A method for improving accuracy of a time estimate from a memory device is disclosed. The cover includes a top, a plurality of sides extending from the top and a wide base perimeter extending from the plurality of sides. The latch slides into engagement with the undercut after electrical contacts on the battery have connected with electrical contacts in the battery compartment. The linear prism 4 has two inclined surfaces 4A having an approximately equal tilt angle against a plane which is perpendicular to a thickness direction of the light diffusing member 1. At least one of the engaging jaws of the second piston member cooperates with the retainer ring to form a gap therebetween, which prevents a stress concentration on portions of the circumferential groove adjacent to the axial cutouts, upon an axial movement of the piston, so that the required thickness of the retainer ring can be reduced, leading to size reduction of the piston.