Scientists finally achieved this goal in the 1960s, with the noble gas krypton. All noble gases (think of "neon" lights) emit strong, colored light when excited, and krypton happens to emit a real beauty, a sharp beacon of orange light that's easy to measure. So a meter became 1,650,763.73 wavelengths of this orange light from a krypton-86 atom. That's an e-mailable definition, since all krypton atoms are identical, and scientist could just pick up a krypton discharge tube if he needed it. Scientists had finally relegated the platinum-iridium rod to the velvet casket of a museum.