the present invention when the argon gas is discontinued altogether the combination of the water vapor from chamber 12 or an alternate independent source and the additional water vapor created by the glow discharge bombardment of the web will maintain the glow by itself at a much reduced cost the water vapor in compartment 12 can also be used in high index compartment 21 to stabilize the required pressure and background atmosphere therein to properly carry out the deposition of high index material over a significant period of time the high index deposition process starts out in an atmosphere of oxygen and water vapor as the process progresses oxygen is consumed and resupplied at a constant rate from oxygen supply 65 the initial water vapor is also consumed in the process with an attendant drop in pressure in compartment 21 in prior art methods and apparatus where the duration of the process is relatively short this loss of water vapor is compensated for by increasing the flow of oxygen into the compartment for long duration processes however as contemplated by the present invention oxygen alone cannot adequately stabilize the process thus in the present invention water vapor is made available to compartment 21 to augment the water vapor taken from the walls of the compartment.