180 THREE-DIMENSIONAL PHOTOGRAPHYproperly separated, then all other images should be in their correct
positions. This does occur.
We have established seven feet as the ortho screen viewing dis-
tance for the 50 inch picture made with a Realist camera. There-
fore it follows that any object which was originally seven feet from
the camera will appear to be seven feet from the spectator or at
the same distance as the screen. This being true, it is obvious
that the two images of that object will be exactly superimposed
upon the screen.
POINT 3.-By superimposing the images of objects originally
seven feet from the camera, the correct relationship of all other
pairs of images will he automatically obtained.
But are you supposed to keep notes so that you will know which
objects are just seven feet away? What if there is no object seven
feet from the camera when you make the picture?
The solution is simple. You already know that by control of the
mask openings it is possible to place a frame or “window” around
the scene and this window may be placed at any desired distance.
All that is necessary is to have your mounts so prepared that they
have a seven foot window. Then you simply register the images
of the mask apertures. This is desirable anyway, because when the
two mask apertures are not superimposed, there is a sideband at
each side of the screen where scintillation takes place which may
cause headaches among the spectators.
POINT 4.-By superimposing the aperture images of mounts
which are cut for a seven foot window, correct registration of the
picture is automaticnlly obtained. (Provided of course the mount-
ing has been done with the requisite accuracy).
There are no such mounts available at present, but we have had
excellent results using the “normal” separation Permamount
which is so near that correct registration leaves a sideband of only
about a half inch more or less.
Thus it is possible to have an automatic guide to registration
which serves the additional purpose of eliminating scintillation.
When you have established the correct field angle through pic-
ture size and spectator distance and when you have the two images
in correct relationship upon the screen, you have achieved para-
projection! That is all there is to it, and it provides a projected
picture in which the departures &om the original are not visually