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

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

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

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only to reappear
as soon as you shift your glance.

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But don't go rinsing out your eyes!

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What you are seeing is a common phenomenon

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known as a floater.

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The scientific name for these objects
is Muscae volitantes,

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Latin for "flying flies,"

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

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

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Rather, they exist inside your eyeball.

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

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but they are not alive.

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Floaters are tiny objects
that cast shadows on the retina,

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

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They might be bits of tissue,

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red blood cells,

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or clumps of protein.

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And because they're suspended
within the vitreous humor,

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

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floaters drift along
with your eye movements,

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and seem to bounce a little
when your eye stops.

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Floaters may be only
barely distinguishable most of the time.

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

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

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will result in a more
sharply defined shadow.

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And floaters are particularly noticeable

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when you are looking
at a uniform bright surface,

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like a blank computer screen,

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snow,

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or a clear sky,

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

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

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

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with a single overhead light bulb,

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which also makes
the shadow appear clearer.

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There is another visual phenomenon
that looks similar to floaters

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but is in fact unrelated.

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If you've seen tiny dots of light
darting about

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when looking at a bright blue sky,

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

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

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Here, you are not seeing shadows

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but little moving windows
letting light through to your retina.

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The windows are actually caused
by white blood cells

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moving through the capillaries
along your retina's surface.

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

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

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Because the space
and the white blood cells

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are both more transparent to blue light

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than the red blood cells
normally present in capillaries,

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we see a moving dot of light
wherever this happens,

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

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Under ideal viewing conditions,

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

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

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

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

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While everybody's eyes experience
these sort of effects,

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the number and type vary greatly.

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In the case of floaters,

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

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However, abnormally numerous

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or large floaters
that interfere with vision

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

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But the majority of the time
entoptic phenomena,

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such as floaters and blue sky sprites,

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are just a gentle reminder
that what we think we see

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depends just as much
on our biology and minds

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as it does on the external world.