26 OPTICS AND PRECISION OPTICAL ELEMENTS; COMPONENTS USED IN OPTICAL, ELECTRONIC AND SCIENTIFIC INSTRUMENTS-NAMELY, PRISMS, LENSES, OPTICAL FLATS AND RETICLES; QUARTZ MICROSCOPE SLIDES AND COVER SLIPS; VARIOUS MATERIALS, INCLUDING GLASS, QUARTZ, CRYSTALS, AND METALS FABRICATED TO HIGH OPTICAL AND MECHANICAL TOLERANCES