You need to look at a bright blue light such as the sky. After a moment, you will see a dance of translucent dots just outside the center of the vision. The dots are short-lived, visible for a second or less, traveling short distances along seemingly random, curvy paths and brighter than the background. If the eye is stopped, the points are still moving around. The dots are white blood cells moving in the capillaries in front of the retina of the eye.
Why does this happen? Blue light is well absorbed by the red blood cells that fill the capillaries. The eye and brain "edit out" the shadow lines of the capillaries, partially by dark adaptation of the photoreceptors lying the capillaries. 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. This is called the blue field entoptic phenomenon. Most people are able to see this phenomenon. However, it is rather weak, and many people don’t notice it until asked to pay attention. This phenomenon is used to measure blood flow in the capillaries of the retina, known as Blue field entoptoscope.
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