Effects of Eye Position on Stereo Perception

[P3D Peter Homer <P.J.Homer@xxxxxxxxxxxxxxxxx>]

This topic would seem to be relevent to this group when it concerns stereo
perception. The eyes converge on objects they are viewing and it was quite
a logical early view that this convergence gave information on depth by a
kind of triangulation. But I had a photocopy from a book on psychology that
stated that that theory and experimentatation showed that convergence of
the eyes played little or no part in depth perception . Stereo viewing with
stereoscopes and particularily free viewing would seem to support this .
Although perhaps because of the belief that convergence was neccessary for
stereo perception it seems it was originaly thought that convergence was
neccessary in a stereoscope as well.
 I have seen a diagram of Wheatstones stereoscope from above which show the
viewers eyes converging on a point behind the mirrors. Apparently his and
later Brewsters reason for using half lenses originaly was not so much to
diverege the lines of sight out to pairs larger than the interocular. But
to diverge them on the other side of the lenses so that they converge on a
point between the pairs.
  Modern stereoscopes for 35mm slides are not constructed with half lenses
and unless what happens with those with adjustable eyepieces is that we
actualy set them so that we are looking through the inner halves of the
lenses rather than the centre as we would think . What about those with
large fixed eyepieces would only those people with the right eye seperation
to see throught the inner halves be able to see a proper stereeo image?.
With parrallel "free viewing"  we can be even more certain that our eyes
are not convergent but we see a stereo image and as our brain has not
evolved for viewing stereo pairs but real objects it is likely that this
mechanism is involved in the viewing of such real objects. Rather than the
convergence of our eyes and I have seen an explanation in another book on
psychology as to what this probably is. P.J.Homer



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