[00:06] Have you ever noticed something swimming
in your field of vision?
[00:10] It may look like a tiny worm
or a transparent blob,
[00:14] and whenever you try to get
a closer look, it disappears,
[00:18] only to reappear
as soon as you shift your glance.
[00:21] But don't go rinsing out your eyes!
[00:23] What you are seeing is a common phenomenon
[00:26] known as a floater.
[00:28] The scientific name for these objects
is Muscae volitantes,
[00:32] Latin for "flying flies,"
[00:34] and true to their name,
they can be somewhat annoying.
[00:37] But they're not actually bugs
or any kind of external objects at all.
[00:42] Rather, they exist inside your eyeball.
[00:45] Floaters may seem to be alive,
since they move and change shape,
[00:48] but they are not alive.
[00:50] Floaters are tiny objects
that cast shadows on the retina,
[00:54] the light-sensitive tissue
at the back of your eye.
[00:58] They might be bits of tissue,
[00:59] red blood cells,
[01:00] or clumps of protein.
[01:02] And because they're suspended
within the vitreous humor,
[01:06] the gel-like liquid
that fills the inside of your eye,
[01:09] floaters drift along
with your eye movements,
[01:11] and seem to bounce a little
when your eye stops.
[01:15] Floaters may be only
barely distinguishable most of the time.
[01:19] They become more visible
the closer they are to the retina,
[01:22] just as holding your hand closer
to a table with an overhead light
[01:26] will result in a more
sharply defined shadow.
[01:29] And floaters are particularly noticeable
[01:31] when you are looking
at a uniform bright surface,
[01:34] like a blank computer screen,
[01:36] snow,
[01:37] or a clear sky,
[01:38] where the consistency of the background
makes them easier to distinguish.
[01:43] The brighter the light is,
the more your pupil contracts.
[01:46] This has an effect similar
to replacing a large diffuse light fixture
[01:50] with a single overhead light bulb,
[01:53] which also makes
the shadow appear clearer.
[01:56] There is another visual phenomenon
that looks similar to floaters
[01:59] but is in fact unrelated.
[02:02] If you've seen tiny dots of light
darting about
[02:04] when looking at a bright blue sky,
[02:07] you've experienced what is known
as the blue field entoptic phenomenon.
[02:12] In some ways,
this is the opposite of seeing floaters.
[02:15] Here, you are not seeing shadows
[02:17] but little moving windows
letting light through to your retina.
[02:21] The windows are actually caused
by white blood cells
[02:24] moving through the capillaries
along your retina's surface.
[02:28] These leukocytes can be so large
that they nearly fill a capillary
[02:32] causing a plasma space
to open up in front of them.
[02:37] Because the space
and the white blood cells
[02:39] are both more transparent to blue light
[02:41] than the red blood cells
normally present in capillaries,
[02:44] we see a moving dot of light
wherever this happens,
[02:48] following the paths of your capillaries
and moving in time with your pulse.
[02:52] Under ideal viewing conditions,
[02:54] you might even see what looks
like a dark tail following the dot.
[02:58] This is the red blood cells
that have bunched up behind the leukocyte.
[03:03] Some science museums have an exhibit
which consists of a screen of blue light,
[03:07] allowing you to see these blue sky sprites
much more clearly than you normally would.
[03:13] While everybody's eyes experience
these sort of effects,
[03:16] the number and type vary greatly.
[03:18] In the case of floaters,
[03:20] they often go unnoticed,
as our brain learns to ignore them.
[03:24] However, abnormally numerous
[03:26] or large floaters
that interfere with vision
[03:29] may be a sign of a more serious condition,
requiring immediate medical treatment.
[03:34] But the majority of the time
entoptic phenomena,
[03:37] such as floaters and blue sky sprites,
[03:40] are just a gentle reminder
that what we think we see
[03:43] depends just as much
on our biology and minds
[03:46] as it does on the external world.