In addition to Walters information, consider this too. As Walter said, you must have your eye as close as possible to the knife edge. When you do see the shadow of the knife edge move across the mirror, note that if the knife edge shadow appears to move in the same direction as it is actually moving, you are inside the radius of curvature and must move the knife edge further away from the mirror. If the shadow appears to move in the opposite direction, you are outside the radius of curvature and must move closer to the mirror.

When you are exactly at the radius of the mirror, you should not be able to tell which way the knife edge is moving and the mirror will appear to darken evenly (null) all over. This darkening evenly all over only holds true if the mirror is truly spherical. If the mirror is not spherical, for example an ellipsoid, the outer sections and the center will darken evenly at different distances of the knife edge from the mirror. A Couder mask, such as described in "Tex" is used to facilitate determining the null points of various zones and thus determine the actual shape of the mirror.

Jarvis Krumbein,