Answer
See below.
Work Step by Step
Each image is projected onto a screen through its own polarization filter, either vertical or horizontal, and the images merge.
A Polaroid only admits light whose direction of polarization is along its transmission axis.
When the two Polaroids are aligned, anything that passes through the first gets through the second with no problem.
When the two Polaroids are at right angles to each other, anything that passes through the first is automatically absorbed by the second.
In 3-D projection, it is required that each eye see a slightly different image.
This is accomplished by wearing a different polarization filter in front of each eye. Only one of the two images reaches that eye. For example, if one of the images is projected through a vertical polarization filter, the light is vertically polarized, and will reach the eye that has a vertically-oriented Polaroid in front of it.
The other eye, with a horizontally-oriented polaroid in front of it, will not see that image at all, but will see the slightly different one that was projected through a horizontal polaroid.
This is discussed on page 558.
