198 THREE-DIMENSIONAL PHOTOGRAPHY
materials are used the screen will show both images no matter
whether you wear the viewing goggles or not. Incidentally this is
more serious than you might think because a great many stereo
projectors have been returned after trial simply because the buyer
used his old beaded movie screen-and didn’t get results.
The screen MUST have a metallic surface, and the easiest way
to get this is to use an aluminum screen. You can spray a smooth
panel of pressed wood or the like for the best surface, but for a
portable screen it is advisable to buy one.
For normal home use the advised size is 50x50 inches. There
should be some means whereby the screen surface is stretched
tightly as waves or wrinkles in the screen surface are annoying.
For the best stereo the surface of the screen should not be visible
as such. One screen with the conventional spring roller has three
stretcher rods at the back to hold the surface flat, Another con-
sists of a folding square frame of aluminum truss rods supported
by inverted vee legs. The screen snaps to all sides of the square
frame and so remains stretched tightly in all directions. This
screen also has a slight vertical tilt so that the “hot spot” common
to metallic screen is directed away from the line of vision, a detail
which makes for considerably improved projection conditions.
GoGGLEs.-Cardboard framed goggles are obtainable at very low
prices and are intended for use by large gatherings. For home use,
where the spectators will rarely number more than 25 it is highly
advisable to purchase the required number of plastic frame goggles
in which the polarizing material is cemented between transparent
plastic sheets and the whole set in a plastic mount. These are more
comfortable and the lenses are far more easily kept clean and
scratch free than those of cheaper construction.
PROJECTION ROUTINE
Once you get set in projection routine you should have no
trouble at all.- Be sure the projector is clean. This includes the lenses and
the condensers. Go over the instrument periodically and clean it
thoroughly. - Never give a show without a preview. Select the slides to be
shown and look at all of them before the spectators see the screen.