Abstract: The degree to which a rotating object like a machine tool spindle undesirably wobbles in directions both transverse and parallel to its axis of rotation is measured preferably by providing a laser beam or other beam energy source which directs the beam toward a beam reflecting surface, most advantageously a curved surface, like a spherical or cylindrical surface. The reflecting surface is most advantageously formed by a spherical body mounted for adjustment in a plane perpendicular to the rotating axis involved, so that the center of the curved surface can be positioned as close as possible to the axis involved before error measurements are taken. In such case, the distance the beam traverses between the beam source and the point on the curved surface struck by the beam is a measure of the degree to which the spindle undesirably wobbles in directions both transverse to and parallel to the spindle axis.