The refractive index of my normal everyday glasses is such that it separates the colors of red and blue by making the red appear closer and the blue appear farther away, even when looking at a flat monitor screen or, say, a stained glass window. By moving my head these two colors look "out of register', so to speak, and I must mentally learn to disregard this anomaly. The interesting thing is that by turning my head, these colors still create the illusion of depth even when I have one eye closed. Weird. (I find this useful for doing sketches in 3-D by drawing with complementary highlighter markers-- the images appear to me to 'lift' off the page, for whatever that's worth).