A window being tested on an optical flat. The window is perfectly flat on the near side, but banks away on the far side. The left side is 650 nm (red) causing the fringes to be a little wider but fewer than the right side, which is 532 nm (green). The red and green are nearly opposites, harmonically, so the red fringes appear in almost the same locations where the dark fringes appear on the green side. (532 nm is the exact harmonic opposite of 709.4 nm.