A FLOATING MARK GIVES NEW MEANING TO STEREO
PROJECTION IN THE CLASS ROOM
by
Professor K. B. Jackson B.A.Sc. University of Toronto.
Those of us who have been confronted with the task of explaining stereo-
scopic depth perception and measurement realize the inadequacy of the conven-
tional diagrams and descriptions and the necessity of introducing our audiences
to the experience of seeing depth, and movement in depth — the “floating
mark".
Stereoscopic projection for the demonstration of space perception has been
available for many years, first using red and blue-green filters for separation
and subsequently, and with much better results, using polaroid. And many useful
space diagrams have been produced for stereo projection to illustrate problems
in solid geometry, spherical trigonometry, astronomy, navigation, and photo-
grammetry, as well as a wide range of stereoscopic photographs, from photo-
micrographs to high altitude high obliques with immense bases and impossible
depths.
Two difficulties in the use of these projected stereo models soon become
apparent. First, the lecturer cannot point in depth and his attempts to do so are
annoyingly unsatisfactory, and second, audiences without previous experience
in stereoscopic observation find difficulty in adjusting their convergence suf-
ficiently to scan the model throughout its depth, particularly if the relief or
differential parallax is excessive.
It is the purpose of this paper to demonstrate how these difficulties can be
overcome by the introduction of a projected floating mark, which enables the
lecturer to direct the attention of his audience to any point i» the stereo model
and, by the adjustments required, eliminates causes of the observers’ difficulties,
namely, abnormal convergence (or divergence) and want of correspondence. It
becomes obvious that, with this equipment, the use and meaning of stereo pro-
jection can be greatly extended, not merely as a visual aid in any three-dimen-
sional problem, but also as a means of stereoscopic training and assessment, and,
particularly in photogrammetry, as a method of demonstrating the actual meas-
urement of absolute and differential parallax, and the existence and distribu-
tion of want of correspondence due to a lack of relative orientation. It enables
the instructor to lead his class through the necessary experience of stereoscopic
vision, and provides him with useful variations for training and testing their
tolerance and sentivity.
The equipment is designed to use standard lantern slides and projectors and
consists of two co-ordinated adjustable projector supports and an auxiliary point-
projector. The latter focuses a small bright spot on the screen, and is mounted
on an adjustable head so that the spot may be made to scan the screen.
The slide projectors are provided with polaroid filters, and the audience
with polaroid viewers, so that each eye sees only one of the stereoscopic pair,
but the point-projector is not polarized and therefore both eyes see the spot.
The position of the spot is obviously fixed in depth by the screen, whereas
the stereoscopic model may be distributed in front, on, or behind the screen,
depending upon the lateral adjustment of the stereoscopic pair. Its position in
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