outside atmosphere such a construction makes it possible to carry out the deposition process at a reduced pressure when a pump not shown is connected to the gas outlet the aforesaid cylindrical passage may also be used in a rotary embodiment so that the substrate tube can be rotated continuously or in steps during the deposition process during the vapour deposition process a reaction zone 6 moves back and forth along the length of the hollow glass substrate tube 2 between the reversal point 11 near the supply side 3 and the reversal point 12 near the discharge side 4 which length is also referred to as the deposition length 5 inside the hollow glass substrate tube 2 so as to form glass layers the width 7 of the reaction zone 6 is smaller than the deposition length 5 the present invention is in particular suitable for use in a pcvd type deposition process in which the reaction zone is a low pressure plasma the term u201c low pressure u201d is understood to mean that the plasma is created at a pressure of about 1 20 mbar in the substrate tube while glass forming gases which may or may not be doped are being supplied to the supply side 3 of the hollow glass substrate tube 2 glass layers not shown are deposited along the deposition length 5 on the inner surface of the hollow glass substrate tube 2 a number of glass layers deposited using a more or less constant composition of.