194 THREE-DIMENSIONAL PHOTOGRAPHY.
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Fig. 12-8. Convergent vision with two prisms.tunately all such viewers using optical elements must be held
perfectly steady or the two images will dance. This is the fault
which led to the discard of this method of viewing.
Another form of projection is one in which a shutter or a pair
of shutters pass between the film and the screen in alternate se-
quence. Each spectator has a similar shutter mounted in a frame
which can be held in the hand. The shutters are all synchronized,
usually by being connected into a common AC circuit. By this
synchronization of shutters, the pictures are projected alternately
and the eyes are synchronously obscured so that each eye sees only
its own image. As the frequency of the shutter alternation is con-
siderably higher than that required for continuity of image through
persistence of vision (the motion picture principle) there is no
noticeable flicker. It is claimed that this system is remarkable for
its great brilliance, but inasmuch as only one lamp illuminates the
screen at one time, and as both lamps illuminate the screen in the
polarized system, the doubling of light output. just about cancels
out the filter absorption.
This system demands that the shutters operate in the same cycle.
SS'Fig. 12-9. Screen viewer with image-inverting swan prisms.