0:00:06.830,0:00:10.424
Have you ever noticed something swimming[br]in your field of vision?

0:00:10.448,0:00:14.061
It may look like a tiny worm[br]or a transparent blob,

0:00:14.085,0:00:18.052
and whenever you try to get[br]a closer look, it disappears,

0:00:18.076,0:00:20.763
only to reappear[br]as soon as you shift your glance.

0:00:21.727,0:00:23.792
But don't go rinsing out your eyes!

0:00:23.816,0:00:26.393
What you are seeing is a common phenomenon

0:00:26.417,0:00:27.449
known as a floater.

0:00:28.451,0:00:32.593
The scientific name for these objects[br]is Muscae volitantes,

0:00:32.616,0:00:34.755
Latin for "flying flies,"

0:00:34.779,0:00:37.909
and true to their name,[br]they can be somewhat annoying.

0:00:37.933,0:00:42.045
But they're not actually bugs[br]or any kind of external objects at all.

0:00:42.069,0:00:44.331
Rather, they exist inside your eyeball.

0:00:45.209,0:00:48.742
Floaters may seem to be alive,[br]since they move and change shape,

0:00:48.766,0:00:50.013
but they are not alive.

0:00:50.832,0:00:54.649
Floaters are tiny objects[br]that cast shadows on the retina,

0:00:54.673,0:00:57.228
the light-sensitive tissue[br]at the back of your eye.

0:00:58.077,0:00:59.590
They might be bits of tissue,

0:00:59.614,0:01:00.722
red blood cells,

0:01:00.746,0:01:02.681
or clumps of protein.

0:01:02.705,0:01:06.206
And because they're suspended[br]within the vitreous humor,

0:01:06.230,0:01:09.277
the gel-like liquid[br]that fills the inside of your eye,

0:01:09.301,0:01:11.580
floaters drift along[br]with your eye movements,

0:01:11.604,0:01:14.331
and seem to bounce a little[br]when your eye stops.

0:01:15.291,0:01:18.532
Floaters may be only[br]barely distinguishable most of the time.

0:01:19.460,0:01:22.711
They become more visible[br]the closer they are to the retina,

0:01:22.735,0:01:26.217
just as holding your hand closer[br]to a table with an overhead light

0:01:26.241,0:01:28.727
will result in a more[br]sharply defined shadow.

0:01:29.555,0:01:31.580
And floaters are particularly noticeable

0:01:31.604,0:01:34.582
when you are looking[br]at a uniform bright surface,

0:01:34.606,0:01:36.219
like a blank computer screen,

0:01:36.243,0:01:37.446
snow,

0:01:37.470,0:01:38.814
or a clear sky,

0:01:38.838,0:01:43.083
where the consistency of the background[br]makes them easier to distinguish.

0:01:43.107,0:01:46.761
The brighter the light is,[br]the more your pupil contracts.

0:01:46.785,0:01:50.698
This has an effect similar[br]to replacing a large diffuse light fixture

0:01:50.722,0:01:53.018
with a single overhead light bulb,

0:01:53.042,0:01:55.323
which also makes[br]the shadow appear clearer.

0:01:56.189,0:01:59.744
There is another visual phenomenon[br]that looks similar to floaters

0:01:59.768,0:02:01.319
but is in fact unrelated.

0:02:02.382,0:02:04.760
If you've seen tiny dots of light[br]darting about

0:02:04.784,0:02:06.977
when looking at a bright blue sky,

0:02:07.001,0:02:11.098
you've experienced what is known[br]as the blue field entoptic phenomenon.

0:02:12.273,0:02:15.140
In some ways,[br]this is the opposite of seeing floaters.

0:02:15.947,0:02:17.578
Here, you are not seeing shadows

0:02:17.602,0:02:21.028
but little moving windows[br]letting light through to your retina.

0:02:21.982,0:02:24.796
The windows are actually caused[br]by white blood cells

0:02:24.820,0:02:27.886
moving through the capillaries[br]along your retina's surface.

0:02:28.696,0:02:32.753
These leukocytes can be so large[br]that they nearly fill a capillary

0:02:32.777,0:02:35.907
causing a plasma space[br]to open up in front of them.

0:02:37.132,0:02:39.142
Because the space[br]and the white blood cells

0:02:39.166,0:02:41.696
are both more transparent to blue light

0:02:41.720,0:02:44.961
than the red blood cells[br]normally present in capillaries,

0:02:44.985,0:02:48.041
we see a moving dot of light[br]wherever this happens,

0:02:48.065,0:02:51.924
following the paths of your capillaries[br]and moving in time with your pulse.

0:02:52.829,0:02:54.786
Under ideal viewing conditions,

0:02:54.810,0:02:58.912
you might even see what looks[br]like a dark tail following the dot.

0:02:58.936,0:03:02.578
This is the red blood cells[br]that have bunched up behind the leukocyte.

0:03:03.428,0:03:07.679
Some science museums have an exhibit[br]which consists of a screen of blue light,

0:03:07.703,0:03:12.242
allowing you to see these blue sky sprites[br]much more clearly than you normally would.

0:03:13.195,0:03:16.195
While everybody's eyes experience[br]these sort of effects,

0:03:16.219,0:03:18.799
the number and type vary greatly.

0:03:18.823,0:03:20.315
In the case of floaters,

0:03:20.339,0:03:23.487
they often go unnoticed,[br]as our brain learns to ignore them.

0:03:24.311,0:03:26.335
However, abnormally numerous

0:03:26.359,0:03:29.045
or large floaters[br]that interfere with vision

0:03:29.069,0:03:33.664
may be a sign of a more serious condition,[br]requiring immediate medical treatment.

0:03:34.569,0:03:37.597
But the majority of the time[br]entoptic phenomena,

0:03:37.621,0:03:40.210
such as floaters and blue sky sprites,

0:03:40.234,0:03:43.623
are just a gentle reminder[br]that what we think we see

0:03:43.647,0:03:46.766
depends just as much[br]on our biology and minds

0:03:46.790,0:03:49.044
as it does on the external world.