The outer glass envelope contains a smaller quartz envelope, with a Tungsten filament inside. The quartz envelope also contains Iodine vapour. The Iodine reacts with Tungsten that evaporates and deposits the Tungsten back on the filament. The filament can be run at a temperature above that possible with Tungsten-in-Argon incandescent bulbs, thus providing more illumination for the wattage consumed.
Unfortunately the filament is not locked in place but can swing freely, and likely to shorten its life.