arrangement of perforations 5 as well as control of the pressure in the water reception chamber 11 determines the presence of a background 17 impressed areas 16 or the absence of one perforations 15 in the variation illustrated in fig6 the curtain of water 7 from the injector 6 according to the invention passes between two perpendicular spouts i e a fixed spout 18 and a movable spout 19 the movable spout 19 is part of a lip or washer 20 which extends along the whole length of the injector 6 and which is linked to a rear counter pressure chamber 21 this chamber is supplied with water at a pressure which should be higher than the pressure level in chamber 11 and consequently in the forward chamber 22 thus the hydraulic thrust in the rear chamber 21 propels the lip 20 along its whole length in the direction indicated in fig6 by the arrow 23 thus tending to reduce the gap between the fixed spout 18 and the movable spout 19 i e reducing the thickness of the curtain of water 7 as it progresses toward the fixed spout 18 arrow 23 the movable lip 20 is halted by stopper screws 24 whose position can be adjusted from the outside for instance with a button or a wheel 25 on the one hand the position of the movable lip 20 can be controlled i e the width of the gap 31 separating the two spouts 18 and 19 so that the curtain of water 7 can be reduced to a very fine depth for instance it can be reduced as low as forty microns on the other hand the spreading of counter pressure in the chamber 21 along the whole length of the movable lip 20 ensures that.