Q: During a thunderstorm my partner and I were watching the clouds for lightning and both noticed an unusual effect. Curving against the grey clouds we saw many random, bright sparkles, somewhat akin to 鈥渟now鈥� on a TV screen. What caused this effect, which seemed to come from inside our eyes?
A: I have observed this phenomenon several times when gazing at a bright sky, especially from an aeroplane window. What one is seeing is one鈥檚 own blood cells flowing through translucent blood vessels in front of the eye鈥檚 retina.
Normally we do not see the unchanging pattern of these blood vessels because the brain knows that they are there and compensates for them.
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The combination of a bright light (especially if it is blue) and the continuously changing position of the blood cells causes them to become noticeable. In fact, the sparks that you see will appear to pulsate in time with your own pulse.
This phenomenon is the subject of a demonstration at the New York Hall of Science, in which visitors can gaze into a box containing a bright blue light. In addition, it has even been suggested as a diagnostic tool in which patients with certain types of blood diseases can estimate their own blood cell counts.