282 THREE-DIMENSIONAL PHOTOGRAPHYslides, have been offered in limited quantity. These are for use in
normal projectors and require red-green viewers instead of the
polarizing type, They are fairly satisfactory, although some peo-
ple see a color bombardment when the red-green anaglyph is used.
They do provide stereo projection for those who have no regular
stereo equipment. Unfortunately they are limited to commercially
available subjects which is a grave disadvantage.
It is rumored that similar slides and movies are to be made
available using the Vectograph principle. This system uses polar-
ization extinction and has no color bombardment effect. They
are highly satisfactory, but again the “commercial subject” limita-
tion is a disadvantage. It is said that when the Vectograph films
do make their appearance they will be in color, whereas the red-
green are perforce monochromatic.
In the absence of specific information, no more will be said
about these stereo projection slides and films.
However, there are certain factors regarding the recent history
of stereo motion pictures which will be of interest, so we shall
give them passing attention. The first inyolves the often dis-
cussed “free vision” viewing, or the viewing of stereo motion pic-
tures without the aid of any kind of viewing device, and the great
publicity which has been given the subject, to say nothing of the
multitude of spurious methods which have been proposed and
which have actually been used.
Free-Vision Screen.-This is based upon the principle of a mul-
titude of intersecting stereo beams, so disposed that the spectator
will receive one left and one right beam in the corresponding
eyes. It can readily be understood that as long as the essential con-
ditions prevail, it will also be possible for a spectator in sorii‘e
position to receive alternate beams, thus producing a pseudoscopic
relief rather than stereoscopic.
It will also be obvious that there will be certain positions of the
spectator where neither the stereoscopic nor the pseudoscopic re-
lief will be apparent.
Such a screen has been reported from Europe and has received
a great deal of publicity. However, the technical report reveals a
significant condition. The seats in the auditorium are specially
positioned in groups, and the spectator must move his head about
until he locates the correct position for relief viewing. These two