Before getting back to that polarization thread.... Today a friend tested my stereopsis with an old red-button Realist slide viewer. In order for me to see the stereo "fusion" and hidden stereo image, I had to turn the adjuster lever all the way to one end to horizontally superimpose the left and right images. I think it uses mirrors inside. Would this mean that I should get some "prism" in my eyeglasses prescription? How much would it cost? Remember I'm a homeless bum who can't simply walk into an optometrist's office for an exam and a pair of glasses. The fact I've never had "prism" prescribed should indicate professional callousness if it would ever have been a major benefit to me.