This is a quick 'n dirty way of building a cheap polariscope. I call it a 'Make it Glow box', because when you put a translucent object on it, it looks like it emits light itself, since the direct background light is blocked by the polarization filters. You can also photograph the objects by holding the second polarizer in front of your camera, or by using a standard polarizing filter on the camera.
How does it work?
Light from below is polarized in one direction using a polarization filter. A translucent object on top of this first polarization filter scatters light in all directions and depending on the material depolarizes the light. A second polarization filter perpendicular on the first one blocks all direct polarized light, but the light scattered through the object is still visible.
What can we do with it?
A polariscope can be used to see through stuff, in the past it was used for seed testing, it is used for showing 'stress' patterns in for instance glasses or other transparent materials, and for testing gems for authenticity.