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[telscope@xxxxxxxxxx (Peter Abrahams)]

STEREOSCOPY  (part 1)
        Humans use a variety of cues to perceive depth in a visual field.
Binocular vision provides an impression of depth that is independent of
light and shadow, object size and overlay, and color changes with distance.
Promotions for the earliest binoculars, from Germany, used the term
'plasticity' to describe the modeled or sculpted effect that they gave.
Hyperstereoscopy is the enhancement of sense of depth, and it is used in
some rangefinders, in photography, radiology, and mapmaking, and by
thrillseekers who believe that 3d photography is better 
than religion.
        Simple instruments that enhance depth perception use mirrors to effect an
increased interocular distance.  The telestereoscope was invented in 1857 by
Hermann von Helmholtz (1821-1894,) who also wrote the much used textbook
'Handbuch der physiologische Optik', and devised the ophthalmoscope for
viewing the interior of the eye and the ophthalmometer for viewing
reflections of images in the eye; in between major contributions to
theoretical physics.  Helmholtz' telestereoscope has two mirrors a distance
apart, at 45 degrees to the line of sight so that they reflect the image
towards each other.  Two more mirrors are placed at the same angle, in front
of the observer's eyes to reflect the images into the eyes.  An identical
device, with 'object' mirrors 18" apart, is used by radiologists to examine
x-rays in 3-d.  In the newsgroup sci.optics, a post from late 1995 reported
a vendor at the Grand Canyon selling viewing time on a telestereoscope with
mirrors 6 feet apart.  Any reader who has used such a device would find a
narration of the experience to be most welcome.
        More interesting are instruments that use dioptrics to deliver a magnified
image in enhanced stereo.  These resemble binoculars with objectives perhaps
18 inches apart.  These battery commanders rangefinders of WWII are
(infrequently) found at gun shows and military collectors conventions.
Zeiss made a similar device around the turn of the century, and a German
model from WWII is depicted in the March/April 1990 Stereo World.  John
Herschel, in The Telescope, his book from 1861, described a mirror
telestereoscope with a single lens adjacent to the 'object' mirror, and
eyepieces behind the mirror facing the eyes, which he said had been
successfully constructed by A. S. Herschel. These wonderful devices give
views of great beauty, and are worth the effort it takes to use or acquire
one.  
        The foregoing is a shameless attempt to induce discussion on this subject.
Information, references, and personal experiences are needed by the author
to fatten up future epistles on the subject.
Peter Abrahams
1948 Mapleleaf Rd.
Lake Oswego, Oregon  97034
503-636-2988
e-mail:  telscope@xxxxxxxxxx

telscope@xxxxxxxxxx (Peter Abrahams)          
the history of the telescope, 
     the prism binocular, and the microscope


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