26 INSTRUMENTS AND APPARATUS FOR MEASURING LENGTHS, THICKNESSES AND/OR ANGLES; INSTRUMENTS AND APPARATUS FOR MEASURING LENGTHS, THICKNESSES AND/OR ANGLES AND HAVING AS A PART THEREOF MECHANICAL, OPTICAL, AND ELECTRONIC AMPLIFICATION MEANS; GAUGES, MASTER GAUGES, GAUGE BLOCKS FOR MEASURING DIMENSIONS OF OUTER OR INNER SECTIONS AND THICKNESSES RESPECTIVELY