Re: Seeing fluid on the surface of your eye - how does this work?

Thank you for your thoughts and suggestions. Your idea to do an
additional experiment is good, but the one you suggest would again
produce results observable only to myself, and I am already sure these
are tears I'm seeing. I am familiar with floaters, and these droplets
behave entirely differently, and are directly affected by blinking,
while floaters are not. I believe my previous comments show that,
assuming my ability to observe and report is reliable (I assert this
is true!) the phenomena I am observing are clearly identifiable as
tear droplets, and distinguishable from any phenomenon originating
inside the eye.

In order for an experiment to be useful in convincing a skeptical
other, the results would have to be observable to that other. The
experiment would also have to be designed to duplicate the
circumstances of the original observation as closely as possible.
Since I am myopic, perhaps my setup with the cards should be
duplicated by a number of other myopic individuals, and their
observations recorded. There would be other factors which also would
have to be considered, such as lighting conditions, etc. However, I
don't think I want to become involved in setting up such a multi-
person study at the present time. An alternative might be to make a
setup which works in a way similar to a human eye, substituting a CCD
detector for the biological retina, and recording the result as a
digital image, but I'm not going to go that far right now. Thank you
again for your very thoughtful suggestions.
- Dan

jwill@xxxxxxxxxxxx wrote:
Hi.

You can't tell tears from floaters by anything you so far have
described, because the eye moves when you blink, and you
don't know whether the objects you see are more or less dense
than the fluid supporting them. I have seen floaters, and I
can't tell them from something on my cornea by any simple
experiment.

However, you can do a direct test of the tear hypothesis by
taking a piece of transparent plastic (Saran wrap) or some other
thin, transparent sheet, making a tiny
cut or other mark on it, wetting one side with saline solution (in
case you accidnetally touch your eye), and
putting it very near your cornea while you do the card trick.
You could suspend your film on a
spectacle frame, for example. Put the cards beyond the film.
If you see the mark you made, then it is POSSIBLE
you might be seeing something on your cornea.

Also, if you can see stuff in your tears with the cards, you
should be able to see the same stuff by looking in a good mirror with
a magnifying glass and a bright light. If there are blood cells,
specks of
solid mucous, pigmented protozoa, or anything else highly absorbant
in your tear film, you should visit an ophthalmologist, because this
would be an indication of a possible injury or disease condition.

Keep in mind that a pinhole light source has no magnification, so
something 2 cm from your retina (viz., on your cornea) would
have to be quite big, a mm or so, for you to see the silhouette image
of it.
On the other hand, something right against your retina would
be so close that it would span several retinal photoreceptors and
thus have a pinhole image with easily visible detail.

Touching a little on the physics of this problem, an inverse distance
law for resolution of detail holds for an object illuminated by a
pinhole
light source, just as it holds for any other source.

daniwe...@xxxxxxxxx wrote:
On Jul 13, 5:42=EF=BF=BDpm, "jw...@xxxxxxxxxxxx" <jw...@xxxxxxxxxxxx> w=
rote:
On Jul 8, 11:15=EF=BF=BDpm, daniwe...@xxxxxxxxx wrote:

... I notice I am able to observe the
droplets of fluid on the surface of my eye (especially when I blink=
)
which I ordinarily can't see. What am I doing to the light, and why
can I see the fluid? Thank you.

You are assuming this is fluid on the surface of your eye: =EF=BF=BDI=
t can't
be, because the absorbance of anything in corneal fluids (barring
some sort of disease) is too low. =EF=BF=BD What you are seeing is de=
ad blood
cells
and other detritus in the vitreous humor, casting shadows right
against
your retina. =EF=BF=BD The analogy to a pinhole camera is correct: =
=EF=BF=BDThe small
hole
allows the point-spread function to be very narrow, allowing high-
resolution
vision of this stuff.

You also may be able to see strands of broken fibers which used to
anchor the
vitreous humor to the eyeball. =EF=BF=BD As you age, your vitreous hu=
mor turns
slowly
from firm jelly to watery liquid, starting near the retina. =EF=BF=BD=
It
fragments and
detaches from the retina. =EF=BF=BDSo, the stuff you see will become =
more
heavily
populated ("dirtier") as you age. =EF=BF=BD This won't interfere with=
vision,
because it
in not in the retinal object plane. =EF=BF=BD But, refractive index
differences in the vitreous
humor may make your vision intermittently blurry as you age; this
resembles
the effect of mucous or other substances in the tears but is not.

Thanks to everyone for your thoughtful replies!

I believe I am certainly seeing droplets of fluid on the surface of my
eye for the following reasons:

1. Blinking permanently changes their number, size, shape and
position.

2. If I squint, I am able to "corral" them together.

When my head is positioned vertically, relative to the ground, the
droplets seem to drift chiefly downward (and slightly sideways at
times) after I blink. When I position my head sideways, the droplets
seem to describe a curved path after I blink, initially fast toward my
lower eyelid, and then slower, curving away towards the ground. At no
time do they seem to drift up, away from the ground. I think their
movement is being influenced both by gravity and by surface tension
and/or other fluid mechanical behaviors, which, however, I have no
technical knowledge of. But it seems clear these are tear droplets,
rather than floaters, which, being inside the eye, would not be so
influenced and modified by blinking.

- Dan

.

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