RE: Sclerotic rings

["Al Fraser and Laurie Fletcher" <alflaurf@abs.net>]

There has been a discussion about the utility of sclerotic rings for the
support of eyes.  Parts of that are:



> Gordon Martin Human wrote:
>
> > Given that eyes are filled with fluid, and fluids are by nature
> > incompressible, I don't see this as a valid hypotheses.
>
> Fluids are incompressible, but fluid-filled sacks can be deformed, so it
> isn't a priori crazy that a compensatory structure might be of use
> in maintaining visual acuity (whether the pressure comes from depth,
motion
> or whatever).

and David Marjanovic wrote:

Ask any professor of physics -- fluids ARE compressible, just less so than
gases and spongy solids.
........

Yes, water (the relevant fluid) is compressible.  The volume coefficient of
compressibility per atmosphere of pressure is approximately 0.00005.
Therefore, at say, 3000 feet deep (about 100 atmospheres pressure), water
will compress fractionally by 0.005.  That means that the linear dimensions
of water (or a sack of water) will change by 0.17% as the water goes from
the surface to 3000 feet deep.  That is very little compression, and hence
water is described, as an approximation, as incompressible.  Much fluid
mechanics of water is done well with the approximation that the
compressibility coefficient is zero.

Sclerotic rings are useless to support eyes under hydrostatic presure.
First, as stated, the compression is very slight.  Second, pressure is
isotropic - equal in all directions, so a structure behind an eye would not
support the eye as there would be no net force vector on the structure.


Al Fraser