What are those floaty things in your eye? - Michael Mauser

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 pits of tissue,

red blood cells,

or clumps of protein.

Any 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.