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  • Have you ever noticed something swimming in your field of vision?

  • It may look like a tiny worm or a transparent blob,

  • and whenever you try to get a closer look, it disappears,

  • only to reappear as soon as you shift your glance.

  • But don't go rinsing out your eyes!

  • What you are seeing is a common phenomenon known as a floater.

  • The scientific name for these objects is Muscae volitantes,

  • Latin for "flying flies,"

  • and true to their name, they can be somewhat annoying.

  • But they're not actually bugs or any kind of external objects at all.

  • Rather, they exist inside your eyeball.

  • Floaters may seem to be alive, since they move and change shape,

  • but they are not alive.

  • Floaters are tiny objects that cast shadows on the retina,

  • the light-sensitive tissue at the back of your eye.

  • They might be bits of tissue,

  • red blood cells,

  • or clumps of protein.

  • And because they're suspended within the vitreous humor,

  • the gel-like liquid that fills the inside of your eye,

  • floaters drift along with your eye movements,

  • and seem to bounce a little when your eye stops.

  • Floaters may be only barely distinguishable most of the time.

  • They become more visible the closer they are to the retina,

  • just as holding your hand closer to a table with an overhead light

  • will result in a more sharply defined shadow.

  • And floaters are particularly noticeable

  • when you are looking at a uniform bright surface,

  • like a blank computer screen,

  • snow,

  • or a clear sky,

  • where the consistency of the background makes them easier to distinguish.

  • The brighter the light is, the more your pupil contracts.

  • This has an effect similar to replacing a large diffuse light fixture

  • with a single overhead light bulb,

  • which also makes the shadow appear clearer.

  • There is another visual phenomenon that looks similar to floaters

  • but is in fact unrelated.

  • If you've seen tiny dots of light darting about

  • when looking at a bright blue sky,

  • you've experienced what is known as the blue field entoptic phenomenon.

  • In some ways, this is the opposite of seeing floaters.

  • Here, you are not seeing shadows

  • but little moving windows letting light through to your retina.

  • The windows are actually caused by white blood cells

  • moving through the capillaries along your retina's surface.

  • These leukocytes can be so large that they nearly fill a capillary

  • causing a plasma space to open up in front of them.

  • Because the space in the white blood cells

  • are both more transparent to blue light

  • than the red blood cells normally present in capillaries,

  • we see a moving dot of light wherever this happens,

  • following the paths of your capillaries and moving in time with your pulse.

  • Under ideal viewing conditions,

  • you might even see what looks like a dark tail following the dot.

  • This is the red blood cells that have bunched up behind the leukocyte.

  • Some science museums have an exhibit which consists of a screen of blue light,

  • allowing you to see these blue sky sprites much more clearly than you normally would.

  • While everybody's eyes experience these sort of effects,

  • the number and type vary greatly.

  • In the case of floaters,

  • they often go unnoticed as our brain learns to ignore them.

  • However, abnormally numerous

  • or large floaters that interfere with vision

  • may be sign of a more serious condition, requiring immediate medical treatment.

  • But the majority of the time entoptic phenomena,

  • such as floaters and blue sky sprites,

  • are just a gentle reminder that what we think we see

  • depends just as much on our biology and minds

  • as it does on the external world.

Have you ever noticed something swimming in your field of vision?

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B1 中級

TED-ED】目に浮いているものは何?- マイケル・マウザー (【TED-Ed】What are those floaty things in your eye? - Michael Mauser)

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    新年好 に公開 2021 年 01 月 14 日
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