prism question

Before getting back to that polarization thread....

Today a friend tested my stereopsis with an old red-button Realist
slide viewer. In order for me to see the stereo "fusion" and hidden
stereo image, I had to turn the adjuster lever all the way to one end
to horizontally superimpose the left and right images. I think it
uses mirrors inside.

Would this mean that I should get some "prism" in my eyeglasses
prescription?

How much would it cost? Remember I'm a homeless bum who can't simply
walk into an optometrist's office for an exam and a pair of glasses.
The fact I've never had "prism" prescribed should indicate
professional callousness if it would ever have been a major benefit to
me.

 

So I should walk around with mirrors on my sunglasses. Or maybe I
should find an optometrist or opthamologist on the planet Vulcan.

 

Here is a private reply I received:

"The Realist viewer doesn't have prisms in it. - The lever moves the
lenses closer or further apart. This has a slight prismatic effect.
Most people with normal eye spacing can view the 3D picture without
difficulty if the lever is kept in the middle. - Normal eye spacing
is about 63mm. The slide's film chips are mounted so that objects at
infinity in a landscape picture are about 63mm apart. So ideally,
your eyes will be parallel when looking at an object in the viewer
which is supposed to appear far away. Objects which are supposed to
appear closer to the viewer are closer together on the film chips but
the disparity between very near and far objects is no more than 1 to 2
mm.

"Prisms in glasses are often used to to prevent double vision - but
cross-eyed people sometimes find ways to avoid the confusion of double
vision on their own (for example by pointing the blind spot of one eye
parallel to the central vision of the other eye.) - Stereo Sue talks
about this in her book."