RH writes- A pseudoscope is an optical instrument made with two prisms. The mirrors in the prisms reverse the image in each eye causing some items viewed through it to look quite unusual: convex objects become concave, things pointing towards you may look like they are pointing away and vice versa. This site describes more illusions that can be seen with it and how to make one for around ten dollars (holding up 2 $5 prisms). Link.
Make a Pseudoscope