The third method involves moving the eye in relation to the flat. When moving the eye from a zero-degree angle of incidence to an oblique angle, the fringes will appear to move. If the testing surface is concave, the fringes will appear to move toward the center. If the surface is convex, the fringes will move away from the center. To get a truly accurate reading of the surface, the test should usually be performed in at least two different directions. As grid lines, the fringes only represent part of a grid, so a valley running across the surface may only show as a slight bend in the fringe if it is running parallel to the valley. However, if the optical flat is rotated 90 degrees and retested, the fringes will run perpendicular to the valley and it will show up as a row of "V" or "U" shaped contours in the fringes. By testing in more than one orientation, a better map of the surface can be made.