STEREO PROJECTION 179
put on the Polaroid goggles each eye will see only one picture, the
two eyes see two different pictures but the visual impression is that
of a single picture. But at first you do not wear these goggles, so
you see both pictures upon the screen.
How are these two pictures to be aligned?
This question has received more nonsensical answers than could
be imagined; and most of them are wholly without explanation
other than that the projectionist prefers his own method. But let
us think a moment. Our problem is to duplicate insofar as possible
the conditions of direct vision, isn’t it? Well then what are these
conditions of normal, direct vision we wish to duplicate? First of
all, when viewing a distant object the axes of the eyes are substan-
tially parallel.
POINT 1.-Images of o6jects at a distance should be separated by
a distance roughly eqid to the average interpupillay. In practice,
this is made about two to two and a quarter inches. So these
images are placed at a separation of roughly two inches. The pro-
jector horizontal adjustment control is used for this purpose,
The two pictures are independent, so either image may be
placed at the left. If the right image is placed at the left, then the
eyes will “cross” at some point about halfway between the screen
and the spectator.
POINT 2.-Care must be taken lo have the images in their cor-
rect relationship.
Cover one lens of the projector. Look at the screen through the
viewing goggles. Cover one eye with the free hand. Does the image
disappear? If your left eye is covered and if the image disappears
it is the left image. Now while watching the screen, uncover the
other projector lens, The second image should appear at the right
of the first one (provided the first one proved to be the left as ex-
plained above). If the second image appears at the left of the first
image, then use the projector adjustment to bring the two into
coincidence and continue in the same direction until they are
again separated by about two inches.
Of course you do not want to go through this long rigamarole
every time you get ready to project a picture. So let’s follow
through with the reasoning.
When the two pictures are so arranged that infinity images are
frankie
(Frankie)
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