moving yellow dots. If I recall correctly, each dot seemed to possess a V-shaped "wake" like a speeding boat makes upon the water. (this supposedly comes from stimulation of the retina's
edge-detector neural computation, and all moving objects have these "wakes.")

The effect is called "Scheerer's phenomenon". The dots are leukocytes which move along through the blood-filled capillaries. I noticed that the velocity of all of these dots was varying in synchronism with my heartbeat. As my blood pressure changed during each heartbeat, the dots moved fast and slow. The moving dots seemed to wander randomly, yet many of them executed a typical maneuver: a wiggling, sinusoidal trajectory. Apparently there are many capillaries on the retina which have the shape of a snake, a sine-wave, and when a leukocyte travels through that channel, it executes a sinusoidal "wiggle" motion. The capillaries are said to be normally invisible because they are full of hemocytes (red
blood cells), and these hemocytes are too close to each other, and too far away from the retina to create individual shadows. Therefore, like the capillaries themselves, our retinas "edit" the blood cells out the view perceived by our brains. On the other hand, the leukocytes (White blood
cells) are large, and they act like gaps in the columns of blood which fill the capillaries. These "moving holes" in the blood are made visible when we stare at a uniformly illuminated surface. Even better, stare at a point-source illuminator (laser, or light passed through a single glass fiber) which has been extremely de-focused by a powerful lens (such as a
telescope eyepiece.) Doing so is an improvement, since as a result, the light rays behave as parallel lines at the place where they strike the retina, and so the shadows of the leukocytes will be very sharp. (They actually will be ring-like diffraction patterns.) If instead we star at the blue sky, then the light rays behave as "diffuse light" at the location where they strike the retina, and the shadows of the leukocytes
will be fuzzier and more difficult to notice."