Spherometer Diagram. Turn the screw gently downwards till it touches the convex surface. Let a be the reading taken from the circular scale which is in line with the vertical scale.
The surface of the convex mirror is round like the back of a spoon and so the central leg must be shorter and must be adjusted up and down until all four tips touch the surface at the same time. You could measure its diameter or the circumference of a great circle, the surface area or the radius.