English: A precision glass window ground to a flatness of λ/10, resting upon an λ/10 optical flat, illuminated with both the green 532 nm light from a laser and white light from an incandescent bulb, showing interference fringes which reveal the unevenness of the surface. Image 1 shows that the fringes are fairly straight, with some small deviations, indicated that the surface of the window in very close to λ/10 flatness. The flat has slightly chamfered edges, which shows up as bends near the ends of the fringes. The bottom side of the flat is mirrored, which not ideal, masking some of the fringes. Image 2 shows the interference fringes in white light. Image 3 is after an hour of wringing, as the surfaces come closer to parallel. Image 3 is after 2 hours or wringing. Images 5 and 6 are taken after the surfaces have fully wrung, becoming parallel. Both the white-light test and the finger-pressure test show that the surface of the window is concave.