9 OPTICAL APPARATUS AND INSTRUMENTS, NAMELY, BAND PASS FILTER, EDGE FILTERS, POLARIZING BEAM SPLITTER PLATES, GAIN FLATTENING FILTERS, COUPLERS WITH BAND SPLITTERS, WIDE-BAND BLOCKERS, INTERFERENCE FILTERS, INFRARED FILTERS, LASER MIRRORS, BEAM SPLITTERS, COLD MIRRORS/FILTERS, AND NATURAL DENSITY FILTERS