In 1874, Henry Fleuss made what was possibly the first Nitrox dive using a rebreather. Hyperoxic myopia has occurred in closed circuit oxygen rebreather divers with prolonged exposures. Two crewmen in diving suits rode astride, each equipped with an oxygen rebreather apparatus. This makes hypoxia a deadly problem for rebreather divers. His thesis was the first scientific research using an autonomous rebreather diving equipment. Other scuba systems that were invented before the "Aqua-Lung" were "closed circuit" (or "rebreather"). The Divex Shadow rebreather is used in diving operations. The Mk15.5 was his rebreather of choice for dives deeper than . The suit also featured an underwater rebreather. The material used in the rebreather to remove exhaled carbon dioxide was sodium hydroxide. The mouthpiece is connected to the rest of the rebreather by breathing hoses. Gas extension for the depth compensated passive addition rebreather is approximately proportional to metabolic usage. The crew is individually supplied with simple rebreather chest-pack apparatuses for emergency situations. He led development of a type of rebreather. Mixed-gas and rebreather technologies can now be used in many sites.