4" x 3" sheet
This green viewing film is used to view the magnetic fields of permanent magnets. The material turns dark when magnetic field lines are running perpendicular to its surface, and turns light when the magnetic field lines are running parallel to the surface.
In the lower picture, you can see the film on top of two blocks of the same 1/4" cube magnets. In the block on the left, the poles of the magnets are facing the film, so the tops of the magnets appear dark. On the block of cubes on the right, the poles are facing up and down in the picture, parallel to the film. You can see the difference in the magnetic field patterns on the film.